The relation of insects to man as simple parasites has long been studied, and until very recent years the bulk of the literature of medical entomology referred to this phase of the subject. This is now completely overshadowed by the fact that so many of these parasitic forms are more than simple parasites, they are transmitters of other microscopic parasites which are pathogenic to man. Yet the importance of insects as parasites still remains and must be considered in a discussion of the relation of insects to the health of man. In taking up the subject we shall first consider some general features of the phenomenon of animal parasitism.

Parasitism is an adaptation which has originated very often among living organisms and in widely separated groups. It would seem simple to define what is meant by a "parasite" but, in reality, the term is not easily limited. It is often stated that a parasite is "An organism which lives at the expense of another," but this definition is applicable to a predatory species or, in its broadest sense, to all organisms. For our purpose we may say with Braun: "A parasite is an organism which, for the purpose of obtaining food, takes up its abode, temporarily or permanently, on or within another living organism".

Thus, parasitism is a phase of the broad biological phenomenon of symbiosis, or living together of organisms. It is distinguished from mutualism, or symbiosis in the narrow sense, by the fact that only one party to the arrangement obtains any advantage, while the other is to a greater or less extent injured.

Of parasites we may distinguish on the basis of their location on or in the host, ecto-parasites, which live outside of the body; and endo-parasites, which live within the body. On account of their method of breathing the parasitic arthropods belong almost exclusively to the first of these groups.

On the basis of relation to their host, we find temporary parasites, those which seek the host only occasionally, to obtain food; and the stationary or permanent parasites which, at least during certain stages, do not leave their host.

Facultative parasites are forms which are not normally parasitic, but which, when accidentally ingested, or otherwise brought into the body, are able to exist for a greater or less period of time in their unusual environment. These are generally called in the medical literature "pseudoparasites" but the term is an unfortunate one.

We shall now take up the different groups of arthropods, discussing the more important of the parasitic forms attacking man. The systematic relationship of these forms, and key for determining important species will be found in Chapter XII.

Acarina or Mites

The Acarina, or mites, form a fairly natural group of arachnids, characterized, in general, by a sac-like, unsegmented body which is generally fused with the cephalothorax. The mouth-parts have been united to form a beak or rostrum.

The representatives of this group undergo a marked metamorphosis. Commonly, the larvÆ on hatching from the egg, possess but three pairs of legs, and hence are called hexapod larvÆ. After a molt, they transform into nymphs which, like the adult, have four pairs of legs and are called octopod nymphs. These after a period of growth, molt one or more times and, acquiring external sexual organs, become adult.

Most of the mites are free-living, but there are many parasitic species and as these have originated in widely separated families, the Acarina form an especially favorable group for study of the origin of parasitism. Such a study has been made by Ewing (1911), who has reached the following conclusions:

"We have strong evidence indicating that the parasitic habit has originated independently at least eleven times in the phylogeny of the Ararina. Among the zoophagous parasites, the parasitic habit has been developed from three different types of free-living Acarina: (a) predaceous forms, (b) scavengers, (c) forms living upon the juices of plants."

Ewing also showed that among the living forms of Acarina we can trace out all the stages of advancing parasitism, semiparasitism, facultative parasitism, even to the fixed and permanent type, and finally to endoparasitism.

Of the many parasitic forms, there are several species which are serious parasites of man and we shall consider the more important of these. Infestation by mites is technically known as acariasis.

The TrombidiidÆ, or Harvest Mites

In many parts of this country it is impossible for a visitor to go into the fields and, particularly, into berry patches and among tall weeds and grass in the summer or early fall without being affected by an intolerable itching, which is followed, later, by a breaking out of wheals, or papules, surrounded by a bright red or violaceous aureola, (fig.43). It is often regarded as a urticaria or eczema, produced by change of climate, an error in diet, or some condition of general health.

Sooner or later, the victim finds that it is due to none of these, but to the attacks of an almost microscopic red mite, usually called "jigger" or "chigger" in this country. As the term "chigger" is applied to one of the true fleas, Dermatophilus penetrans, of the tropics, these forms are more correctly known as "harvest mites." Natives of an infested region may be so immune or accustomed to its attacks as to be unaware of its presence, though such immunity is by no means possessed by all who have been long exposed to the annoyance.

The harvest mites, or chiggers, attacking man are larval forms, possessing three pairs of legs (fig.44). Their systematic position was at first unknown and they were classed under a special genus Leptus, a name which is very commonly still retained in the medical literature. It is now known that they are the larval forms of various species of the genus Trombidium, a group of predaceous forms, the adults of which feed primarily on insects and their eggs. In this country the species best known are those to be found late in summer, as larvÆ at the base of the wings of houseflies or grasshoppers.

There is much uncertainty as to the species of the larvÆ attacking man but it is clear that several are implicated. Bruyant has shown that in France the larvÆ Trombidium inapinatum and Trombidium holosericeum are those most frequently found. The habit of attacking man is abnormal and the larvÆ die after entering the skin. Normally they are parasitic on various insects.

Most recent writers agree that, on man, they do not bore into the skin, as is generally supposed, but enter a hair follicle or sebaceous gland and from the bottom of this, pierce the cutis with their elongate hypopharynx. According to Braun, there arises about the inserted hypopharynx a fibrous secretion—the so-called "beak" which is, in reality, a product of the host. Dr. J. C. Bradley, however, has made careful observations on their method of attack, and he assures us that the mite ordinarily remains for a long time feeding on the surface of the skin, where it produces the erythema above described. During this time it is not buried in the skin but is able to retreat rapidly into it through a hair follicle or sweat gland. The irritation from the mites ceases after a few days, but not infrequently the intolerable itching leads to so much scratching that secondary infection follows.

Relief from the irritation may be afforded by taking a warm salt bath as soon as possible after exposure or by killing the mites by application of benzine, sulphur ointment or carbolized vaseline. When they are few in number, they can be picked out with a sterile needle.

Much may be done in the way of warding off their attacks by wearing gaiters or close-woven stockings extending from ankle to the knee. Still more efficacious is the sprinkling of flowers of sulphur in the stockings and the underclothes from a little above the knee, down. The writers have known this to make it possible for persons who were especially susceptible to work with perfect comfort in badly infested regions. Powdered naphthalene is successfully used in the same way and as Chittenden (1906) points out, is a safeguard against various forms of man-infesting tropical insect pests.

The question of the destruction of the mites in the field is sometimes an important one, and under some conditions, is feasible. Chittenden states that much can be accomplished by keeping the grass, weeds, and useless herbage mowed closely, so as to expose the mites to the sun. He believes that in some cases good may be done by dusting the grass and other plants, after cutting, with flowers of sulphur or by spraying with dilute kerosene emulsion in which sulphur has been mixed. More recently (1914) he calls attention to the value of cattle, and more especially sheep, in destroying the pests by tramping on them and by keeping the grass and herbage closely cropped.

Ixodoidea or Ticks

Until recently, the ticks attracted comparatively little attention from entomologists. Since their importance as carriers of disease has been established, interest in the group has been enormously stimulated and now they rank second only to the mosquitoes in the amount of detailed study that has been devoted to them.

The ticks are the largest of the Acarina. They are characterized by the fact that the hypostome, or "tongue" (fig.45) is large and file-like, roughened by sharp teeth. They possess a breathing pore on each side of the body, above the third or fourth coxÆ (fig.45b).

There are two distinct families of the Ixodoidea, differing greatly in structure, life-history and habits. These are the ArgasidÆ and the IxodidÆ. We shall follow Nuttall (1908) in characterizing these two families and in pointing out their biological differences, and shall discuss briefly the more important species which attack man. The consideration of the ticks as carriers of disease will be reserved for a later chapter.

ArgasidÆ

In the ticks belonging to the family ArgasidÆ, there is comparatively little sexual dimorphism, while this is very marked in the IxodidÆ. The capitulum, or so-called "head" is ventral, instead of terminal; the palpi are leg-like, with the segments subequal; the scutum, or dorsal shield, is absent; eyes, when present, are lateral, on supracoxal folds. The spiracles are very small; coxÆ unarmed; tarsi without ventral spurs, and the pulvilli are absent or rudimentary.

In habits and life history the ArgasidÆ present striking characteristics. In the first place, they are long-lived, a factor which counts for much in the maintenance of the species. They are intermittent feeders, being comparable with the bed-bug in this respect. There are two or more nymphal stages, and they may molt after attaining maturity. The female lays comparatively few eggs in several small batches.

Nuttall (1911) concludes that "The ArgasidÆ represent the relatively primitive type of ticks because they are less constantly parasitic than are the IxodidÆ. Their nymphs and adults are rapid feeders and chiefly infest the habitat of their hosts. * * * Owing to the ArgasidÆ infesting the habitats of their hosts, their resistance to prolonged starvation and their rapid feeding habits, they do not need to bring forth a large progeny, because there is less loss of life in the various stages, as compared with the IxodidÆ, prior to their attaining maturity."

Of the ArgasidÆ, we have in the United States, several species which have been reported as attacking man.

Argas persicus, the famous "Miana bug" (fig.46), is a very widely distributed species, being reported from Europe, Asia, Africa, and Australia. It is everywhere preeminently a parasite of fowls. According to Nuttall it is specifically identical with Argas americanus Packard or Argas miniatus Koch, which is commonly found on fowls in the United States, in the South and Southwest. Its habits are comparable to those of the bed-bug. It feeds intermittently, primarily at night, and instead of remaining on its host, it then retreats to cracks and crevices. Hunter and Hooker (1908) record that they have found the larva to remain attached for five or eight days before dropping. Unlike the IxodidÆ, the adults oviposit frequently.

The most remarkable feature of the biology of this species is the great longevity, especially of the adult. Hunter and Hooker report keeping larvÆ confined in summer in pill boxes immediately after hatching for about two months while under similar conditions those of the Ixodid, Boophilus annulatus lived for but two or three days. Many writers have recorded keeping adults for long periods without food. We have kept specimens in a tin box for over a year and a half and at the end of that time a number were still alive. Laboulliene kept unfed adults for over three years. In view of the effectiveness of sulphur in warding off the attacks of TrombidiidÆ, it is astonishing to find that Lounsbury has kept adults of Argas persicus for three months in a box nearly filled with flowers of sulphur, with no apparent effect on them.

We have already called attention to the occasional serious effects of the bites of this species. While such reports have been frequently discredited there can be no doubt that they have foundation in fact. The readiness with which this tick attacks man, and the extent to which old huts may be infested makes it especially troublesome.

Otiobius (Ornithodoros) megnini, the "spinose ear-tick" (figs. 47,48), first described from Mexico, as occurring in the ears of horses, is a common species in our Southwestern States and is recorded by Banks as occurring as far north as Iowa.

The species is remarkable for the great difference between the spiny nymph stage and the adult. The life history has been worked out by Hooker (1908). Seed ticks, having gained entrance to the ear, attach deeply down in the folds, engorge, and in about five days, molt; as nymphs with their spinose body they appear entirely unlike the larvÆ. As nymphs they continue feeding sometimes for months. Finally the nymph leaves the host, molts to form the unspined adult, and without further feeding is fertilized and commences oviposition.

The common name is due to the fact that in the young stage the ticks occur in the ear of their hosts, usually horses or cattle. Not uncommonly it has been reported as occurring in the ear of man and causing very severe pain. Stiles recommends that it be removed by pouring some bland oil into the ear.

Banks (1908) reports three species of Ornithodoros—O. turicata, coriaceus and talaje—as occurring in the United States. All of these attack man and are capable of inflicting very painful bites.

IxodidÆ

The ticks belonging to the family IxodidÆ (figs. 49and50) exhibit a marked sexual dimorphism. The capitulum is anterior, terminal, instead of ventral as in the ArgasidÆ; the palpi are relatively rigid (except in the subfamily IxodinÆ), with rudimentary fourth segment; scutum present; eyes, when present, dorsal, on side of scutum. The spiracles are generally large, situated well behind the fourth coxÆ; coxÆ generally with spurs; pulvilli always present.

In habits and life history the typical IxodidÆ differ greatly from the ArgasidÆ. They are relatively short-lived, though some recent work indicates that their longevity has been considerably under-estimated. Typically, they are permanent feeders, remaining on the host, or hosts, during the greater part of their life. They molt twice only, on leaving the larval and the nymphal stages. The adult female deposits a single, large batch of eggs. Contrasting the habits of the IxodidÆ to those of the ArgasidÆ, Nuttall (1911) emphasizes that the IxodidÆ are more highly specialized parasites. "The majority are parasitic on hosts having no fixed habitat and consequently all stages, as a rule, occur upon the host."

As mere parasites of man, apart from their power to transmit disease, the IxodidÆ are much less important than the ArgasidÆ. Many are reported as occasionally attacking man and of these the following native species may be mentioned.

Ixodes ricinus, the European castor bean tick (figs. 49,50), is a species which has been often reported from this country but Banks (1908) has shown that, though it does occur, practically all of the records apply to Ixodes scapularis or Ixodes cookei. In Europe, Ixodes ricinus is very abundant and very commonly attacks man. At the point of penetration of the hypostome there is more or less inflammation but serious injury does not occur unless there have been introduced pathogenic bacteria or, unless the tick has been abruptly removed, leaving the capitulum in the wound. Under the latter circumstances, there may be an abscess formed about the foreign body and occasionally, serious results have followed. Under certain conditions the tick, in various stages, may penetrate under the skin and produce a tumor, within which it may survive for a considerable period of time.

Ixodes cookei is given by Banks as "common on mammals in the Eastern States as far west as the Rockies." It is said to affect man severely.

Amblyomma americanum, (fig.158c), the "lone star tick," is widely distributed in the United States. Its common name is derived from the single silvery spot on the scutum of the female. Hunter and Hooker regard this species as, next to Boophilus annulatus, the most important tick in the United States. Though more common on cattle, it appears to attack mammals generally, and "in portions of Louisiana and Texas it becomes a pest of considerable importance to moss gatherers and other persons who spend much time in the forests."

Amblyomma cajennense, noted as a pest of man in central and tropical America, is reported from various places in the south and southwestern United States.

