The fact that certain arthropods are poisonous, or may affect the health of man as direct parasites has always received attention in the medical literature. We come now to the more modern aspect of our subject,—the consideration of insects and other arthropods as transmitters and disseminators of disease.

The simplest way in which arthropods may function in this capacity is as simple carriers of pathogenic organisms. It is conceivable that any insect which has access to, and comes in contact with such organisms and then passes to the food, or drink, or to the body of man, may in a wholly accidental and incidental manner convey infection. That this occurs is abundantly proved by the work of recent years. We shall consider as typical the case against the house-fly, which has attracted so much attention, both popular and scientific. The excellent general treatises of Hewitt (1910), Howard (1911), and Graham-Smith (1913), and the flood of bulletins and popular literature render it unnecessary to consider the topic in any great detail.

The House-fly As a Carrier of Disease

Up to the past decade the house-fly has usually been regarded as a mere pest. Repeatedly, however, it had been suggested that it might disseminate disease. We have seen that as far back as the sixteenth century, Mercurialis suggested that it was the agent in the spread of bubonic plague, and in 1658, Kircher reiterated this view. In 1871, Leidy expressed the opinion that flies were probably a means of communicating contagious diseases to a greater degree than was generally suspected. From what he had observed regarding gangrene in hospitals, he thought flies should be carefully excluded from wounds. In the same year, the editor of the London Lancet, referring to the belief that they play a useful rÔle in purifying the air said, "Far from looking upon them as dipterous angels dancing attendance on Hygeia, regard them rather in the light of winged sponges spreading hither and thither to carry out the foul behests of Contagion."

These suggestions attracted little attention from medical men, for it is only within very recent years that the charges have been supported by direct evidence. Before considering this evidence, it is necessary that we define what is meant by "house-fly" and that we then consider the life-history of the insect.

There are many flies which are occasionally to be found in houses, but according to various counts, from 95 per cent to 99 per cent of these in warm weather in the Eastern United States belong to the one species Musca domestica (fig.108). This is the dominant house-fly the world over and is the one which merits the name. It has been well characterized by Schiner (1864), whose description has been freely translated by Hewitt, as follows:

"Frons of male occupying a fourth part of the breadth of the head. Frontal stripe of female narrow in front, so broad behind that it entirely fills up the width of the frons. The dorsal region of the thorax dusty grey in color with four equally broad longitudinal stripes. Scutellum gray with black sides. The light regions of the abdomen yellowish, transparent, the darkest parts at least at the base of the ventral side yellow. The last segment and a dorsal line blackish brown. Seen from behind and against the light, the whole abdomen shimmering yellow, and only on each side of the dorsal line on each segment a dull transverse band. The lower part of the face silky yellow, shot with blackish brown. Median stripe velvety black. AntennÆ brown. Palpi black. Legs blackish brown. Wings tinged with pale gray with yellowish base. The female has a broad velvety back, often reddishly shimmering frontal stripe, which is not broader at the anterior end than at the bases of the antennÆ, but become so very much broader above that the light dustiness of the sides is entirely obliterated. The abdomen gradually becoming darker. The shimmering areas on the separate segments generally brownish. All the other parts are the same as in the male."

The other species of flies found in houses in the Eastern United States which are frequently mistaken for the house or typhoid fly may readily be distinguished by the characters of the following key:

It is almost universally believed that the adults of Musca domestica hibernate, remaining dormant throughout the winter in attics, around chimneys, and in sheltered but cold situations. This belief has been challenged by Skinner (1913), who maintains that all the adult flies die off during the fall and early winter and that the species is carried over in the pupal stage, and in no other way. The cluster-fly, Pollenia rudis, undoubtedly does hibernate in attics and similar situations and is often mistaken for the house-fly. In so far as concerns Musca domestica, the important question as to hibernation in the adult stage is an open one. Many observations by one of the writers (Johannsen) tend to confirm Dr. Skinner's conclusion, in so far as it applies to conditions in the latitude of New York State. Opposed, is the fact that various experimenters, notably Hewitt (1910) and Jepson (1909) wholly failed to carry pupÆ through the winter.