Dermacentor variabilis is a common dog tick of the eastern United States. It frequently attacks man, but the direct effects of its bite are negligible.

The "Rocky Mountain spotted fever tick" (Dermacentor andersoni according to Stiles, D. venustus according to Banks) is, from the viewpoint of its effects on man, the most important of the ticks of the United States. This is because, as has been clearly established, it transmits the so-called "spotted fever" of man in our northwestern states. This phase of the subject will be discussed later and it need merely be mentioned here, that this species has been reported as causing painful injuries by its bites. Dr. Stiles states that he has seen cases of rather severe lymphangitis and various sores and swellings developing from this cause. In one case, of an individual bitten near the elbow, the arm became very much swollen and the patient was confined in bed for several days. The so-called tick paralysis produced by this species is discussed in a preceding chapter.

There are many other records of various species of ticks attacking man, but the above-mentioned will serve as typical and it is not necessary to enter into greater detail.Treatment of Tick Bites—When a tick attaches to man the first thing to be done is to remove it without leaving the hypostome in the wound to fester and bring about secondary effects. This is best accomplished by applying to the tick's body some substance which will cause it to more readily loosen its hold. Gasoline or petroleum, oil or vaseline will serve. For removing the spinose ear-tick, Stiles recommends pouring some bland oil into the ear. Others have used effectively a pledget of cotton soaked in chloroform.

In general, the treatment recommended by Wellman for the bites of Ornithodoros moubata will prove helpful. It consists of prolonged bathing in very hot water, followed by the application of a strong solution of bicarbonate of soda, which is allowed to dry upon the skin. He states that this treatment is comforting. For severe itching he advises smearing the bites with vaseline, which is slightly impregnated with camphor or menthol. Medical aid should be sought when complications arise.

The DermanyssidÆ are Gamasid mites which differ from others of the group in that they are parasitic on vertebrates. None of the species normally attack man, but certain of them, especially the poultry mite, may be accidental annoyances.

Dermanyssus gallinÆ (fig.51), the red mite of poultry, is an exceedingly common and widespread parasite of fowls. During the day it lives in cracks and crevices of poultry houses, under supports of roosts, and in litter of the food and nests, coming out at night to feed. They often attack people working in poultry houses or handling and plucking infested fowls. They may cause an intense pruritis, but they do not produce a true dermatosis, for they do not find conditions favorable for multiplication on the skin of man.

TarsonemidÆ

The representatives of the family TarsonemidÆ are minute mites, with the body divided into cephalothorax and abdomen. There is marked sexual dimorphism. The females possess stigmata at the anterior part of the body, at the base of the rostrum, and differ from all other mites in having on each side, a prominent clavate organ between the first and second legs. The larva, when it exists, is hexapodous and resembles the adult. A number of the species are true parasites on insects, while others attack plants. Several of them may be accidental parasites of man.

Pediculoides ventricosus (fig. 52and53) is, of all the TarsonemidÆ reported, the one which has proved most troublesome to man. It is a predaceous species which attacks a large number of insects but which has most commonly been met with by man through its fondness for certain grain-infesting insects, notably the Angoumois grain moth, Sitotroga cerealella, and the wheat straw-worm, Isosoma grande. In recent years it has attracted much attention in the United States and its distribution and habits have been the object of detailed study by Webster (1901).

There is a very striking sexual dimorphism in this species. The non-gravid female is elongate, about 200µ by 70µ (fig.52), with the abdomen slightly striated longitudinally. The gravid female (fig.53) has the abdomen enormously swollen, so that it is from twenty to a hundred times greater than the rest of the body. The species is viviparous and the larvÆ undergo their entire growth in the body of the mother. They emerge as sexually mature males and females which soon pair. The male (fig.54) is much smaller, reaching a length of only 320µ but is relatively broad, 80µ, and angular. Its abdomen is very greatly reduced.

As far back as 1850 it was noted as causing serious outbreaks of peculiar dermatitis among men handling infested grain. For some time the true source of the difficulty was unknown and it was even believed that the grain had been poisoned. Webster has shown that in this country (and probably in Europe as well) its attacks have been mistaken for those of the red bugs or "chiggers" (larval TrombiidÆ). More recently a number of outbreaks of a mysterious "skin disease" were traced to the use of straw mattresses, which were found to be swarming with these almost microscopic forms which had turned their attentions to the occupants of the beds. Other cases cited were those of farmers running wheat through a fanning mill, and of thrashers engaged in feeding unthrashed grain into the cylinder of the machine.

The medical aspects of the question have been studied especially by Schamberg and Goldberger and from the latter's summary (1910) we derive the following data. Within twelve to sixteen hours after exposure, itching appears and in severe cases, especially where exposure is continued night after night by sleeping on an infested bed, the itching may become almost intolerable. Simultaneously, there appears an eruption which characteristically consists of wheals surrounded by a vesicle (fig.55). The vesicle as a rule does not exceed a pin head in size but may become as large as a pea. Its contents rapidly become turbid and in a few hours it is converted into a pustule. The eruption is most abundant on the trunk, slight on the face and extremities and almost absent on the feet and hands. In severe cases there may be constitutional disturbances marked, at the outset, by chilliness, nausea, and vomiting, followed for a few days by a slight elevation of temperature, with the appearance of albumin in the urine. In some cases the eruption may simulate that of chicken-pox or small-pox.

Treatment for the purpose of killing the mites is hardly necessary as they attach feebly to the surface and are readily brushed off by friction of the clothes. "Antipruritic treatment is always called for; warm, mildly alkaline baths or some soothing ointment, such as zinc oxide will be found to fulfil this indication." Of course, reinfestation must be guarded against, by discarding, or thoroughly fumigating infested mattresses, or by avoiding other sources. Goldberger suggests that farm laborers who must work with infested wheat or straw might protect themselves by anointing the body freely with some bland oil or grease, followed by a change of clothes and bath as soon as their work is done. We are not aware of any experiments to determine the effect of flowers of sulphur, but their efficiency in the case of "red bugs" suggests that they are worth a trial against Pediculoides.

Various species of TyroglyphidÆ (fig.150f) may abound on dried fruits and other products and attacking persons handling them, may cause a severe dermatitis, comparable to that described above for Pediculoides ventricosus. Many instances of their occurrence as such temporary ectoparasites are on record. Thus, workers who handle vanilla pods are subject to a severe dermatitis, known as vanillism, which is due to the attacks of Tyroglyphus siro, or a closely related species. The so-called "grocer's itch" is similarly caused by mites infesting various products. Castellani has shown that in Ceylon, workers employed in the copra mills, where dried cocoanut is ground up for export, are much annoyed by mites, which produce the so-called "copra itch." The skin of the hands, arms and legs, and sometimes of the whole body, except the face, is covered by fairly numerous, very pruriginous papules, often covered by small, bloody crusts due to scratching. The condition is readily mistaken for scabies. It is due to the attacks of Tyroglyphus longior castellanii which occur in enormous numbers in some samples of the copra.

SarcoptidÆ

The SarcoptidÆ are minute whitish mites, semi-globular in shape, with a delicate transversely striated cuticula. They lack eyes and tracheÆ. The mouth-parts are fused at the base to form a cone which is usually designated as the head. The legs are short and stout, and composed of five segments. The tarsi may or may not possess a claw and may terminate in a pedunculated sucker, or simple long bristle, or both. The presence or absence of these structures and their distribution are much used in classification. The mites live on or under the skin of mammals and birds, where they produce the disease known as scabies, mange, or itch. Several species of the SarcoptidÆ attack man but the most important of these, and the one pre-eminent as the "itch mite" is Sarcoptes scabiei.

The female of Sarcoptes scabiei, of man, is oval and yellowish white; the male more rounded and of a somewhat reddish tinge, and much smaller. The body is marked by transverse striÆ which are partly interrupted on the back. There are transverse rows of scales, or pointed spines, and scattered bristles on the dorsum.

The male (fig.56) which is from 200-240µ in length, and 150-200µ in breadth, possesses pedunculated suckers on each pair of legs except the third, which bears, instead, a long bristle. The female (fig.56) 300-450µ in length and 250-350µ in breadth, has the pedunculated suckers on the first and second pairs of legs, only, the third and fourth terminating in bristles.

The mite lives in irregular galleries from a few millimeters to several centimeters in length, which it excavates in the epidermis (fig.57). It works especially where the skin is thin, such as between the fingers, in the bend of the elbows and knees, and in the groin, but it is by no means restricted to these localities. The female, alone, tunnels into the skin; the males remain under the superficial epidermal scales, and seldom are found, as they die soon after mating.

As she burrows into the skin the female deposits her eggs, which measure about 150 × 100µ. FÜrstenberg says that each deposits an average of twenty-two to twenty-four eggs, though Gudden reports a single burrow as containing fifty-one. From these there develop after about seven days, the hexapod larvÆ. These molt on the sixteenth day to form an octopod nymph, which molts again the twenty-first day. At the end of the fourth week the nymphs molt to form the sexually mature males and the so-called pubescent females. These pair, the males die, and the females again cast their skin, and become the oviparous females. Thus the life cycle is completed in about twenty-eight days.

The external temperature exercises a great influence on the development of the mites and thus, during the winter, the areas of infestation not only do not spread, but they become restricted. As soon as the temperature rises, the mites increase and the infestation becomes much more extensive.

In considering the possible sources of infestation, and the chances of reinfestation after treatment, the question of the ability of the mite to live apart from its host is a very important one. Unfortunately there are few reliable data on this subject. Gerlach found that, exposed in the dry, warm air of a room they became very inactive within twenty-four hours, that after two days they showed only slight movement, and that after three or four days they could not be revived by moisture and warming. The important fact was brought out that in moist air, in folded soiled underwear, they survived as long as ten days. Bourguignon found that under the most favorable conditions the mites of Sarcoptes scabiei equi would live for sixteen days.

The disease designated the "itch" or "scabies," in man has been known from time immemorial, but until within less than a hundred years it was almost universally attributed to malnutrition, errors of diet, or "bad blood." This was in spite of the fact that the mite was known to Mouffet and that Bonomo had figured both the adult and the egg and had declared the mite the sole cause of the disease. In 1834 the Corsican medical student, Francis Renucci, demonstrated the mite before a clinic in Saint Louis Hospital in Paris and soon thereafter there followed detailed studies of the life history of the various itch mites of man and animals.

The disease is a cosmopolitan one, being exceedingly abundant in some localities. Its spread is much favored where large numbers of people are crowded together under insanitary conditions and hence it increases greatly during wars and is widely disseminated and abundant immediately afterwards. Though more commonly to be met with among the lower classes, it not infrequently appears among those of the most cleanly, careful habits, and it is such cases that are most liable to wrong diagnosis by the physician.

Infection occurs solely through the passage, direct or indirect, of the young fertilized females to the skin of a healthy individual. The adult, oviparous females do not quit their galleries and hence do not serve to spread the disease. The young females move about more or less at night and thus the principal source of infestation is through sleeping in the same bed with an infested person, or indirectly through bedclothes, or even towels or clothing. Diurnal infestation through contact or clothing is exceptional. Many cases are known of the disease being contracted from animals suffering from scabies, or mange.

When a person is exposed to infestation, the trouble manifests itself after eight or ten days, though there usually elapses a period of twenty to thirty days before there is a suspicion of anything serious. The first symptom is an intense itching which increases when the patient is in bed. When the point of irritation is examined the galleries may usually be seen as characteristic sinuous lines, at first whitish in color but soon becoming blackish because of the contained eggs and excrement. The galleries, which may not be very distinct in some cases, may measure as much as four centimeters in length. Little vesicles, of the size of a pin head are produced by the secretions of the feeding mite; they are firm, and projecting, and contain a limpid fluid. Figures58 and59 show the typical appearance of scabies on the hands, while figure60 shows a severe general infestation. The intolerable itching induces scratching and through this various complications may arise. The lesions are not normally found on the face and scalp, and are rare on the back.

Formerly, scabies was considered a very serious disease, for its cause and method of treatment were unknown, and potentially it may continue indefinitely. Generation after generation of the mites may develop and finally their number become so great that the general health of the individual is seriously affected. Now that the true cause of the disease is known, it is easily controlled.

Treatment usually consists in softening the skin by friction with soap and warm water, followed by a warm bath, and then applying some substance to kill the mites. Stiles gives the following directions, modified from Bourguignon's, as "a rather radical guide, to be modified according to facilities and according to the delicacy of the skin or condition of the patient":

1. The patient, stripped naked, is energetically rubbed all over (except the head) for twenty minutes, with green soap and warm water. 2. He is then placed in a warm bath for thirty minutes, during which time the rubbing is continued. 3. The parasiticide is next rubbed in for twenty minutes and is allowed to remain on the body for four or five hours; in the meantime the patient's clothes are sterilized, to kill the eggs or mites attached to them. 4. A final bath is taken to remove the parasiticide.

The parasiticide usually relied on is the officinal sulphur ointment of the United States pharmacopoeia. When infestation is severe it is necessary to repeat treatment after three or four days in order to kill mites which have hatched from the eggs.

The above treatment is too severe for some individuals and may, of itself, produce a troublesome dermatitis. We have seen cases where the treatment was persisted in and aggravated the condition because it was supposed to be due to the parasite. For delicate-skinned patients the use of balsam of Peru is very satisfactory, and usually causes no irritation whatever. Of course, sources of reinfection should be carefully guarded against.

Sarcoptes scabiei crustosÆ, which is a distinct variety, if not species, of the human itch mite, is the cause of so-called Norwegian itch. This disease is very contagious, and is much more resistant than the ordinary scabies. Unlike the latter, it may occur on the face and scalp.

Sarcoptes scabiei not only attacks man but also occurs on a large number of mammals. Many species, based on choice of host, and minute differences in size and secondary characters, have been established, but most students of the subject relegate these to varietal rank. Many of them readily attack man, but they have become sufficiently adapted to their normal host so that they are usually less persistent on man.