The house-fly breeds by preference in horse manure. Indeed, Dr. Howard, whose extensive studies of the species especially qualify him for expressing an opinion on the subject, has estimated that under ordinary city and town conditions, more than ninety per cent of the flies present in houses have come from horse stables or their vicinity. They are not limited to such localities, by any means, for it has been found that they would develop in almost any fermenting organic substance. Thus, they have been bred from pig, chicken, and cow manure, dirty waste paper, decaying vegetation, decaying meat, slaughter-house refuse, sawdust-sweepings, and many other sources. A fact which makes them especially dangerous as disease-carriers is that they breed readily in human excrement.

The eggs are pure white, elongate ovoid, somewhat broader at the anterior end. They measure about one millimeter (1-25 inch) in length. They are deposited in small, irregular clusters, one hundred and twenty to one hundred and fifty from a single fly. A female may deposit as many as four batches in her life time. The eggs hatch in from eight to twenty-four hours.

The newly hatched larva, or maggot (fig.108), measures about two millimeters (1-12 inch) in length. It is pointed at the head end and blunt at the opposite end, where the spiracular openings are borne. It grows rapidly, molts three times and reaches maturity in from six to seven days, under favorable conditions.

The pupal stage, like that of related flies, is passed in the old larval skin which, instead of being molted, becomes contracted and heavily chitinized, forming the so-called puparium (fig.108). The pupal stage may be completed in from three to six days.

Thus during the warm summer months a generation of flies may be produced in ten to twelve days. Hewitt at Manchester, England, found the minimum to be eight days but states that larvÆ bred in the open air in horse manure which had an average daily temperature of 22.5° C., occupied fourteen to twenty days in their development, according to the air temperature.

After emergence, a period of time must elapse before the fly is capable of depositing eggs. This period has been tuned the preoviposition period. Unfortunately we have few exact data regarding this period. Hewitt found that the flies became sexually mature in ten to fourteen days after their emergence from the pupal state and four days after copulation they began to deposit their eggs; in other words the preoviposition stage was fourteen days or longer. Griffith (1908) found this period to be ten days. Dr. Howard believes that the time "must surely be shorter, and perhaps much shorter, under midsummer conditions, and in the freedom of the open air." He emphasizes that the point is of great practical importance, since it is during this period that the trapping and other methods of destroying the adult flies, will prove most useful.

Howard estimates that there may be nine generations of flies a year under outdoor conditions in places comparable in climate to Washington. The number may be considerably increased in warmer climates.

The rate at which flies may increase under favorable conditions is astounding. Various writers have given estimates of the numbers of flies which may develop as the progeny of a single individual, providing all the eggs and all the individual flies survived. Thus, Howard estimates that from a single female, depositing one hundred and twenty eggs on April 15th, there may be by September 10th, 5,598,720,000,000 adults. Fortunately, living forms do not produce in any such mathematical manner and the chief value of the figures is to illustrate the enormous struggle for existence which is constantly taking place in nature.

Flies may travel for a considerable distance to reach food and shelter, though normally they pass to dwellings and other sources of food supply in the immediate neighborhood of their breeding places. Copeman, Howlett and Merriman (1911) marked flies by shaking them in a bag containing colored chalk. Such flies were repeatedly recovered at distances of eight to one thousand yards and even at a distance of seventeen hundred yards, nearly a mile.

Hindle and Merriman (1914) continued these experiments on a large scale at Cambridge, England. They "do not think it likely that, as a rule, flies travel more than a quarter of a mile in thickly-housed areas." In one case a single fly was recovered at a distance of 770 yards but a part of this distance was across open fen-land. The surprising fact was brought out that flies tend to travel either against or across the wind. The actual direction followed may be determined either directly by the action of the wind (positive anemotropism), or indirectly owing to the flies being attracted by any odor that it may convey from a source of food. They conclude that it is likely that the chief conditions favoring the disposal of flies are fine weather and a warm temperature. The nature of the locality is another considerable factor. Hodge (1913) has shown that when aided by the wind they may fly to much greater distances over the water. He reports that at Cleveland, Ohio, the cribs of the water works, situated a mile and a quarter, five miles, and six miles out in Lake Erie are invaded by a regular plague of flies when the wind blows from the city. Investigation showed that there was absolutely nothing of any kind in which flies could breed on the crib.