Notoedres cati (usually known as Sarcoptes minor) is a species of itch mites which produce an often fatal disease of cats. The body is rounded and it is considerably smaller than Sarcoptes scabiei, the female (fig.61) measuring 215-230µ long and 165-175µ wide; the males 145-150µ by 120-125µ. The most important character separating Notoedres from Sarcoptes is the position of the anus, which is dorsal instead of terminal. The mite readily transfers to man but does not persist, the infestation usually disappearing spontaneously in about two weeks. Infested cats are very difficult to cure, unless treatment is begun at the very inception of the outbreak, and under ordinary circumstances it is better to kill them promptly, to avoid spread of the disease to children and others who may be exposed.

DemodecidÆ

The DemodecidÆ are small, elongate, vermiform mites which live in the hair follicles of mammals. The family characteristics will be brought out in the discussion of the species infesting man, Demodex folliculorum.

Demodex folliculorum (fig.62) is to be found very commonly in the hair follicles and sebaceous glands of man. It is vermiform in appearance, and with the elongate abdomen transversely striated so as to give it the appearance of segmentation. The female is 380-400µ long by 45µ; the male 300µ by 40µ. The three-jointed legs, eight in number, are reduced to mere stubs in the adult. The larval form is hexopod. These mites thus show in their form a striking adaptation to their environment. In the sebaceous glands and hair follicles they lie with their heads down (fig.63). Usually there are only a few in a gland, but Gruby has counted as many as two hundred.

The frequency with which they occur in man is surprising. According to European statistics they are found in 50 per cent to 60 per cent or even more. Gruby found them in forty out of sixty persons examined. These figures are very commonly quoted, but reliable data for the United States seem to be lacking. Our studies indicate that it is very much less common in this country than is generally assumed.

The Demodex in man does not, as a rule, cause the slightest inconvenience to its host. It is often stated that they give rise to comedons or "black-heads" but there is no clear evidence that they are ever implicated. Certain it is that they are not the usual cause. A variety of the same, or a very closely related species of Demodex, on the dog gives rise to the very resistant and often fatal follicular mange.

Hexapoda or True Insects

The Hexapoda, or true insects, are characterized by the fact that the adult possesses three pairs of legs. The body is distinctly segmented and is divided into head, thorax, and abdomen.

The mouth-parts in a generalized form, consist of an upper lip, or labrum, which is a part of the head capsule, and a central unpaired hypopharynx, two mandibles, two maxillÆ and a lower lip, or labium, made up of the fused pair of second maxillÆ. These parts may be greatly modified, dependent upon whether they are used for biting, sucking, piercing and sucking, or a combination of biting and sucking.

Roughly speaking, insects may be grouped into those which undergo complete metamorphosis and those which have incomplete metamorphosis. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes sexually mature.

Representatives of several orders have been reported as accidental or faculative parasites of man, but the true parasites are restricted to four orders. These are the Siphunculata; the Hemiptera, the Diptera and the Siphonaptera.

Siphunculata

The order Siphunculata was established by Meinert to include the true sucking lice. These are small wingless insects, with reduced mouth-parts, adapted for sucking; thorax apparently a single piece due to indistinct separation of its three segments: the compound eyes reduced to a single ommatidium on each side. The short, powerful legs are terminated by a single long claw. Metamorphosis incomplete.

There has been a great deal of discussion regarding the structure of the mouth-parts, and the relationships of the sucking lice, and the questions cannot yet be regarded as settled. The conflicting views are well represented by Cholodkovsky (1904 and 1905) and by Enderlein (1904).

Following Graber, it is generally stated that the mouth-parts consist of a short tube furnished with hooks in front, which constitutes the lower lip, and that within this is a delicate sucking tube derived from the fusion of the labrum and the mandibles. Opposed to this, Cholodkovsky and, more recently, Pawlowsky, (1906), have shown that the piercing apparatus lies in a blind sac under the pharynx and opening into the mouth cavity (fig.64). It does not form a true tube but a furrow with its open surface uppermost. Eysell has shown that, in addition, there is a pair of chitinous rods which he regards as the homologues of the maxillÆ.

When the louse feeds, it everts the anterior part of the mouth cavity, with its circle of hooks. The latter serve for anchoring the bug, and the piercing apparatus is then pushed out.

Most writers have classed the sucking lice as a sub-order of the Hemiptera, but the more recent anatomical and developmental studies render this grouping untenable. An important fact, bearing on the question, is that, as shown by Gross, (1905), the structure of the ovaries is radically different from that of the Hemiptera.

Lice infestation and its effects are known medically as pediculosis. Though their continued presence is the result of the grossest neglect and filthiness, the original infestation may be innocently obtained and by people of the most careful habits.

Three species commonly attack man. Strangely enough, there are very few accurate data regarding their life history.

Pediculus humanus (fig.65), the head louse, is the most widely distributed. It is usually referred to in medical literature as Pediculus capitis, but the Linnean specific name has priority. In color it is of a pale gray, blackish on the margins. It is claimed by some authors that the color varies according to the color of the skin of the host. The abdomen is composed of seven distinct segments, bearing spiracles laterally. There is considerable variation in size. The males average 1.8 mm. and the females 2.7 mm. in length.

The eggs, fifty to sixty in number, stick firmly to the hairs of the host and are known as nits. They are large and conspicuous, especially on dark hair and are provided with an operculum, or cap, at the free end, where the nymphs emerge. They hatch in about six days and about the eighteenth day the young lice are sexually mature.

The head lice live by preference on the scalp of their host but occasionally they are found on the eyelashes and beard, or in the pubic region. They may also occur elsewhere on the body. The penetration of the rostrum into the skin and the discharge of an irritating saliva produce a severe itching, accompanied by the formation of an eczema-like eruption (fig.66). When the infestation is severe, the discharge from the pustules mats down the hair, and scabs are formed, under which the insects swarm. "If allowed to run, a regular carapace may form, called trichoma, and the head exudes a foetid odor. Various low plants may grow in the trichoma, the whole being known as plica palonica."—Stiles.

Sources of infestation are various. School children may obtain the lice from seatmates, by wearing the hats or caps of infested mates, or by the use, in common, of brushes and combs. They may be obtained from infested beds or sleeper berths. Stiles reports an instance in which a large number of girls in a fashionable boarding school developed lousiness a short time after traveling in a sleeping car.

Treatment is simple, for the parasites may readily be controlled by cleanliness and washing the head with a two per cent solution of carbolic acid or even kerosene. The latter is better used mixed with equal parts of olive oil, to avoid irritation. The treatment should be applied at night and followed the next morning by a shampoo with soap and warm water. It is necessary to repeat the operation in a few days. Xylol, used pure, or with the addition of five per cent of vaseline, is also very efficacious. Of course, the patient must be cautioned to stay away from a lighted lamp or fire while using either the kerosene or xylol. While these treatments will kill the eggs or nits, they will not remove them from the hairs. Pusey recommends repeated washings with vinegar or 25 per cent of acetic acid in water, for the purpose of loosening and removing the nits.

Treatment of severe infestations in females is often troublesome on account of long hair. For such cases the following method recommended by Whitfield (1912) is especially applicable:

The patient is laid on her back on the bed with her head over the edge, and beneath the head is placed a basin on a chair so that the hair lies in the basin. A solution of 1 in 40 carbolic acid is then poured over the hair into the basin and sluiced backwards and forwards until the whole of the hair is thoroughly soaked with it. It is especially necessary that care should be taken to secure thorough saturation of the hair over the ears and at the nape of the neck, since these parts are not only the sites of predilection of the parasites but they are apt to escape the solution. This sluicing is carried out for ten minutes by the clock. At the end of ten minutes the hair is lifted from the basin and allowed to drain, but is not dried or even thoroughly wrung out. The whole head is then swathed with a thick towel or better, a large piece of common house flannel, which is fastened up to form a sort of turban, and is allowed to remain thus for an hour. It can then be washed or simply allowed to dry, as the carbolic quickly disperses. At the end of this period every pediculus and what is better, every ovum is dead and no relapse will occur unless there is exposure to fresh contagion. Whitfield states that there seem to be no disadvantages in this method, which he has used for years. He has never seen carboluria result from it, but would advise first cutting the hair of children under five years of age.

Pediculus corporis (= P. vestimenti) the body louse, is larger than the preceding species, the female measuring 3.3 mm., and the male 3 mm. in length. The color is a dirty white, or grayish. P. corporis has been regarded by some authorities as merely a variety of P. humanus but Piaget maintains there are good characters separating the two species.

The body louse lives in the folds and seams of the clothing of its host, passing to the skin only when it wishes to feed. Brumpt states that he has found enormous numbers of them in the collars of glass-ware or grains worn by certain naked tribes in Africa.

Exact data regarding the life-history of this species have been supplied, in part, by the work of Warburton (1910), cited by Nuttall. He found that Pediculus corporis lives longer than P. humanus under adverse conditions. This is doubtless due to its living habitually on the clothing, whereas humanus lives upon the head, where it has more frequent opportunities of feeding. He reared a single female upon his own person, keeping the louse enclosed in a cotton-plugged tube with a particle of cloth to which it could cling. The tube was kept next to his body, thus simulating the natural conditions of warmth and moisture under which the lice thrive. The specimen was fed twice daily, while it clung to the cloth upon which it rested. Under these conditions she lived for one month. Copulation commenced five days after the female had hatched and was repeated a number of times, sexual union lasting for hours. The female laid one hundred and twenty-four eggs within twenty-five days.

The eggs hatched after eight days, under favorable conditions, such as those under which the female was kept. They did not hatch in the cold. Eggs kept near the person during the day and hung in clothing by the bedside at night, during the winter, in a cold room, did not hatch until the thirty-fifth day. When the nymphs emerge from the eggs, they feed at once, if given a chance to do so. They are prone to scatter about the person and abandon the fragment of cloth to which the adult clings.

The adult stage is reached on the eleventh day, after three molts, about four days apart. Adults enter into copulation about the fifth day and as the eggs require eight days for development, the total cycle, under favorable conditions, is about twenty-four days. Warburton's data differ considerably from those commonly quoted and serve to emphasize the necessity for detailed studies of some of the commonest of parasitic insects.

Body lice are voracious feeders, producing by their bites and the irritating saliva which they inject, rosy elevations and papules which become covered with a brownish crust. The intense itching provokes scratching, and characteristic white scars (fig.67) surrounded by brownish pigment (fig.68) are formed. The skin may become thickened and take on a bronze tinge. This melanoderma is especially marked in the region between the shoulders but it may become generalized, a prominent characteristic of "vagabond's disease." According to Dubre and Beille, this melanoderma is due to a toxic substance secreted by the lice, which indirectly provokes the formation of pigment.

Control measures, in the case of the body louse, consist in boiling or steaming the clothes or in some cases, sterilizing by dry heat. The dermatitis may be relieved by the use of zinc-oxide ointment, to which Pusey recommends that there be added, on account of their parasiticidal properties, sulphur and balsam of Peru, equal parts, 15 to 30 grains to the ounce.

Phthirius pubis (=P. inguinalis), the pubic louse, or so-called "crab louse," differs greatly from the preceding in appearance. It is characterized by its relatively short head which fits into a broad depression in the thorax. The latter is broad and flat and merges into the abdomen. The first pair of legs is slender and terminated by a straight claw. The second and third pairs of legs are thicker and are provided with powerful claws fitted for clinging to hairs. The females (fig.69) measure 1.5 to 2 mm. in length by 1.5 mm. in breadth. The male averages a little over half as large. The eggs, or nits, are fixed at the base of the hairs. Only a few, ten to fifteen are deposited by a single female, and they hatch in about a week's time. The young lice mature in two weeks.

The pubic louse usually infests the hairs of the pubis and the perineal region. It may pass to the arm pits or even to the beard or moustache. Rarely, it occurs on the eyelids, and it has even been found, in a very few instances, occurring in all stages, on the scalp. Infestation may be contracted from beds or even from badly infested persons in a crowd. We have seen several cases which undoubtedly were due to the use of public water closets. It produces papular eruption and an intense pruritis. When abundant, there occurs a grayish discoloration of the skin which Duguet has shown is due to a poisonous saliva injected by the louse, as is the melanoderma caused by the body louse.

The pubic louse may be exterminated by the measures recommended for the head louse, or by the use of officinal mercurial ointment.

Hemiptera

Several species of Hemiptera-Heteroptera are habitual parasites of man, and others occur as occasional or accidental parasites. Of all these, the most important and widespread are the bed-bugs, belonging to the genus Cimex (= Acanthia).The Bed-bugs—The bed-bugs are characterized by a much flattened oval body, with the short, broad head unconstricted behind, and fitting into the strongly excavated anterior margin of the thorax. The compound eyes are prominent, simple eyes lacking. AntennÆ four-jointed, the first segment short, the second long and thick, and the third and fourth slender. The tarsi are short and three segmented.

It is often assumed in the literature of the subject that there is but a single species of Cimex attacking man, but several such species are to be recognized. These are distinguishable by the characters given in Chapter XII. We shall consider especially Cimex lectularius, the most common and widespread species.

Cimex lectularius (= Acanthia lectularia, Clinocoris lectularius), is one of the most cosmopolitan of human parasites but, like the lice, it has been comparatively little studied until recent years, when the possibility that it may be concerned with the transmission of various diseases has awakened interest in the details of its life-history and habits.

The adult insect (fig.70) is 4-5 mm. long by 3 mm. broad, reddish brown in color, with the beak and body appendages lighter in color. The short, broad and somewhat rectangular head has no neck-like constriction but fits into the broadly semilunar prothorax. The four segmented labium or proboscis encloses the lancet-like maxillÆ and mandibles. The distal of the four antennal segments is slightly club-shaped. The prothorax is characteristic of the species, being deeply incised anteriorly and with its thin lateral margins somewhat turned up. The mesothorax is triangular, with the apex posteriorly, and bears the greatly atrophied first pair of wings. There is no trace of the metathoracic pair. The greatly flattened abdomen has eight visible segments, though in reality the first is greatly reduced and has been disregarded by most writers. The body is densely covered with short bristles and hairs, the former being peculiarly saber-shaped structures sharply toothed at the apex and along the convex side (fig.159b).