The omnivorous habits of the house-fly are matters of everyday observation. From our view point, it is sufficient to emphasize that from feeding on excrement, on sputum, on open sores, or on putrifying matter, the flies may pass to the food or milk upon the table or to healthy mucous membranes, or uncontaminated wounds. There is nothing in its appearance to tell whether the fly that comes blithely to sup with you is merely unclean, or whether it has just finished feeding upon dejecta teeming with typhoid bacilli.

The method of feeding of the house-fly has an important bearing on the question of its ability to transmit pathogenic organisms. Graham-Smith (1910) has shown that when feeding, flies frequently moisten soluble substances with "vomit" which is regurgitated from the crop. This is, of course, loaded with bacteria from previous food. When not sucked up again these drops of liquid dry, and produce round marks with an opaque center and rim and an intervening less opaque area. Fly-specks, then, consist of both vomit spots and feces. Graham-Smith shows a photograph of a cupboard window where, on an area six inches square, there were counted eleven hundred and two vomit marks and nine fecal deposits.

From a bacteriologist's viewpoint a discussion of the possibility of a fly's carrying bacteria would seem superfluous. Any exposed object, animate or inanimate, is contaminated by bacteria and will transfer them if brought into contact with suitable culture media, whether such substance be food, or drink, open wounds, or the sterile culture media of the laboratory. A needle point may convey enough germs to produce disease. Much more readily may the house-fly with its covering of hairs and its sponge-like pulvilli (fig.109) pick up and transfer bits of filth and other contaminated material.

For popular instruction this inevitable transfer of germs by the house-fly is strikingly demonstrated by the oft copied illustration of the tracks of a fly on a sterile culture plate. Two plates of gelatine or, better, agar medium are prepared. Over one of these a fly (with wings clipped) is allowed to walk, the other is kept as a check. Both are put aside at room temperature, to be examined after twenty-four to forty-eight hours. At the end of that time, the check plate is as clear as ever, the one which the fly has walked is dotted with colonies of bacteria and fungi. The value in the experiment consists in emphasizing that by this method we merely render visible what is constantly occurring in nature.

A comparable experiment which we use in our elementary laboratory work is to take three samples of clean (preferably, sterile) fresh milk in sterile bottles. One of them is plugged with a pledget of cotton, into the second is dropped a fly from the laboratory and into the third is dropped a fly which has been caught feeding upon garbage or other filth. After a minute or two the flies are removed and the vials plugged as was number one. The three are then set aside at room temperature. When examined after twenty-four hours the milk in the first vial is either still sweet or has a "clean" sour odor; that of the remaining two is very different, for it has a putrid odor, which is usually more pronounced in the case of sample number three.

Several workers have carried out experiments to determine the number of bacteria carried by flies under natural conditions. One of the most extended and best known of these is the series by Esten and Mason (1908). These workers caught flies from various sources in a sterilized net, placed them in a sterile bottle and poured over them a known quantity of sterilized water, in which they were shaken so as to wash the bacteria from their bodies. They found the number of bacteria on a single fly to range from 550 to 6,600,000. Early in the fly season the numbers of bacteria on flies are comparatively small, while later the numbers are comparatively very large. The place where flies live also determines largely the numbers that they carry. The lowest number, 550, was from a fly caught in the bacteriological laboratory, the highest number, 6,600,000 was the average from eighteen swill-barrel flies. Torrey (1912) made examination of "wild" flies from a tenement house district of New York City. He found "that the surface contamination of these 'wild' flies may vary from 570 to 4,400,000 bacteria per insect, and the intestinal bacterial content from 16,000 to 28,000,000."

Less well known in this country is the work of Cox, Lewis, and Glynn (1912). They examined over four hundred and fifty naturally infected house-flies in Liverpool during September and early October. Instead of washing the flies they were allowed to swim on the surface of sterile water for five, fifteen, or thirty minutes, thus giving natural conditions, where infection occurs from vomit and dejecta of the flies, as well as from their bodies. They found, as might be expected, that flies from either insanitary or congested areas of the city contain far more bacteria than those from the more sanitary, less congested, or suburban areas. The number of aerobic bacteria from the former varied from 800,000 to 500,000,000 per fly and from the latter from 21,000 to 100,000. The number of intestinal forms conveyed by flies from insanitary or congested areas was from 10,000 to 333,000,000 as compared with from 100 to 10,000 carried by flies from the more sanitary areas.