The peculiar disagreeable odor of the adult bed-bug is due to the secretion of the stink glands which lie on the inner surface of the mesosternum and open by a pair of orifices in front of the metacoxÆ, near the middle line. In the nymphs, the thoracic glands are not developed but in the abdomen there are to be found three unpaired dorsal stink glands, which persist until the fifth molt, when they become atrophied and replaced by the thoracic glands. The nymphal glands occupy the median dorsal portion of the abdomen, opening by paired pores at the anterior margin of the fourth, fifth and sixth segments. The secretion is a clear, oily, volatile fluid, strongly acid in reaction. Similar glands are to be found in most of the Hemiptera-Heteroptera and their secretion is doubtless protective, through being disagreeable to the birds. In the bed-bug, as Marlatt points out, "it is probably an illustration of a very common phenomenon among animals, i.e., the persistence of a characteristic which is no longer of any special value to the possessor." In fact, its possession is a distinct disadvantage to the bed-bug, as the odor frequently reveals the presence of the bugs, before they are seen.

The eggs of the bed-bug (fig.70) are pearly white, oval in outline, about a millimeter long, and possess a small operculum or cap at one end, which is pushed off when the young hatches. They are laid intermittently, for a long period, in cracks and crevices of beds and furniture, under seams of mattresses, under loose wall paper, and similar places of concealment of the adult bugs. Girault (1905) observed a well-fed female deposit one hundred and eleven eggs during the sixty-one days that she was kept in captivity. She had apparently deposited some of her eggs before being captured.

The eggs hatch in six to ten days, the newly emerged nymphs being about 1.5 mm. in length and of a pale yellowish white color. They grow slowly, molting five times. At the last molt the mesathoracic wing pads appear, characteristic of the adult. The total length of the nymphal stage varies greatly, depending upon conditions of food supply, temperature and possibly other factors. Marlatt (1907) found under most favorable conditions a period averaging eight days between molting which, added to an equal egg period, gave a total of about seven weeks from egg to adult insect. Girault (1912) found the postembryonic period as low as twenty-nine days and as high as seventy days under apparently similar and normal conditions of food supply. Under optimum and normal conditions of food supply, beginning August 27, the average nymphal life was 69.9 days; average number of meals 8.75 and the molts 5. Under conditions allowing about half the normal food supply the average nymphal life was from 116.9 to 139 days. Nymphs starved from birth lived up to 42 days. We have kept unfed nymphs, of the first stage, alive in a bottle for 75 days. The interesting fact was brought out that under these conditions of minimum food supply there were sometimes six molts instead of the normal number.

The adults are remarkable for their longevity, a factor which is of importance in considering the spread of the insect and methods of control. Dufour (1833) (not De Geer, as often stated) kept specimens for a year, in a closed vial, without food. This ability, coupled with their willingness to feed upon mice, bats, and other small mammals, and even upon birds, accounts for the long periods that deserted houses and camps may remain infested. There is no evidence that under such conditions they are able to subsist on the starch of the wall paper, juices of moistened wood, or the moisture in the accumulations of dust, as is often stated.

There are three or four generations a year, as Girault's breeding experiments have conclusively shown. He found that the bed-bug does not hibernate where the conditions are such as to allow it to breed and that breeding is continuous unless interrupted by the lack of food or, during the winter, by low temperature.

Bed-bugs ordinarily crawl from their hiding places and attack the face and neck or uncovered parts of the legs and arms of their victims. If undisturbed, they will feed to repletion. We have found that the young nymph would glut itself in about six minutes, though some individuals fed continuously for nine minutes, while the adult required ten to fifteen minutes for a full meal. When gorged, it quickly retreats to a crack or crevice to digest its meal, a process which requires two or three days. The effect of the bite depends very greatly on the susceptibility of the individual attacked. Some persons are so little affected that they may be wholly ignorant of the presence of a large number of bugs. Usually the bite produces a small hard swelling, or wheal, whitish in color. It may even be accompanied by an edema and a disagreeable inflammation, and in such susceptible individuals the restlessness and loss of sleep due to the presence of the insects may be a matter of considerable importance. Stiles (1907) records the case of a young man who underwent treatment for neurasthenia, the diagnosis being agreed upon by several prominent physicians; all symptoms promptly disappeared, however, immediately following a thorough fumigation of his rooms, where nearly a pint of bed-bugs were collected.

It is natural to suppose that an insect which throughout its whole life is in such intimate relationship with man should play an important rÔle in the transmission of disease. Yet comparatively little is definitely known regarding the importance of the bed-bug in this respect. It has been shown that it is capable of transmitting the bubonic plague, and South American trypanosomiasis. Nuttall succeeded in transmitting European relapsing fever from mouse to mouse by its bite. It has been claimed that Oriental sore, tuberculosis, and even syphilis may be so carried. These phases of the subject will be considered later.

The sources of infestation are many, and the invasion of a house is not necessarily due to neglect, though the continued presence of the pests is quite another matter. In apartments and closely placed houses they are known to invade new quarters by migration. They are frequently to be met with in boat and sleeper berths, and even the plush seats of day coaches, whence a nucleus may be carried in baggage to residences. They may be brought in the laundry or in clothes of servants.

Usually they are a great scourge in frontier settlements and it is generally believed that they live in nature under the bark of trees, in lumber, and under similar conditions. This belief is founded upon the common occurrence of bugs resembling the bed-bug, in such places. As a matter of fact, they are no relation to bed-bugs but belong to plant-feeding forms alone (fig.19 c,d).

It is also often stated that bed-bugs live in poultry houses, in swallows nests, and on bats, and that it is from these sources that they gain access to dwellings. These bugs are specifically distinct from the true bed-bug, but any of them may, rarely, invade houses. Moreover, chicken houses are sometimes thoroughly infested with the true Cimex lectularius.

Control measures consist in the use of iron bedsteads and the reduction of hiding places for the bugs. If the infestation is slight they may be exterminated by a vigilant and systematic hunt, and by squirting gasoline or alcohol into cracks and crevices of the beds, and furniture. Fumigation must be resorted to in more general infestations.

The simplest and safest method of fumigation is by the use of flowers of sulphur at the rate of two pounds to each one thousand cubic feet of room space. The sulphur should be placed in a pan, a well made in the top of the pile and a little alcohol poured in, to facilitate burning. The whole should be placed in a larger pan and surrounded by water so as to avoid all danger of fire. Windows should be tightly closed, beds, closets and drawers opened, and bedding spread out over chairs in order to expose them fully to the fumes. As metal is tarnished by the sulphur fumes, ornaments, clocks, instruments, and the like should be removed. When all is ready the sulphur should be fired, the room tightly closed and left for twelve to twenty-four hours. Still more efficient in large houses, or where many hiding places favor the bugs, is fumigation with hydrocyanic acid gas. This is a deadly poison and must be used under rigid precautions. Through the courtesy of Professor Herrick, who has had much experience with this method, we give in the Appendix, the clear and detailed directions taken from his bulletin on "Household Insects."

Fumigation with formaldehyde gas, either from the liquid or "solid" formalin, so efficient in the case of contagious diseases, is useless against bed-bugs and most other insects.Other Bed-bugs—Cimex hemipterus (= C. rotundatus) is a tropical and subtropical species, occurring in both the old and new world. Patton and Cragg state that it is distributed throughout India, Burma, Assam, the Malay Peninsula, Aden, the Island of Mauritius, Reunion, St. Vincent and Porto Rico. "It is widely distributed in Africa, and is probably the common species associated there with man." Brumpt also records it for Cuba, the Antilles, Brazil, and Venezuela.

This species, which is sometimes called the Indian bed-bug, differs from C. lectularius in being darker and in having a more elongate abdomen. The head also is shorter and narrower, and the prothorax has rounded borders.

It has the same habits and practically the same life cycle as Cimex lectularius. Mackie, in India, has found that it is capable of transmitting the Asiatic type of recurrent fever. Roger suggested that it was also capable of transmitting Kala-azar and Patton has described in detail the developmental stages of Leishmania, the causative organism of Kala-azar, in the stomach of this bug, but Brumpt declares that the forms described are those of a common, non-pathogenic flagellate to be found in the bug, and have nothing to do with the human disease. Brumpt has shown experimentally that Cimex hemipterus may transmit Trypanosoma cruzi in its excrement.

Cimex boueti, occurring in French Guinea, is another species attacking man. Its habits and general life history are the same as for the above species. It is 3 to 4.5 mm. in length, has vestigial elytra, and much elongated antennÆ and legs. The extended hind legs are about as long as the body.

Cimex columbarius, a widely distributed species normally living in poultry houses and dove cotes, C.inodorus, infesting poultry in Mexico, C.hirundinis, occurring in the nests of swallows in Europe and Oeciacus vicarius (fig.19i) occurring in swallows' nests in this country, are species which occasionally infest houses and attack man.

Conorhinus sanguisugus, the cone-nosed bed-bug. We have seen in our consideration of poisonous insects, that various species of Reduviid bugs readily attack man. Certain of these are nocturnal and are so commonly found in houses that they have gained the name, of "big bed-bugs." The most noted of these, in the United States, is Conorhinus sangiusugus (fig.71), which is widely distributed in our Southern States.

Like its near relatives, Conorhinus sangiusugus is carnivorous in habit and feeds upon insects as well as upon mammalian and human blood. It is reported as often occurring in poultry houses and as attacking horses in barns. The life history has been worked out in considerable detail by Marlatt, (1902), from whose account we extract the following.

The eggs are white, changing to yellow and pink before hatching. The young hatch within twenty days and there are four nymphal stages. In all these stages the insect is active and predaceous, the mouth-parts (fig.72) being powerfully developed. The eggs are normally deposited, and the early stages are undoubtedly passed, out of doors, the food of the immature forms being other insects. Immature specimens are rarely found indoors. It winters both in the partly grown and adult stage, often under the bark of trees or in any similar protection, and only in its nocturnal spring and early summer flights does it attack men. Marlatt states that this insect seems to be decidedly on the increase in the region which it particularly infests,—the plains region from Texas northward and westward. In California a closely related species of similar habits is known locally as the "monitor bug."

The effect of the bite of the giant bed-bug on man is often very severe, a poisonous saliva apparently being injected into the wound. We have discussed this phase of the subject more fully under the head of poisonous insects.

Conorhinus megistus is a Brazilian species very commonly attacking man, and of special interest since Chagas has shown that it is the carrier of a trypanosomiasis of man. Its habits and life history have been studied in detail by Neiva, (1910).

This species is now pre-eminently a household insect, depositing its eggs in cracks and crevices in houses, though this is a relatively recent adaptation. The nymphs emerge in from twenty to forty days, depending upon the temperature. There are five nymphal stages, and as in the case of true bed-bugs, the duration of these is very greatly influenced by the availability of food and by temperature. Neiva reckons the entire life cycle, from egg to egg, as requiring a minimum of three hundred and twenty-four days.

The nymphs begin to suck blood in three to five days after hatching. They usually feed at night and in the dark, attacking especially the face of sleeping individuals. The bite occasions but little pain. The immature insects live in cracks and crevices in houses and invade the beds which are in contact with walls, but the adults are active flyers and attack people sleeping in hammocks. The males as well as the females are blood suckers.

Like many blood-sucking forms, Conorhinus megistus can endure for long periods without food. Neiva received a female specimen which had been for fifty-seven days alive in a tightly closed box. They rarely feed on two consecutive days, even on small quantities of blood, and were never seen to feed on three consecutive days.

Methods of control consist in screening against the adult bugs, and the elimination of crevices and such hiding places of the nymphs. Where the infestation is considerable, fumigation with sulphur is advisable.

Parasitic Diptera or Flies

Of the Diptera or two-winged flies, many species occasionally attack man. Of these, a few are outstanding pests, many of them may also serve to disseminate disease, a phase of our subject which will be considered later. We shall now consider the most important of the group from the viewpoint of their direct attacks on man.

PsychodidÆ or Moth-Flies

The PsychodidÆ or Moth-flies, include a few species which attack man, and at least one species, Phlebotomus papatasii, is known to transmit the so-called "three-day fever" of man. Another species is supposed to be the vector of Peruvian verruga.

The family is made up of small, sometimes very small, nematocerous Diptera, which are densely covered with hairs, giving them a moth-like appearance. The wings are relatively large, oval or lanceolate in shape, and when at rest are held in a sloping manner over the abdomen, or are held horizontally in such a way as to give the insect a triangular outline. Not only is the moth-like appearance characteristic, but the venation of the wings (fig.163, d) is very peculiar and, according to Comstock, presents an extremely generalized form. All of the longitudinal veins separate near the base of the wing except veins R₂ and R₃ and veins M₁ and M₂. Cross veins are wanting in most cases.

Comparatively little is known regarding the life-history and habits of the PsychodidÆ, but one genus, Phlebotomus, contains minute, blood-sucking species, commonly known as sand-flies. The family is divided into two subfamilies, the PsychodinÆ and the PhlebotominÆ. The second of these, the PhlebotominÆ, is of interest to us.The PhlebotominÆ—The PhlebotominÆ differ from the PsychodinÆ in that the radical sector branches well out into the wing rather than at the base of the wing. They are usually less hairy than the PsychodinÆ. The ovipositor is hidden and less strongly chitinized. The species attacking man belong to the genus Phlebotomus, small forms with relatively large, hairy wings which are held upright, and with elongate proboscis. The mandibles and maxillÆ are serrated and fitted for biting.

According to Miss Summers (1913) there are twenty-nine known species of the genus Phlebotomus, five European, eleven Asiatic, seven African and six American. One species only, Phlebotomus vexator, has been reported for the United States. This was described by Coquillett, (1907), from species taken on Plummer's Island, Maryland. It measures only 1.5 mm. in length. As it is very probable that this species is much more widely distributed, and that other species of these minute flies will be found to occur in our fauna, we quote Coquillett's description.

Phlebotomus vexator, Coq.: Yellow, the mesonotum brown, hairs chiefly brown; legs in certain lights appear brown, but are covered with a white tomentum; wings hyaline, unmarked; the first vein (R₁) terminates opposite one-fifth of the length of the first submarginal cell (cell R₂); this cell is slightly over twice as long as its petiole; terminal, horny portion of male claspers slender, bearing many long hairs; the apex terminated by two curved spines which are more than one-half as long as the preceding part, and just in front of these are two similar spines, while near the middle of the length of this portion is a fifth spine similar to the others. Length 1.5 mm.

The life-history of the Phlebotomus flies has been best worked out for the European Phlebotomus papatasii and we shall briefly summarize the account of Doerr and Russ (1913) based primarily on work on this species. The European Phlebotomus flies appear at the beginning of the warm season, a few weeks after the cessation of the heavy rains and storms of springtime. They gradually become more abundant until they reach their first maximum, which in Italy is near the end of July (Grassi). They then become scarcer but reach a second maximum in September. At the beginning of winter they vanish completely, hibernating individuals not being found.

After fertilization there is a period of eight to ten days before oviposition. The eggs are then deposited, the majority in a single mass covered by a slimy secretion from the sebaceous glands. The larvÆ emerge in fourteen to twenty days. There is uncertainty as to the length of larval life, specimens kept in captivity remaining fifty or more days without transforming. Growth may be much more rapid in nature. The larvÆ do not live in fluid media but in moist detritus in dark places. Marett believes that they live chiefly on the excrement of pill-bugs (OniscidÆ) and lizards. Pupation always occurs during the night. The remnants of the larval skin remain attached to the last two segments of the quiescent pupa and serve to attach it to the stone on which it lives. The pupal stage lasts eleven to sixteen days, the adult escaping at night.

Only the females suck blood. They attack not only man but all warm-blooded animals and, according to recent workers, also cold-blooded forms, such as frogs, lizards, and larvÆ. Indeed, Townsend (1914) believes that there is an intimate relation between Phlebotomus and lizards, or other reptiles the world over. The Phlebotomus passes the daylight hours within the darkened recesses of the loose stone walls and piles of rock in order to escape wind and strong light. Lizards inhabit the same places, and the flies, always ready to suck blood in the absence of light and wind, have been found more prone to suck reptilian than mammalian blood.

On hot summer nights, when the wind is not stirring, the Phlebotomus flies, or sand-flies, as they are popularly called, invade houses and sleeping rooms in swarms and attack the inmates. As soon as light begins to break the flies either escape to the breeding places, or cool, dark places protected from the wind, or a part of them remain in the rooms, hiding behind pictures, under garments, and in similar places. Wherever the Phlebotomus flies occur they are an intolerable nuisance. On account of their small size they can easily pass through the meshes of ordinary screens and mosquito curtains. They attack silently and inflict a very painful, stinging bite, followed by itching. The ankles, dorsum of the feet, wrists, inner elbow, knee joint and similar places are favorite places of attack, possibly on account of their more delicate skin.

Special interest has been attracted to these little pests in recent years, since it has been shown that they transmit the European "pappatici fever" or "three day fever." More recently yet, it appears that they are the carriers of the virus of the Peruvian "verruga." This phase of the subject will be discussed later.

Control measures have not been worked out. As Newstead says, "In consideration of the facts which have so far been brought to light regarding the economy of Phlebotomus, it is clearly evident that the task of suppressing these insects is an almost insurmountable one. Had we to deal with insects as large and as accessible as mosquitoes, the adoption of prophylactic measures would be comparatively easy, but owing to the extremely minute size and almost flea-like habits of the adult insects, and the enormous area over which the breeding-places may occur, we are faced with a problem which is most difficult of solution." For these reasons, Newstead considers that the only really prophylactic measures which can at present be taken, are those which are considered as precautionary against the bites of the insects.

Of repellents, he cites as one of the best a salve composed of the following:

Of sprays he recommends as the least objectionable and at the same time one of the most effective, formalin. "The dark portions and angles of sleeping apartments should be sprayed with a one per cent. solution of this substance every day during the season in which the flies are prevalent. A fine spraying apparatus is necessary for its application and an excessive amount must not be applied. It is considered an excellent plan also to spray the mosquito curtains regularly every day towards sunset; nets thus treated are claimed to repel the attacks of these insects." This effectiveness of formalin is very surprising for, as we have seen, it is almost wholly ineffective against bed-bugs, mosquitoes, house flies and other insects, where it has been tried.

A measure which promises to be very effective, where it can be adopted, is the use of electric fans so placed as to produce a current of air in the direction of the windows of sleeping apartments. On account of the inability of the Phlebotomus flies to withstand even slight breezes, it seems very probable that they would be unable to enter a room so protected.

CulicidÆ or Mosquitoes

From the medical viewpoint, probably the most interesting and important of the blood-sucking insects are the mosquitoes. Certainly this is true of temperate zones, such as those of the United States. The result is that no other group of insects has aroused such widespread interest, or has been subjected to more detailed study than have the mosquitoes, since their rÔle as carriers of disease was made known. There is an enormous literature dealing with the group, but fortunately for the general student, this has been well summarized by a number of workers. The most important and helpful of the general works are those of Howard (1901), Smith (1904), Blanchard (1905), Mitchell (1907), and especially of Howard, Dyar, and Knab, whose magnificent monograph is still in course of publication.

Aside from their importance as carriers of disease, mosquitoes are notorious as pests of man, and the earlier literature on the group is largely devoted to references to their enormous numbers and their blood-thirstiness in certain regions. They are to be found in all parts of the world, from the equator to the Arctic and Antarctic regions. LinnÆus, in the "Flora Lapponica," according to Howard, Dyar and Knab, "dwells at some length upon the great abundance of mosquitoes in Lapland and the torments they inflicted upon man and beast. He states that he believes that nowhere else on earth are they found in such abundance and he compares their numbers to the dust of the earth. Even in the open, you cannot draw your breath without having your mouth and nostrils filled with them; and ointments of tar and cream or of fish grease are scarcely sufficient to protect even the case-hardened cuticle of the Laplander from their bite. Even in their cabins, the natives cannot take a mouthful of food or lie down to sleep unless they are fumigated almost to suffocation." In some parts of the Northwestern and Southwestern United States it is necessary to protect horses working in the fields by the use of sheets or burlaps, against the ferocious attacks of these insects. It is a surprising fact that even in the dry deserts of the western United States they sometimes occur in enormous numbers.

Until comparatively recent years, but few species of mosquitoes were known and most of the statements regarding their life-history were based upon the classic work of Reaumur (1738) on the biology of the rain barrel mosquito, Culex pipiens. In 1896, Dr. Howard refers to twenty-one species in the United States, now over fifty are known; Giles, in 1900, gives a total of two hundred and forty-two for the world fauna, now over seven hundred species are known. We have found eighteen species at Ithaca, N. Y.

All of the known species of mosquitoes are aquatic in the larval stage, but in their life-histories and habits such great differences occur that we now know that it is not possible to select any one species as typical of the group. For our present purpose we shall first discuss the general characteristics and structure of mosquitoes, and shall then give the life-history of a common species, following this by a brief consideration of some of the more striking departures from what have been supposed to be the typical condition.

The CulicidÆ are slender, nematocerous Diptera with narrow wings, antennÆ plumose in the males, and usually with the proboscis much longer than the head, slender, firm and adapted for piercing in the female. The most characteristic feature is that the margins of the wings and, in most cases, the wing veins possess a fringe of scale-like hairs. These may also cover in part, or entirely, the head, thorax, abdomen and legs. The females, only, suck blood.

On account of the importance of the group in this country and the desirability of the student being able to determine material in various stages, we show in the accompanying figures the characters most used in classification.

The larvÆ (fig.73) are elongate, with the head and thorax sharply distinct. The larval antennÆ are prominent, consisting of a single cylindrical and sometimes curved segment. The outer third is often narrower and bears at its base a fan-shaped tuft of hairs, the arrangement and abundance of which is of systematic importance. About the mouth are the so-called rotary mouth brushes, dense masses of long hairs borne by the labrum and having the function of sweeping food into the mouth. The form and arrangement of thoracic, abdominal, and anal tufts of hair vary in different species and present characteristics of value. On either side of the eighth abdominal segment is a patch of scales varying greatly in arrangement and number and of much value in separating species. Respiration is by means of tracheÆ which open at the apex of the so-called anal siphon, when it is present. In addition, there are also one or two pairs of tracheal gills which vary much in appearance in different species. On the ventral side of the anal siphon is a double row of flattened, toothed spines whose number and shape are likewise of some value in separating species. They constitute the comb or pecten.

The pupa (fig.139, b) unlike that of most insects, is active, though it takes no food. The head and thorax are not distinctly separated, but the slender flexible abdomen in sharply marked off. The antennÆ, mouth-parts, legs, and wings of the future adult are now external, but enclosed in chitinous cases. On the upper surface, near the base of the wings are two trumpets, or breathing tubes, for the pupal spiracles are towards the anterior end instead of at the caudal end, as in the larva. At the tip of the abdomen is a pair of large chitinous swimming paddles.

As illustrative of the life cycle of a mosquito we shall discuss the development of a common house mosquito, Culex pipiens, often referred to in the Northern United States as the rain barrel mosquito. Its life cycle is often given as typical for the entire group, but, as we have already emphasized, no one species can serve this purpose.

The adults of Culex pipiens hibernate throughout the winter in cellars, buildings, hollow trees, or similar dark shelters. Early in the spring they emerge and deposit their eggs in a raft-like mass. The number of eggs in a single mass is in the neighborhood of two hundred, recorded counts varying considerably. A single female may deposit several masses during her life time. The duration of the egg stage is dependent upon temperature. In the warm summer time the larvÆ may emerge within a day. The larvÆ undergo four molts and under optimum conditions may transform into pupÆ in about a week's time. Under the same favorable conditions, the pupal stage may be completed in a day's time. The total life cycle of Culex pipiens, under optimum conditions, may thus be completed in a week to ten days. This period may be considerably extended under less favorable conditions of temperature and food supply.

Culex pipiens breeds continuously throughout the summer, developing in rain barrels, horse troughs, tin cans, or indeed in any standing water about houses, which lasts for a week or more. The catch basins of sewers furnish an abundant supply of the pests under some conditions. Such places, the tin gutters on residences, and all possible breeding places must be considered in attempts to exterminate this species.

Other species of mosquitoes may exhibit radical departures from Culex pipiens in life-history and habits. To control them it is essential that the biological details be thoroughly worked out for, as Howard, Dyar, and Knab have emphasized, "much useless labor and expense can be avoided by an accurate knowledge of the habits of the species." For a critical discussion of the known facts the reader is referred to their monograph. We shall confine ourselves to a few illustrations.

The majority of mosquitoes in temperate climates hibernate in the egg stage, hatching in the spring or even mild winter days in water from melting snow. It is such single-brooded species which appear in astounding numbers in the far North. Similarly, in dry regions the eggs may stand thorough dessication, and yet hatch out with great promptness when submerged by the rains. "Another provision to insure the species against destruction in such a case, exists in the fact * * * that not all the eggs hatch, a part of them lying over until again submerged by subsequent rains." In temperate North America, a few species pass the winter in the larval state. An interesting illustration of this is afforded by Wyeomia smithii, whose larvÆ live in pitcher plants and are to be found on the coldest winter days imbedded in the solid ice. Late in the spring, the adults emerge and produce several broods during the summer.

In the United States, one of the most important facts which has been brought out by the intensive studies of recent years is that certain species are migratory and that they can travel long distances and become an intolerable pest many miles from their breeding places. This was forcibly emphasized in Dr. Smith's work in New Jersey, when he found that migratory mosquitoes, developing in the salt marshes along the coast, are the dominant species largely responsible for the fame of the New Jersey mosquito. The species concerned are Aedes sollicitans, A. cantator and A. tÆniorhynchus. Dr. Smith decided that the first of these might migrate at least forty miles inland. It is obvious that where such species are the dominant pest, local control measures are a useless waste of time and money. Such migratory habits are rare, however, and it is probable that the majority of mosquitoes do not fly any great distance from their breeding places.

While mosquitoes are thought of primarily as a pest of man, there are many species which have never been known to feed upon human or mammalian blood, no matter how favorable the opportunity. According to Howard, Dyar, and Knab, this is true of Culex territans, one of the common mosquitoes in the summer months in the Northern United States. There are some species, probably many, in which the females, like the males, are plant feeders. In experimental work, both sexes are often kept alive for long periods by feeding them upon ripe banana, dried fig, raisins, and the like, and in spite of sweeping assertions that mosquitoes must have a meal of blood in order to stimulate the ovaries to development, some of the common blood-sucking species, notably Culex pipiens, have been bred repeatedly without opportunity to feed upon blood.

The effect of the bite varies greatly with different species and depends upon the susceptibility of the individual bitten. Some persons are driven almost frantic by the attacks of the pests when their companions seem almost unconscious of any inconvenience. Usually, irritation and some degree of inflammation appear shortly following the bite. Not infrequently a hardened wheal or even a nodule forms, and sometimes scratching leads to secondary infection and serious results.

The source of the poison is usually supposed to be the salivary glands of the insect. As we have already pointed out, (p. 34), Macloskie believed that one lobe of the gland, on each side, was specialized for forming the poison, while a radically different view is that of Schaudinn, who believed that the irritation is due to the expelled contents of the oesophageal diverticula, which contain a gas and a peculiar type of fungi or bacteria. In numerous attempts, Schaudinn was unable to produce any irritation by applying the triturated salivary glands to a wound, but obtained the typical result when he used the isolated diverticula.

The irritation of the bite may be relieved to some extent by using ammonia water, a one per cent. alcoholic solution of menthol, or preparations of cresol, or carbolic acid. Dr. Howard recommends rubbing the bite gently with a piece of moist toilet soap. Castellani and Chalmers recommend cleansing inflamed bites with one in forty carbolic lotion, followed by dressing with boracic ointment. Of course, scratching should be avoided as much as possible.