Pathogenic bacteria and those allied to the food poisoning group were only obtained from the congested or moderately congested areas and not from the suburban areas, where the chances of infestation were less.

The interesting fact was brought out that flies caught in milk shops apparently carry and obtain more bacteria than those from other shops with exposed food in a similar neighborhood. The writers explained this as probably due to the fact that milk when accessible, especially during the summer months, is suitable culture medium for bacteria, and the flies first inoculate the milk and later reinoculate themselves, and then more of the milk, so establishing a vicious circle.

They conclude that in cities where food is plentiful flies rarely migrate from the locality in which they are bred, and consequently the number of bacteria which they carry depends upon the general standard of cleanliness in that locality. Flies caught in a street of modern, fairly high class, workmen's dwellings forming a sanitary oasis in the midst of a slum area, carried far less bacteria than those caught in the adjacent neighborhood.

Thus, as the amount of dirt carried by flies in any particular locality, measured in the terms of bacteria, bears a definite relation to the habits of the people and to the state of the streets, it demonstrates the necessity of efficient municipal and domestic cleanliness, if the food of the inhabitants is to escape pollution, not only with harmless but also with occasional pathogenic bacteria.

The above cited work is of a general nature, but, especially in recent years, many attempts have been made to determine more specifically the ability of flies to transmit pathogenic organisms. The critical reviews of Nuttall and Jepson (1909), Howard (1911), and Graham-Smith (1913) should be consulted by the student of the subject. We can only cite here a few of the more striking experiments.

Celli (1888) fed flies on pure cultures of Bacillus typhosus and declared that he was able to recover these organisms from the intestinal contents and excrement.

Firth and Horrocks (1902), cited by Nuttall and Jepson, "kept Musca domestica (also bluebottles) in a large box measuring 4 × 3 × 3 feet, with one side made of glass. They were fed on material contaminated with cultures of B. typhosus. Agar plates, litmus, glucose broth and a sheet of clean paper were at the same time exposed in the box. After a few days the plates and broth were removed and incubated with a positive result." Graham-Smith (1910) "carried out experiments with large numbers of flies kept in gauze cages and fed for eight hours on emulsions of B. typhosus in syrup. After that time the infested syrup was removed and the flies were fed on plain syrup. B. typhosus was isolated up to 48 hours (but not later) from emulsions of their feces and from plates over which they walked."

Several other workers, notably Hamilton (1903), Ficker (1903), Bertarelli (1910) Faichnie (1909), and Cochrane (1912), have isolated B. typhosus from "wild" flies, naturally infected. The papers of Faichnie and of Cochrane we have not seen, but they are quoted in extenso by Graham-Smith (1913).

On the whole, the evidence is conclusive that typhoid germs not only may be accidentally carried on the bodies of house-flies but may pass through their bodies and be scattered in a viable condition in the feces of the fly for at least two days after feeding. Similar, results have been reached in experiments with cholera, tuberculosis and yaws, the last-mentioned being a spirochÆte disease. Darling (1913) has shown that murrina, a trypanosome disease of horses and mules in the Canal zone is transmitted by house-flies which feed upon excoriated patches of diseased animals and then pass to cuts and galls of healthy animals.

Since it is clear that flies are abundantly able to disseminate viable pathogenic bacteria, it is important to consider whether they have access to such organisms in nature. A consideration of the method of spread of typhoid will serve to illustrate the way in which flies may play an important rÔle.

Typhoid fever is a specific disease caused by Bacillus typhosus, and by it alone. The causative organism is to be found in the excrement and urine of patients suffering from the disease. More than that, it is often present in the dejecta for days, weeks, or even months and years, after the individual has recovered from the disease. Individuals so infested are known as "typhoid carriers" and they, together with those suffering from mild cases, or "walking typhoid," are a constant menace to the health of the community in which they are found.