Repellents of various kinds are used, for warding off the attacks of the insects. We have often used a mixture of equal parts of oil of pennyroyal and kerosene, applied to the hands and face. Oil of citronella is much used and is less objectionable to some persons. A recommended formula is, oil of citronella one ounce, spirits of camphor one ounce, oil of cedar one-half ounce. A last resort would seem to be the following mixture recommended by Howard, Dyar, and Knab for use by hunters and fishermen in badly infested regions, against mosquitoes and blackflies.

Take 2¼ lbs. of mutton tallow and strain it. While still hot add ½ lb. black tar (Canadian tar). Stir thoroughly and pour into the receptacle in which it is to be contained. When nearly cool stir in three ounces of oil of citronella and 1¼ oz. of pennyroyal.

At night the surest protection is a good bed net. There are many types of these in use, but in order to be serviceable and at the same time comfortable it should be roomy and hung in such a way as to be stretched tightly in every direction. We prefer one suspended from a broad, square frame, supported by a right-angled standard which is fastened to the head of the bed. It must be absolutely free from rents or holes and tucked in securely under the mattress or it will serve merely as a convenient cage to retain mosquitoes which gain an entrance. While such nets are a convenience in any mosquito ridden community, they are essential in regions where disease-carrying species abound. Screening of doors, windows and porches, against the pests is so commonly practiced in this country that its importance and convenience need hardly be urged.

Destruction of mosquitoes and prevention of breeding are of fundamental importance. Such measures demand first, as we have seen, the correct determination of the species which is to be dealt with, and a knowledge of its life-history and habits. If it prove to be one of the migratory forms, it is beyond mere local effort and becomes a problem demanding careful organization and state control. An excellent illustration of the importance and effectiveness of work along these lines is afforded by that in New Jersey, begun by the late Dr. John B. Smith and being pushed with vigor by his successor, Dr. Headlee.

In any case, there is necessity for community action. Even near the coast, where the migratory species are dominant, there are the local species which demand attention and which cannot be reached by any measures directed against the species of the salt marshes. The most important of local measures consist in the destruction of breeding places by filling or draining ponds and pools, clearing up of more temporary breeding places, such as cans, pails, water barrels and the like. Under conditions where complete drainage of swamps is impracticable or undesirable, judicious dredging may result in a pool or series of steep-sided pools deep enough to maintain a supply of fish, which will keep down the mosquito larvÆ. Where water receptacles are needed for storage of rain water, they should be protected by careful screening or a film of kerosene over the top of the water, renewed every two weeks or so, so as to prevent mosquitoes from depositing their eggs. When kerosene is used, Water drawn from the bottom of the receptacle will not be contaminated by it to any injurious extent. Where ponds cannot be drained much good will be accomplished by spraying kerosene oil on the surface of the water, or by the introduction of fish which will feed on the larvÆ.

Detailed consideration of the most efficient measures for controlling mosquitoes is to be found in Dr. Howard's Bulletin No. 88 of the Bureau of Entomology, "Preventive and remedial work against mosquitoes" or, in more summarized form, in Farmers' Bulletin No. 444. One of these should be obtained by any person interested in the problems of mosquito control and public health.

The SimuliidÆ, or Black Flies

The SimuliidÆ, or black flies, are small, dark, or black flies, with a stout body and a hump-back appearance. The antennÆ are short but eleven-segmented, the wings broad, without scales or hairs, and with the anterior veins stout but the others very weak. The mouth-parts (fig.74) are fitted for biting.

The larvÆ of the SimuliidÆ (fig.75) are aquatic and, unlike those of mosquitoes, require a well Ærated, or swiftly running water. Here they attach to stones, logs, or vegetation and feed upon various micro-organisms. They pupate in silken cocoons open at the top. Detailed life-histories have not been worked out for most of the species. We shall consider as typical that of Simulium pictipes, an inoffensive species widely distributed in the Eastern United States, which has been studied especially at Ithaca, N.Y. (Johannsen, 1903).

The eggs are deposited in a compact yellowish layer on the surface of rock, on the brinks of falls and rapids where the water is flowing swiftly. They are elongate ellipsoidal in shape, about .4 by .18 mm. As myriads of females deposit in the same place the egg patches may be conspicuous coatings of a foot or much more in diameter. When first laid they are enveloped in a yellowish white slime, which becomes darker, until finally it becomes black just before the emerging of the larvÆ. The egg stage lasts a week.

The larvÆ (fig.75) are black, soft skinned, somewhat cylindrical in shape, enlarged at both ends and attenuated in the middle. The posterior half is much stouter than the anterior part and almost club-shaped. The head bears two large fan-shaped organs which aid in procuring food. Respiration is accomplished by means of three so-called blood gills which are pushed out from the dorsal part of the rectum. The larvÆ occur in enormous numbers, in moss-like patches. If removed from their natural habitat and placed in quiet water they die within three or four hours. Fastened to the rock by means of a disk-like sucker at the caudal end of the body, they ordinarily assume an erect position. They move about on the surface of the rocks, to a limited extent, with a looping gait similar to that of a measuring worm, and a web is secreted which prevents their being washed away by the swiftly flowing water. They feed chiefly upon algÆ and diatoms.

The complete larval stage during the summer months occupies about four weeks, varying somewhat with the temperature and velocity of the water. At the end of this period they spin from cephalic glands, boot-shaped silken cocoons within which they pupate. The cocoon when spun is firmly attached to the rock and also to adjacent cocoons. Clustered continuously over a large area and sometimes one above another, they form a compact, carpet-like covering on the rocks, the reddish-brown color of which is easily distinguishable from the jet-black appearance of the larvÆ. The pupal stage lasts about three weeks. The adult fly, surrounded by a bubble of air, quickly rises to the surface of the water and escapes. The adults (fig.76) are apparently short lived and thus the entire life cycle, from egg to egg is completed in approximately eight weeks.

In the case of Simulium pictipes at Ithaca, N.Y., the first brood of adults emerges early in May and successive generations are produced throughout the summer and early autumn. This species winters in the larval condition. Most of the other species of Simulium which have been studied seem to be single brooded.

While Simulium pictipes does not attack man, there are a number of the species which are blood-sucking and in some regions they are a veritable scourge. In recent years the greatest interest in the group has been aroused by Sambon's hypothesis that they transmit pellagra from man to man. This has not been established, and, indeed, seems very doubtful, but the importance of these insects as pests and the possibility that they may carry disease make it urgent that detailed life-histories of the hominoxious species be worked out.

As pests a vivid account of their attacks is in Agassiz's "Lake Superior" (p.61), quoted by Forbes (1912).

"Neither the love of the picturesque, however, nor the interests of science, could tempt us into the woods, so terrible were the black flies. This pest of flies which all the way hither had confined our ramblings on shore pretty closely to the rocks and the beach, and had been growing constantly worse, here reached its climax. Although detained nearly two days, *** we could only sit with folded hands, or employ ourselves in arranging specimens, and such other operations as could be pursued in camp, and under the protection of a 'smudge.' One, whom scientific ardor tempted a little way up the river in a canoe, after water plants, came back a frightful spectacle, with blood-red rings round his eyes, his face bloody, and covered with punctures. The next morning his head and neck were swollen as if from an attack of erysipelas."

There are even well authenticated accounts on record of death of humans from the attacks of large swarms of these gnats. In some regions, and especially in the Mississippi Valley in this country, certain species of black flies have been the cause of enormous losses to farmers and stockmen, through their attacks on poultry and domestic animals. C.V. Riley states that in 1874 the loss occasioned in one county in Tennessee was estimated at $500,000.

The measures of prevention and protection against these insects have been well summarized by Forbes (1912). They are of two kinds: "the use of repellents intended to drive away the winged flies, and measures for the local destruction of the aquatic larvÆ. The repellents used are either smudges, or surface applications made to keep the flies from biting. The black-fly will not endure a dense smoke, and the well-known mosquito smudge seems to be ordinarily sufficient for the protection of man. In the South, leather, cloth, and other materials which will make the densest and most stifling smoke, are often preserved for this use in the spring. Smudges are built in pastures for the protection of stock, and are kept burning before the doors of barns and stables. As the black-flies do not readily enter a dark room, light is excluded from stables as much as possible during the gnat season. If teams must be used in the open field while gnats are abroad, they may be protected against the attacks of the gnats by applying cotton-seed oil or axle grease to the surface, especially to the less hairy parts of the animals, at least twice a day. A mixture of oil and tar and, indeed, several other preventives, are of practical use in badly infested regions; but no definite test or exact comparison has been made with any them in a way to give a record of the precise results."

"It is easy to drive the flies from houses or tents by burning pyrethrum powder inside; this either kills the flies or stupifies them so that they do not bite for some time thereafter." *** "Oil of tar is commonly applied to the exposed parts of the body for the purpose of repelling the insects, and this preparation is supplied by the Hudson Bay Company to its employees. Minnesota fishermen frequently grease their faces and hands with a mixture of kerosene and mutton tallow for the same purpose." We have found a mixture of equal parts of kerosene and oil of pennyroyal efficient.

Under most circumstances very little can be done to destroy this insect in its early stage, but occasionally conditions are such that a larvicide can be used effectively. Weed (1904), and Sanderson (1910) both report excellent results from the use of phinotas oil, a proprietary compound. The first-mentioned also found that in some places the larvÆ could be removed by sweeping them loose in masses with stiff stable brooms and then catching them downstream on wire netting stretched in the water.

ChironomidÆ or Midges

The flies of this family, commonly known as midges, resemble mosquitoes in form and size but are usually more delicate, and the wing-veins, though sometimes hairy, are not fringed with scales. The venation is simpler than in the mosquitoes and the veins are usually less distinct.

These midges, especially in spring or autumn, are often seen in immense swarms arising like smoke over swamps and producing a humming noise which can be heard for a considerable distance. At these seasons they are frequently to be found upon the windows of dwellings, where they are often mistaken for mosquitoes.

The larvÆ are worm-like, but vary somewhat in form in the different genera. Most of them are aquatic, but a few live in the earth, in manure, decaying wood, under bark, or in the sap of trees, especially in the sap which collects in wounds.

Of the many species of ChironomidÆ, (over eight hundred known), the vast majority are inoffensive. The sub-family CeratopogoninÆ, however, forms an exception, for some of the members of this group, known as sandflies, or punkies, suck blood and are particularly troublesome in the mountains, along streams, and at the seashore. Most of these have been classed under the genus Ceratopogon, but the group has been broken up into a number of genera and Ceratopogon, in the strict sense, is not known to contain any species which sucks the blood of vertebrates.

The CeratopogoninÆ—The CeratopogoninÆ are among the smallest of the Diptera, many of them being hardly a millimeter long and some not even so large. They are ChironomidÆ in which the thorax is not prolonged over the head. The antennÆ are filiform with fourteen (rarely thirteen) segments in both sexes, those of the male being brush-like. The basal segment is enlarged, the last segment never longer than the two preceding combined, while the last five are sub-equal to, or longer than the preceding segment. The legs are relatively stouter than in the other ChironomidÆ. The following three genera of this subfamily are best known as blood suckers in this country.

Of the genus Culicoides there are many species occurring in various parts of the world. A number are known to bite man and animals and it is probable that all are capable of inflicting injury. In some localities they are called punkies, in others, sand-flies, a name sometimes also applied to the species of Simulium and Phlebotomus. Owing to their very small size they are known by some tribes of Indians as No-see-ums. The larvÆ are found in ponds, pools, water standing in hollow tree stumps, and the like. Though probably living chiefly in fresh water, we have found a species occurring in salt water. The larvÆ are small, slender, legless, worm-like creatures (fig.77c) with small brown head and twelve body segments. The pupÆ (fig.77e) are slender, more swollen at the anterior end and terminating in a forked process. They float nearly motionless in a vertical position, the respiratory tubes in contact with the surface film. The adults are all small, rarely exceeding 2¼ mm. in length. The wings are more or less covered with erect setulÆ or hairs and in many species variously spotted and marked with iridescent blotches. The antennÆ have fourteen segments, the palpi usually five. The wing venation and mouth-parts are shown in figures77 and78. Of the twenty or more species of this genus occurring in the United States the following are known to bite: C.cinctus, C.guttipennis, C.sanguisuga, C.stellifer, C.variipennis, C.unicolor.

One of the most widely distributed and commonest species in the Eastern States is C.guttipennis (fig.77a). It is black with brown legs, a whitish ring before the apex of each femur and both ends of each tibia; tarsi yellow, knobs of halteres yellow. Mesonotum opaque, brown, two vittÆ in the middle, enlarging into a large spot on the posterior half, also a curved row of three spots in front of each wing, and the narrow lateral margins, light gray pruinose. Wings nearly wholly covered with brown hairs, gray, with markings as shown in the figure. Length one mm.

Johannseniella Will. is a wide-spread genus related to the foregoing. Its mouth-parts are well adapted for piercing and it is said to be a persistent blood sucker, particularly in Greenland. This genus is distinguished from Culicoides by its bare wings, the venation (fig.163,c), and the longer tarsal claws. There are over twenty North American species.

In the Southwestern United States, Tersesthes torrens Towns. occurs, a little gnat which annoys horses, and perhaps man also, by its bite. It is related to Culicoides but differs in the number of antennal segments and in its wing venation (fig.163, e). The fly measures but two mm. in length and is blackish in color. The antennÆ of the female have thirteen segments, the palpi but three, of which the second is enlarged and swollen.

TabanidÆ or Horse-Flies

The TabanidÆ,—horse-flies, ear-flies, and deer-flies,—are well-known pests of cattle and horses and are often extremely annoying to man. The characteristics of the family and of the principal North American genera are given in the keys of Chapter XII. There are over 2500 recorded species. As in the mosquitoes, the females alone are blood suckers. The males are flower feeders or live on plant juices. This is apparently true also of the females of some of the genera.

The eggs are deposited in masses on water plants or grasses and sedges growing in marshy or wet ground. Those of a common species of Tabanus are illustrated in figure80, a. They are placed in masses of several hundred, light colored when first deposited but turning black. In a week or so the cylindrical larvÆ, tapering at both ends (fig.80, b), escape to the water, or damp earth, and lead an active, carnivorous life, feeding mainly on insect larvÆ, and worms. In the forms which have been best studied the larval life is a long one, lasting for months or even for more than a year. Until recently, little was known concerning the life-histories of this group, but the studies of Hart (1895), and Hine (1903+) have added greatly to the knowledge concerning North American forms.

Many of the species attack man with avidity and are able to inflict painful bites, which may smart for hours. In some instances the wound is so considerable that blood will continue to flow after the fly has left. We have seen several cases of secondary infection following such bites.

The horse-flies have been definitely convicted of transferring the trypanosome of surra from diseased to healthy animals and there is good evidence that they transfer anthrax. The possibility of their being important agents in the conveyal of human diseases should not be overlooked. Indeed, Leiper has recently determined that a species of Chrysops transfers the blood parasite Filaria diurna.

LeptidÆ or Snipe-Flies

The family LeptidÆ is made up of moderate or large sized flies, predaceous in habit. They are sufficiently characterized in the keys of Chapter XII. Four blood-sucking species belonging to three genera have been reported. Of these Symphoromyia pachyceras is a western species. Dr. J. C. Bradley, from personal experience, reports it as a vicious biter.

OestridÆ or Bot-flies

To the family OestridÆ belong the bot and warble-flies so frequently injurious to animals. The adults are large, or of medium size, heavy bodied, rather hairy, and usually resemble bees in appearance.

The larvÆ live parasitically in various parts of the body of mammals, such as the stomach (horse bot-fly), the subcutaneous connective tissue (warble-fly of cattle), or the nasal passage (sheep bot-fly or head maggot).

There are on record many cases of the occurrence of the larvÆ of OestridÆ as occasional parasites of man. A number of these have been collected and reviewed in a thesis by Mme. PÈtrovskaia (1910). The majority of them relate to the following species.

Gastrophilus hÆmorrhoidalis, the red tailed bot-fly, is one of the species whose larvÆ are most commonly found in the stomach of the horse. Schoch (1877) cites the case of a woman who suffered from a severe case of chronic catarrh of the stomach, and who vomited, and also passed from the anus, larvÆ which apparently belonged to this species. Such cases are exceedingly rare but instances of subcutaneous infestation are fairly numerous. In the latter type these larvÆ are sometimes the cause of the peculiar "creeping myasis." This is characterized at its beginning by a very painful swelling which gradually migrates, producing a narrow raised line four to twenty-five millimeters broad. When the larva is mature, sometimes after several months, it becomes stationary and a tumor is formed which opens and discharges the larva along with pus and serum.

Gastrophilus equi is the most widespread and common of the horse bot-flies. Portschinsky reports it as commonly causing subcutaneous myasis of man in Russia.

Hypoderma bovis (= Oestrus bovis), and Hypoderma lineata are the so-called warble-flies of cattle. The latter species is the more common in North America but Dr. C. G. Hewitt has recently shown that H. bovis also occurs. Though warbles are very common in cattle in this country, the adult flies are very rarely seen. They are about half an inch in length, very hairy, dark, and closely resemble common honey-bees.

They deposit their eggs on the hairs of cattle and the animals in licking themselves take in the young larvÆ. These pass out through the walls of the oesophagus and migrate through the tissues of the animal, to finally settle down in the subcutaneous tissue of the back. The possibility of their entering directly through the skin, especially in case of infestation of man, is not absolutely precluded, although it is doubtful.

For both species of Hypoderma there are numerous cases on record of their occurrence in man. Hamilton (1893) saw a boy, six years of age, who had been suffering for some months from the glands on one side of his neck being swollen and from a fetid ulceration around the back teeth of the lower jaw of the same side. Three months' treatment was of no avail and the end seemed near; one day a white object, which was seen to move, was observed in the ulcer at the root of the tongue, and on being extracted was recognized as a full grown larva of Hypoderma. It was of usual tawny color, about half an inch long when contracted, about one third that thickness, and quite lively. The case resulted fatally. The boy had been on a dairy farm the previous fall, where probably the egg (or larva) was in some way taken into his mouth, and the larva found between the base of the tongue and the jaw suitable tissue in which to develop.

Topsent (1901) reports a case of "creeping myasis" caused by H. lineata in the skin of the neck and shoulders of a girl eight years of age. The larva travelled a distance of nearly six and a half inches. The little patient suffered excruciating pain in the place occupied by the larva.

Hypoderma diana infests deer, and has been known to occur in man.

Oestris ovis, the sheep bot-fly, or head maggot, is widely distributed in all parts of the world. In mid-summer the flies deposit living maggots in the nostrils of sheep. These larvÆ promptly pass up the nasal passages into the frontal and maxillary sinuses, where they feed on the mucous to be found there. In their migrations they cause great irritation to their host, and when present in numbers may cause vertigo, paroxysms, and even death. Portschinsky in an important monograph on this species, has discussed in detail its relation to man. He shows that it is not uncommon for the fly to attack man and that the minute living larvÆ are deposited in the eyes, nostrils, lips, or mouth. A typical case in which the larvÆ were deposited in the eye was described by a German oculist Kayser, in 1905. A woman brought her six year old daughter to him and said that the day before, about noontime, a flying insect struck the eye of the child and that since then she had felt a pain which increased towards evening. In the morning the pain ceased but the eye was very red. She was examined at about noon, at which time she was quiet and felt no pain. She was not sensitive to light, and the only thing noticed was a slight congestion and accumulation of secretion in the corner of the right eye. A careful examination of the eye disclosed small, active, white larvÆ that crawled out from the folds of the conjunctiva and then back and disappeared. Five of these larvÆ were removed and although an uncomfortable feeling persisted for a while, the eye became normal in about three weeks.

Some of the other recorded cases have not resulted so favorably, for the eyesight has been seriously affected or even lost.

According to Edmund and Etienne Sergent (1907), myasis caused by the larvÆ of Oestris ovis is very common among the shepherds in Algeria. The natives say that the fly deposits its larvÆ quickly, while on the wing, without pause. The greatest pain is caused when these larvÆ establish themselves in the nasal cavities. They then produce severe frontal headaches, making sleep impossible. This is accompanied by continuous secretion from the nasal cavities and itching pains in the sinuses. If the larvÆ happen to get into the mouth, the throat becomes inflamed, swallowing is painful, and sometimes vomiting results. The diseased condition may last for from three to ten days or in the case of nasal infection, longer, but recovery always follows. The natives remove the larvÆ from the eye mechanically by means of a small rag. When the nose is infested, tobacco fumigations are applied, and in case of throat infestation gargles of pepper, onion, or garlic extracts are used.

Rhinoestrus nasalis, the Russian gad-fly, parasitizes the nasopharyngeal region of the horse. According to Portschinsky, it not infrequently attacks man and then, in all the known cases deposits its larvÆ in the eye, only. This is generally done while the person is quiet, but not during sleep. The fly strikes without stopping and deposits its larva instantaneously. Immediately after, the victim experiences lancinating pains which without intermission increase in violence. There is an intense conjunctivitis and if the larvÆ are not removed promptly the envelopes of the eye are gradually destroyed and the organ lost.

Dermatobia cyaniventris—This fly (fig.83) is widely distributed throughout tropical America, and in its larval stage is well known as a parasite of man. The larvÆ (figs. 81and82) which are known as the "ver macaque," "torcel," "ver moyocuil" or by several other local names, enter the skin and give rise to a boil-like swelling, open at the top, and comparable with the swelling produced by the warble fly larvÆ, in cattle. They cause itching and occasional excruciating pain. When mature, nearly an inch in length, they voluntarily leave their host, drop to the ground and complete their development. The adult female is about 12 mm. in length. The face is yellow, the frons black with a grayish bloom; antennÆ yellow, the third segment four times as long as the second, the arista pectinate. The thorax is bluish black with grayish bloom; the abdomen depressed, brilliant metallescent blue with violet tinge. The legs are yellowish, the squamÆ and wings brownish.

The different types of larvÆ represented in figure 81 were formerly supposed to belong to different species but Blanchard regards them as merely various stages of the same species. It is only very recently that the early stage and the method by which man becomes infested were made known.

About 1900, Blanchard observed the presence of packets of large-sized eggs under the abdomen of certain mosquitoes from Central America; and in 1910, Dr. MoralÈs, of Costa Rica, declared that the Dermatobia deposited its eggs directly under the abdomen of the mosquito and that they were thus carried to vertebrates. Dr. Nunez Tovar observed the mosquito carriers of the eggs and placing larvÆ from this source on animals, produced typical tumors and reared the adult flies. It remained for Surcouf (1913) to work out the full details. He found that the Dermatobia deposits its eggs in packets covered by a very viscid substance, on leaves. These become attached to mosquitoes of the species Janthinosoma lutzi (fig.84) which walk over the leaves. The eggs which adhere to the abdomen, remain attached and are thus transported. The embryo develops, but the young larva (fig.82) remains in the egg until it has opportunity to drop upon a vertebrate fed upon by the mosquito.

MuscidÆ

The following MuscidÆ, characterized elsewhere, deserve special mention under our present grouping of parasitic species. Other important species will be considered as facultative parasites.

Stomoxys calcitrans, the stable-fly, or the biting house-fly, is often confused with Musca domestica and therefore is discussed especially in our consideration of the latter species as an accidental carrier of disease. Its possible relation to the spread of infantile paralysis is also considered later.

The tsetse flies, belonging to the genus Glossina, are African species of blood-sucking MuscidÆ which have attracted much attention because of their rÔle in transmitting various trypanosome diseases of man and animals. They are characterized in Chapter XII and are also discussed in connection with the diseases which they convey.Chrysomyia macellaria, (= Compsomyia), the "screw worm"-fly is one of the most important species of flies directly affecting man, in North America. It is not normally parasitic, however, and hence will be considered with other facultative parasites in Chapter IV.

Auchmeromyia luteola, the Congo floor maggot. This is a muscid of grewsome habits, which has a wide distribution throughout Africa. The fly (fig.86) deposits its eggs on the ground of the huts of the natives. The whitish larvÆ (fig.85) on hatching are slightly flattened ventrally, and each segment bears posteriorly three foot-pads transversely arranged. At night the larvÆ find their way into the low beds or couches of the natives and suck their blood. The adult flies do not bite man and, as far as known, the larvÆ do not play any rÔle in the transmission of sleeping sickness or other diseases.

This habit of blood-sucking by muscid larvÆ is usually referred to as peculiar to Auchmeromyia luteola but it should be noted that the larvÆ of Protocalliphora frequent the nests of birds and feed upon the young. Mr. A.F. Coutant has studied especially the life-history and habits of P.azurea, whose larvÆ he found attacking young crows at Ithaca, N.Y. He was unable to induce the larvÆ to feed on man.

Cordylobia anthropophaga, (Ochromyia anthropophaga), or Tumbu-fly (fig.87) is an African species whose larvÆ affect man much as do those of Dermatobia cyniventris, of Central and South America. The larva (fig.88), which is known as "ver du Cayor" because it was first observed in Cayor, in Senegambia, develops in the skin of man and of various animals, such as dogs, cats, and monkeys. It is about 12 mm. in length, and of the form of the larvÆ of other muscids. Upon the intermediate segments are minute, brownish recurved spines which give to the larva its characteristic appearance. The life-history is not satisfactorily worked out, but Fuller (1914), after reviewing the evidence believes that, as a rule, it deposits its young in the sleeping places of man and animals, whether such be a bed, a board, the floor, or the bare ground. In the case of babies, the maggots may be deposited on the scalp. The minute maggots bore their way painlessly into the skin. As many as forty parasites have been found in one individual and one author has reported finding more than three hundred in a spaniel puppy. Though their attacks are at times extremely painful, it is seldom that any serious results follow.

The Siphonaptera or Fleas

The Siphonaptera, or fleas (fig.89) are wingless insects, with highly chitinized and laterally compressed bodies. The mouth-parts are formed for piercing and sucking. Compound eyes are lacking but some species possess ocelli. The metamorphosis is complete.

This group of parasites, concerning which little was known until recently, has assumed a very great importance since it was learned that fleas are the carriers of bubonic plague. Now over four hundred species are known. Of these, several species commonly attack man. The most common hominoxious species are Pulex irritans, Xenopsylla cheopis, Ctenocephalus canis, Ctenocephalus felis, Ceratophyllus fasciatus and Dermatophilus penetrans, but many others will feed readily on human blood if occasion arises.

We shall treat in this place of the general biology and habits of the hominoxious forms and reserve for the systematic section the discussion of the characteristics of the different genera.

The most common fleas infesting houses in the Eastern United States are the cosmopolitan dog and cat fleas, Ctenocephalus canis (fig.90) and C. felis. Their life cycles will serve as typical. These two species have until recently been considered as one, under the name Pulex serraticeps. See figure92.

The eggs are oval, slightly translucent or pearly white, and measure about .5 mm. in their long diameter. They are deposited loosely in the hairs of the host and readily drop off as the animal moves around. Howard found that these eggs hatch in one to two days. The larvÆ are elongate, legless, white, worm-like creatures. They are exceedingly active, and avoid the light in every way possible. They cast their first skin in from three to seven days and their second in from three to four days. They commenced spinning in from seven to fourteen days after hatching and the imago appeared five days later. Thus in summer, at Washington, the entire life cycle may be completed in about two weeks. (cf.fig. 91,92).

Strickland's (1914) studies on the biology of the rat flea, Ceratophyllus fasciatus, have so important a general bearing that we shall cite them in considerable detail.

He found, to begin with, that there is a marked inherent range in the rate of development. Thus, of a batch of seventy-three eggs, all laid in the same day and kept together under the same conditions, one hatched in ten days; four in eleven days; twenty-five in twelve days; thirty-one in thirteen days; ten in fourteen days; one in fifteen days; and one in sixteen days. Within these limits the duration of the egg period seems to depend mainly on the degree of humidity. The incubation period is never abnormally prolonged as in the case of lice, (Warburton) and varying conditions of temperature and humidity have practically no effect on the percentage of eggs which ultimately hatch.