Human excrement is greedily visited by flies, both for feeding and for ovipositing. The discharges of typhoid patients, or of chronic "carriers," when passed in the open, in box privies, or camp latrines, or the like, serve to contaminate myriads of the insects which may then spread the germ to human food and drink. Other intestinal diseases may be similarly spread. There is abundant epidÆmiological evidence that infantile diarrhoea, dysentery, and cholera may be so spread.

Stiles and Keister (1913) have shown that spores of Lamblia intestinalis, a flagellate protozoan living in the human intestine, may be carried by house-flies. Though this species is not normally pathogenic, one or more species of Entamoeba are the cause of a type of a highly fatal tropical dysentery. Concerning it, and another protozoan parasite of man, they say, "If flies can carry Lamblia spores measuring 10 to 7µ, and bacteria that are much smaller, and particles of lime that are much larger, there is no ground to assume that flies may not carry Entamoeba and Trichomonas spores."

Tuberculosis is one of the diseases which it is quite conceivable may be carried occasionally. The sputum of tubercular patients is very attractive to flies, and various workers, notably Graham-Smith, have found that Musca domestica may distribute the bacillus for several days after feeding on infected material.

A type of purulent opthalmia which is very prevalent in Egypt is often said to be carried by flies. Nuttall and Jepson (1909) consider that the evidence regarding the spread of this disease by flies is conclusive and that the possibility of gonorrhoeal secretions being likewise conveyed cannot be denied.

Many studies have been published, showing a marked agreement between the occurrence of typhoid and other intestinal diseases and the prevalence of house-flies. The most clear-cut of these are the studies of the Army Commission appointed to investigate the cause of epidemics of enteric fever in the volunteer camps in the Southern United States during the Spanish-American War. Though their findings as presented by Vaughan (1909), have been quoted very many times, they are so germane to our discussion that they will bear repetition:

"Flies swarmed over infected fecal matter in the pits and fed upon the food prepared for the soldiers in the mess tents. In some instances where lime had recently been sprinkled over the contents of the pits, flies with their feet whitened with lime were seen walking over the food." Under such conditions it is no wonder that "These pests had inflicted greater loss upon American soldiers than the arms of Spain."

Similar conditions prevailed in South Africa during the Boer War. Seamon believes that very much of the success of the Japanese in their fight against Russia was due to the rigid precautions taken to prevent the spread of disease by these insects and other means.

Veeder has pointed out that the characteristics of a typical fly-borne epidemic of typhoid are that it occurs in little neighborhood epidemics, extending by short leaps from house to house, without regard to water supply or anything else in common. It tends to follow the direction of prevailing winds (cf. the conclusions of Hindle and Merriman). It occurs during warm weather. Of course, when the epidemic is once well under way, other factors enter into its spread.

In general, flies may be said to be the chief agency in the spread of typhoid in villages and camps. In cities with modern sewer systems they are less important, though even under the best of such conditions, they are important factors. Howard has emphasized that in such cities there are still many uncared-for box privies and that, in addition, the deposition of feces overnight in uncared-for waste lots and alleys is common.

Not only unicellular organisms, such as bacteria and protozoa, but also the eggs, embryos and larvÆ of parasitic worms have been found to be transported by house-flies. Ransom (1911) has found that Habronema muscÆ, a nematode worm often found in adult flies, is the immature stage of a parasite occurring in the stomach of the horse. The eggs or embryos passing out with the feces of the horse, are taken up by fly larvÆ and carried over to the imago stage.

Grassi (1883), Stiles (1889), Calandruccio (1906), and especially Nicoll (1911), have been the chief investigators of the ability of house-flies to carry the ova and embryos of human intestinal parasites. Graham-Smith (1913) summarizes the work along this line as follows:

"It is evident from the investigations that have been quoted that house-flies and other species are greatly attracted to the ova of parasitic worms contained in feces and other materials, and make great efforts to ingest them. Unless the ova are too large they often succeed, and the eggs are deposited uninjured in their feces, in some cases up to the third day at least. The eggs may also be carried on their legs or bodies. Under suitable conditions, food and fluids may be contaminated with the eggs of various parasitic worms by flies, and in one case infection of the human subject has been observed. Feces containing tape-worm segments may continue to be a source of infection for as long as a fortnight. Up to the present, however, there is no evidence to show what part flies play in the dissemination of parasitic worms under natural conditions."

Enough has been said to show that the house-fly must be dealt with as a direct menace to public health. Control measures are not merely matters of convenience but are of vital importance.

Under present conditions the speedy elimination of the house-fly is impossible and the first thing to be considered is methods of protecting food and drink from contamination. The first of these methods is the thorough screening of doors and windows to prevent the entrance of flies. In the case of kitchen doors, the flies, attracted by odors, are likely to swarm onto the screen and improve the first opportunity for gaining an entrance. This difficulty can be largely avoided by screening-in the back porch and placing the screen door at one end rather than directly before the door.

The use of sticky fly paper to catch the pests that gain entrance to the house is preferable to the various poisons often used. Of the latter, formalin (40 per cent formaldehyde) in the proportion of two tablespoonfuls to a pint of water is very efficient, if all other liquids are removed or covered, so that the flies must depend on the formalin for drink. The mixture is said to be made more attractive by the addition of sugar or milk, though we have found the plain solution wholly satisfactory, under proper conditions. It should be emphasized that this formalin mixture is not perfectly harmless, as so often stated. There are on record cases of severe and even fatal poisoning from the accidental drinking of solutions.

When flies are very abundant in a room they can be most readily gotten rid of by fumigation with sulphur, or by the use of pure pyrethrum powder either burned or puffed into the air. Herrick (1913) recommends the following method: "At night all the doors and windows of the kitchen should be closed; fresh powder should be sprinkled over the stove, on the window ledges, tables, and in the air. In the morning flies will be found lying around dead or stupified. They may then be swept up and burned." This method has proved very efficaceous in some of the large dining halls in Ithaca.

The writers have had little success in fumigating with the vapors of carbolic acid, or carbolic acid and gum camphor, although these methods will aid in driving flies from a darkened room.

All of these methods are but makeshifts. As Howard has so well put it, "the truest and simplest way of attacking the fly problem is to prevent them from breeding, by the treatment or abolition of all places in which they can breed. To permit them to breed undisturbed and in countless numbers, and to devote all our energy to the problem of keeping them out of our dwellings, or to destroy them after they have once entered in spite of all obstacles, seems the wrong way to go about it."

We have already seen that Musca domestica breeds in almost any fermenting organic material. While it prefers horse manure, it breeds also in human feces, cow dung and that of other animals, and in refuse of many kinds. To efficiently combat the insect, these breeding places must be removed or must be treated in some such way as to render them unsuitable for the development of the larvÆ. Under some conditions individual work may prove effective, but to be truly efficient there must be extensive and thorough coÖperative efforts.

Manure, garbage, and the like should be stored in tight receptacles and carted away at least once a week. The manure may be carted to the fields and spread. Even in spread manure the larvÆ may continue their development. Howard points out that "it often happens that after a lawn has been heavily manured in early summer the occupants of the house will be pestered with flies for a time, but finding no available breeding place these disappear sooner or later. Another generation will not breed in the spread manure."

Hutchinson (1914) has emphasized that the larvÆ of houseflies have deeply engrained the habit of migrating in the prepupal stage and has shown that this offers an important point of attack in attempts to control the pest. He has suggested that maggot traps might be developed into an efficient weapon in the warfare against the house-fly. Certain it is that the habit greatly simplifies the problem of treating the manure for the purpose of killing the larvÆ.

There have been many attempts to find some cheap chemical which would destroy fly larvÆ in horse manure without injuring the bacteria or reducing the fertilizing values of the manure. The literature abounds in recommendations of kerosene, lime, chloride of lime, iron sulphate, and other substances, but none of them have met the situation. The whole question has been gone into thoroughly by Cook, Hutchinson and Scales (1914), who tested practically all of the substances which have been recommended. They find that by far the most effective, economical, and practical of the substances is borax in the commercial form in which it is available throughout the country.