The same investigator found that the most favorable condition for the larva is a low temperature, combined with a high degree of humidity; and that the presence of rubbish in which the larva may bury itself is essential to its successful development. When larvÆ are placed in a bottle containing either wood-wool soiled by excrement, or with feathers or filter paper covered with dried blood they will thrive readily and pupate. They seem to have no choice between dried blood and powdered rat feces for food, and also feed readily on flea excrement. They possess the curious habit of always devouring their molted skins.

An important part of Strickland's experiments dealt with the question of duration of the pupal stage under the influence of temperature and with the longevity and habits of the adult. In October, he placed a batch of freshly formed cocoons in a small dish that was kept near a white rat in a deep glass jar in the laboratory. Two months later one small and feeble flea had emerged, but no more until February, four months after the beginning of the experiment. Eight cocoons were then dissected and seven more found to contain the imago fully formed but in a resting state. The remainder of the batch was then placed at 70° F. for one night, near a white rat. The next day all the cocoons were empty and the fleas were found on the white rat.

Thus, temperature greatly influences the duration of the pupal period, which in Ceratophyllus fasciatus averages seventeen days. Moreover, when metamorphosis is complete a low temperature will cause the imago to remain within the cocoon.

Sexually mature and ovipositing fleas, he fed at intervals and kept alive for two months, when the experiment was discontinued. In the presence of rubbish in which they could bury themselves, unfed rat fleas were kept alive for many months, whereas in the absence of any such substratum they rarely lived a month. In the former case, it was found that the length of life is influenced to some degree by the temperature and humidity. In an experiment carried out at 70° F. and 45 per cent humidity, the fleas did not live for more than four months, while in an experiment at 60° F. and 70 per cent humidity they lived for at least seventeen months. There was no indication that fleas kept under these conditions sucked moisture from surrounding objects, and those kept in bell jars, with an extract of flea-rubbish on filter paper, did not live any longer than those which were not so supplied.

Curiously enough, although the rat is the normal host of Ceratophyllus fasciatus, it was found that when given the choice these fleas would feed upon man in preference to rats. However, none of the fleas laid eggs unless they fed on rat blood.

The experiments of Strickland on copulation and oviposition in the rat flea showed that fleas do not copulate until they are sexually mature and that, at least in the case of Ceratophyllus fasciatus, the reproductive organs are imperfectly developed for some time (more than a week) after emerging from the pupa. When mature, copulation takes place soon after the fleas have fed on their true host—the rat—but not if they have fed on a facultative host only, such as man. Copulation is always followed by oviposition within a very short time.

The effect of the rat's blood on the female with regard to egg-laying, Strickland concludes, is stimulating rather than nutritive, as fleas that were without food for many months were observed to lay eggs immediately after one feed. Similarly, the male requires the stimulus of a meal of rat's blood before it displays any copulatory activity.

Mitzmain (1910) has described in detail the act of biting on man, as observed in the squirrel flea, Ceratophyllus acutus. "The flea when permitted to walk freely on the arm selects a suitable hairy space where it ceases abruptly in its locomotion, takes a firm hold with the tarsi, projects its proboscis, and prepares to puncture the skin. A puncture is drilled by the pricking epipharynx, the saw-tooth mandibles supplementing the movement by lacerating the cavity formed. The two organs of the rostrum work alternately, the middle piece boring, while the two lateral elements execute a sawing movement. The mandibles, owing to their basal attachments, are, as is expressed by the advisory committee on plague investigations in India (Journal of Hygiene, vol. 6, No. 4, p. 499), 'capable of independent action, sliding up and down but maintaining their relative positions and preserving the lumen of the aspiratory channel.' The labium doubles back, the V-shaped groove of this organ guiding the mandibles on either side."

"The action of the proboscis is executed with a forward movement of the head and a lateral and downward thrust of the entire body. As the mouth-parts are sharply inserted, the abdomen rises simultaneously. The hind and middle legs are elevated, resembling oars. The forelegs are doubled under the thorax, the tibia and tarsi resting firmly on the epidermis serve as a support for the body during the feeding. The maxillary palpi are retracted beneath the head and thorax. The labium continues to bend, at first acting as a sheath for the sawing mandibles, and as these are more deeply inserted, it bends beneath the head with the elasticity of a bow, forcing the mandibles into the wound until the maxillÆ are embedded in the skin of the victim. When the proboscis is fully inserted, the abdomen ceases for a time its lateral swinging."

"The acute pain of biting is first felt when the mandibles have not quite penetrated and subsequently during each distinct movement of the abdomen. The swinging of the abdomen gradually ceases as it becomes filled with blood. The sting of the biting gradually becomes duller and less sensitive as feeding progresses. The movements of the elevated abdomen grow noticeably feebler as the downward thrusts of the springy bow-like labium becomes less frequent."

"As the feeding process advances one can discern through the translucent walls of the abdomen a constant flow of blood, caudally from the pharynx, accompanied by a peristaltic movement. The end of the meal is signified in an abrupt manner. The flea shakes its entire body, and gradually withdraws its proboscis by lowering the abdomen and legs and violently twisting the head."

"When starved for several days the feeding of the rat fleas is conducted in a rather vigorous manner. As soon as the proboscis is buried to the full length the abdomen is raised and there ensues a gradual lateral swaying motion, increasing the altitude of the raised end of the abdomen until it assumes the perpendicular. The flea is observed at this point to gain a better foothold by advancing the fore tarsi, and then, gradually doubling back the abdomen, it turns with extreme agility, nearly touching with its dorsal side the skin of the hand upon which it is feeding. Meanwhile, the hungry parasite feeds ravenously."

"It is interesting to note the peculiar nervous action which the rodent fleas exhibit immediately when the feeding process is completed or when disturbed during the biting. Even while the rostrum is inserted to the fullest the parasite shakes its head spasmodically; in a twinkling the mouth is withdrawn and then the flea hops away."

A habit of fleas which we shall see is of significance in considering their agency in the spread of bubonic plague, is that of ejecting blood from the anus as they feed.

Fleas are famous for their jumping powers, and in control measures it is of importance to determine their ability along this line. It is often stated that they can jump about four inches, or, according to the Indian Plague Commission Xenopsylla cheopis cannot hop farther than five inches. Mitzmain (1910) conducted some careful experiments in which he found that the human flea, Pulex irritans, was able to jump as far as thirteen inches on a horizontal plane. The mean average of five specimens permitted to jump at will was seven and three-tenths inches. The same species was observed to jump perpendicularly to a height of at least seven and three-fourths inches. Other species were not able to equal this record.

The effect of the bite of fleas on man varies considerably according to the individual susceptibility. According to Patton and Cragg, this was borne out in a curious manner by the experiments of Chick and Martin. "In these, eight human hosts were tried; in seven, little or no irritation was produced, while in one quite severe inflammation was set up around each bite." Of two individuals, equally accustomed to the insects, going into an infested room, one may be literally tormented by them while the other will not notice them. Indeed it is not altogether a question of susceptibility, for fleas seem to have a special predilection for certain individuals. The typical itching wheals produced by the bites are sometimes followed, especially after scratching, by inflammatory papules.

The itching can be relieved by the use of lotions of carbolic acid (2-3 per cent), camphor, menthol lotion, or carbolated vaseline. If forced to sleep in an infested room, protection from attacks can be in a large measure gained by sprinkling pyrethrum, bubach, or California insect powder between the sheets. The use of camphor, menthol, or oil of eucalyptus, or oil of pennyroyal is also said to afford protection to a certain extent.

In the Eastern United States the occurrence of fleas as household pests is usually due to infested cats and dogs which have the run of the house. We have seen that the eggs are not attached to the host but drop to the floor when they are laid. Verrill, cited by Osborn, states that on one occasion he was able to collect fully a teaspoonful of eggs from the dress of a lady in whose lap a half-grown kitten had been held for a short time. Patton and Cragg record seeing the inside of a hat in which a kitten had spent the night, so covered with flea eggs that it looked "as if it had been sprinkled with sugar from a sifter." It is no wonder that houses in which pets live become overrun with the fleas.

One of the first control measures, then, consists in keeping such animals out of the house or in rigorously keeping them free from fleas. The latter can best be accomplished by the use of strong tar soap or Armour's "Flesope," which may be obtained from most druggists. The use of a three per cent solution of creolin, approximately four teaspoonfuls to a quart of warm water, has also been recommended. While this is satisfactory in the case of dogs, it is liable to sicken cats, who will lick their fur in an effort to dry themselves. Howard recommends thoroughly rubbing into the fur a quantity of pyrethrum powder. This partially stupifies the fleas which should be promptly swept up and burned.

He also recommends providing a rug for the dog or cat to sleep on and giving this rug a frequent shaking and brushing, afterwards sweeping up and burning the dust thus removed.

Since the larvÆ of fleas are very susceptible to exposure, the use of bare floors, with few rugs, instead of carpets or matting, is to be recommended. Thorough sweeping, so as to allow no accumulation of dust in cracks and crevices will prove efficient. If a house is once infested it may be necessary to thoroughly scrub the floors with hot soapsuds, or to spray them with gasoline. If the latter method is adopted, care must be taken to avoid the possibility of fire.

To clear a house of fleas Skinner recommends the use of flake naphthalene. In a badly infested house he took one room at a time, scattering on the floor five pounds of flake naphthalene, and closed it for twenty-four hours. It proved to be a perfect and effectual remedy and very inexpensive, as the naphthalene could be swept up and transferred to other rooms. Dr. Skinner adds, "so far as I am concerned, the flea question is solved and if I have further trouble I know the remedy. I intend to keep the dog and cat."

The late Professor Slingerland very effectively used hydrocyanic acid gas fumigation in exterminating fleas in houses. In one case, where failure was reported, he found on investigation that the house had become thoroughly reinfested from pet cats, which had been left untreated. Fumigation with sulphur is likewise efficient.

The fact that adult fleas are usually to be found on the floor, when not on their hosts, was ingeniously taken advantage of by Professor S. H. Gage in ridding an animal room at Cornell University of the pests. He swathed the legs of a janitor with sticky fly-paper and had him walk back and forth in the room. Large numbers of the fleas were collected in this manner.

In some parts of the southern United States hogs are commonly infested and in turn infest sheds, barns and even houses. Mr. H. E. Vick informs us that it is a common practice to turn sheep into barn-lots and sheds in the spring of the year to collect in their wool, the fleas which abound in these places after the hogs have been turned out.

It is a common belief that adult fleas are attracted to fresh meat and that advantage of this can be taken in trapping them. Various workers, notably Mitzman (1910), have shown that there is no basis for such a belief.The true chiggers—The chigoes, or true chiggers, are the most completely parasitic of any of the fleas. Of the dozen or more known species, one commonly attacks man. This is Dermatophilus penetrans, more commonly known as Sarcopsylla penetrans or Pulex penetrans.

This species occurs in Mexico, the West Indies, Central and South America. There are no authentic records of its occurrence in the United States although, as Baker has pointed out, there is no reason why it should not become established in Florida and Texas. It is usually believed that Brazil was its original home. Sometime about the middle of the nineteenth century it was introduced into West Africa and has spread across that continent.

The males and the immature females of Dermatophilus penetrans (fig.93) closely resemble those of other fleas. They are very active little brown insects about 1-1.2 mm. in size, which live in the dust of native huts and stables, and in dry, sandy soil. In such places they often occur in enormous numbers and become a veritable plague.

They attack not only man but various animals. According to Castellani and Chalmers, "Perhaps the most noted feature is the way in which it attacks pigs. On the Gold Coast it appeared to be largely kept in existence by these animals. It is very easily captured in the free state by taking a little pig with a pale abdomen, and placing it on its back on the ground on which infected pigs are living. After watching a few moments, a black speck will appear on the pig's abdomen, and quickly another and another. These black specks are jiggers which can easily be transferred to a test tube. On examination they will be found to be males and females in about equal numbers."

Both the males and females suck blood. That which characterizes this species as distinguished from other fleas attacking man is that when the impregnated female attacks she burrows into the skin and there swells until in a few days she has the size and appearance of a small pea (fig.94). Where they are abundant, hundreds of the pests may attack a single individual (fig.95). Here they lie with the apex of the abdomen blocking the opening. According to FÜlleborn (1908) they do not penetrate beneath the epidermis. The eggs are not laid in the flesh of the victim, as is sometimes stated, but are expelled through this opening. The female then dies, withers and falls away or is expelled by ulceration. According to Brumpt, she first quits the skin and then, falling to the ground, deposits her eggs. The subsequent development in so far as known, is like that of other fleas.

The chigoe usually enters between the toes, the skin about the roots of the nails, or the soles of the feet, although it may attack other parts of the body. Mense records the occurrence in folds of the epidermis, as in the neighborhood of the anus. They give rise to irritation and unless promptly and aseptically removed there often occurs pus formation and the development of a more or less serious abscess. Gangrene and even tetanus may ensue.

Treatment consists in the careful removal of the insect, an operation more easily accomplished a day or two after its entrance, than at first, when it is unswollen. The ulcerated point should then be treated with weak carbolic acid, or tincture of iodine, or dusted thoroughly with an antiseptic powder.

Castellani and Chalmers recommend as prophylactic measures, keeping the house clean and keeping pigs, poultry, and cattle away therefrom. "High boots should be used, and especial care should be taken not to go to a ground floor bathroom with bare feet. The feet, especially the toes, and under the nails, should be carefully examined every morning to see if any black dots can be discovered, when the jigger should be at once removed, and in this way suppuration will be prevented. It is advisable, also, to sprinkle the floors with carbolic lotion, Jeyes' fluid, or with pyrethrum powder, or with a strong infusion of native tobacco, as recommended by Law and Castellani."

Echidnophaga gallinacea (fig.96) is a widely distributed Hectopsyllid attacking poultry (fig.97). It occurs in the Southern and Southwestern United States and has been occasionally reported as attacking man, especially children. It is less highly specialized than Dermatophilus penetrans, and does not ordinarily cause serious trouble in man.

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