"Borax increases the water-soluble nitrogen, ammonia and alkalinity of manure and apparently does not permanently injure the bacterial flora. The application of manure treated with borax at the rate of 0.62 pound per eight bushels (10 cubic feet) to soil does not injure the plants thus far tested, although its cumulative effect, if any, has not been determined."

As their results clearly show that the substances so often recommended are inferior to borax, we shall quote in detail their directions for treating manure so as to kill fly eggs and maggots.

"Apply 0.62 pound borax or 0.75 pound calcined colemanite to every 10 cubic feet (8 bushels) of manure immediately on its removal from the barn. Apply the borax particularly around the outer edges of the pile with a flour sifter or any fine sieve, and sprinkle two or three gallons of water over the borax-treated manure.

"The reason for applying the borax to the fresh manure immediately after its removal from the stable is that the flies lay their eggs on the fresh manure, and borax, when it comes in contact with the eggs, prevents their hatching. As the maggots congregate at the outer edge of the pile, most of the borax should be applied there. The treatment should be repeated with each addition of fresh manure, but when the manure is kept in closed boxes, less frequent applications will be sufficient. When the calcined colemanite is available, it may be used at the rate of 0.75 pound per 10 cubic feet of manure, and is a cheaper means of killing the maggots. In addition to the application of borax to horse manure to kill fly larvÆ, it may be applied in the same proportion to other manures, as well as to refuse and garbage. Borax may also be applied to the floors and crevices in barns, stables, markets, etc., as well as to street sweepings, and water should be added as in the treatment of horse manure. After estimating the amount of material to be treated and weighing the necessary amount of borax, a measure may be used which will hold the proper amount, thus avoiding the subsequent weighings.

"While it can be safely stated that no injurious action will follow the application of manure treated with borax at the rate of 0.62 pound for eight bushels, or even larger amounts in the case of some plants, nevertheless the borax-treated manure has not been studied in connection with the growth of all crops, nor has its cumulative effect been determined. It is therefore recommended that not more than 15 tons per acre of the borax-treated manure should be applied to the field. As truckmen use considerably more than this amount, it is suggested that all cars containing borax-treated manure be so marked, and that public-health officials stipulate in their directions for this treatment that not over 0.62 pound for eight bushels of manure be used, as it has been shown that larger amounts of borax will injure most plants. It is also recommended that all public-health officials and others, in recommending the borax treatment for killing fly eggs and maggots in manure, warn the public against the injurious effects of large amounts of borax on the growth of plants."

"The amount of manure from a horse varies with the straw or other bedding used, but 12 or 15 bushels per week represent the approximate amount obtained. As borax costs from five to six cents per pound in 100-pound lots in Washington, it will make the cost of the borax practically one cent per horse, per day. And if calcined colemanite is purchased in large shipments the cost should be considerably less."

Hodge (1910) has approached the problem of fly extermination from another viewpoint. He believes that it is practical to trap flies out of doors during the preoviposition period, when they are sexually immature, and to destroy such numbers of them that the comparatively few which survive will not be able to lay eggs in sufficient numbers to make the next generation a nuisance. To the end of capturing them in enormous numbers he has devised traps to be fitted over garbage cans, into stable windows, and connected with the kitchen window screens. Under some conditions this method of attack has proved very satisfactory.

One of the most important measures for preventing the spread of disease by flies is the abolition of the common box privy. In villages and rural districts this is today almost the only type to be found. It is the chief factor in the spread of typhoid and other intestinal diseases, as well as intestinal parasites. Open and exposed to myriads of flies which not only breed there but which feed upon the excrement, they furnish ideal conditions for spreading contamination. Even where efforts are made to cover the contents with dust, or ashes, or lime, flies may continue to breed unchecked. Stiles and Gardner have shown that house-flies buried in a screened stand-pipe forty-eight inches under sterile sand came to the surface. Other flies of undetermined species struggled up through seventy-two inches of sand.

So great is the menace of the ordinary box privy that a number of inexpensive and simple sanitary privies have been designed for use where there are not modern sewer systems. Stiles and Lumsden (1911) have given minute directions for the construction of one of the best types, and their bulletin should be obtained by those interested.

Another precaution which is of fundamental importance in preventing the spread of typhoid, is that of disinfecting all discharges from patients suffering with the disease. For this purpose, quick-lime is the cheapest and is wholly satisfactory. In chamber vessels it should be used in a quantity equal to that of the discharge to be treated. It should be allowed to act for two hours. Air-slaked lime is of no value whatever. Chloride of lime, carbolic acid, or formalin may be used, but are more expensive. Other intestinal diseases demand similar precautions.Stomoxys calcitrans, the stable-fly—It is a popular belief that house-flies bite more viciously just before a rain. As a matter of fact, the true house-flies never bite, for their mouth-parts are not fitted for piercing. The basis of the misconception is the fact that a true biting fly, Stomoxys calcitrans (fig.110), closely resembling the house-fly, is frequently found in houses and may be driven in in greater numbers by muggy weather. From its usual habitat this fly is known as the "stable-fly" or, sometimes as the "biting house-fly."

Stomoxys calcitrans may be separated from the house-fly by the use of the key on p. 145. It may be more fully characterized as follows:

The eyes of the male are separated by a distance equal to one-fourth of the diameter of the head, in the female by one-third. The frontal stripe is black, the cheeks and margins of the orbits silvery-white. The antennÆ are black, the arista feathered on the upper side only. The proboscis is black, slender, fitted for piercing and projects forward in front of the head. The thorax is grayish, marked by four conspicuous, more or less complete black longitudinal stripes; the scutellum is paler; the macrochÆtÆ are black. The abdomen is gray, dorsally with three brown spots on the second and third segments and a median spot on the fourth. These spots are more pronounced in the female. The legs are black, the pulvilli distinct. The wings are hyaline, the vein M₁₊₂ less sharply curved than in the house-fly, the apical cell being thus more widely open (cf. fig.110). Length 7 mm.

This fly is widely distributed, being found the world over. It was probably introduced into the United States, but has spread to all parts of the country. Bishopp (1913) regards it as of much more importance as a pest of domestic animals in the grain belt than elsewhere in the United States. The life-history and habits of this species have assumed a new significance since it has been suggested that it may transmit the human diseases, infantile paralysis and pellagra. In this country, the most detailed study of the fly is that of Bishopp (1913) whose data regarding the life cycle are as follows:

The eggs like those of the house-fly, are about one mm. in length. Under a magnifying glass they show a distinct furrow along one side. When placed on any moist substance they hatch in from one to three days after being deposited.

The larva or maggots (fig.110) have the typical shape and actions of most maggots of the Muscid group. They can be distinguished from those of the house-fly as the stigma-plates are smaller, much further apart, with the slits less sinuous. Development takes place fairly rapidly when the proper food conditions are available and the growth is completed within eleven to thirty or more days.

The pupa (fig.110), like that of related flies, undergoes its development within the contracted and hardened last larval skin, or puparium. This is elongate oval, slightly thicker towards the head end, and one-sixth to one-fourth of an inch in length. The pupal stage requires six to twenty days, or in cool weather considerably longer.

The life-cycle of the stable-fly is therefore considerably longer than that of Musca domestica. Bishopp found that complete development might be undergone in nineteen days, but that the average period was somewhat longer, ranging from twenty-one to twenty-five days, where conditions are very favorable. The longest period which he observed was forty-three days, though his finding of full grown larvÆ and pupÆ in straw during the latter part of March, in Northern Texas, showed that development may require about three months, as he considered that these stages almost certainly developed from eggs deposited the previous December.

The favorite breeding place, where available, seems to be straw or manure mixed with straw. It also breeds in great numbers in horse-manure, in company with Musca domestica.

Newstead considers that in England the stable-fly hibernates in the pupal stage. Bishopp finds that in the southern part of the United States there is no true hibernation, as the adults have been found to emerge at various times during the winter. He believes that in the northern United States the winter is normally passed in the larval and pupal stages, and that the adults which have been observed in heated stables in the dead of winter were bred out in refuse within the warm barns and were not hibernating adults.

Graham-Smith (1913) states that although the stable-fly frequents stable manure, it is probably not an important agent in distributing the organisms of intestinal diseases. Bishopp makes the important observation that "it has never been found breeding in human excrement and does not frequent malodorous places, which are so attractive to the house-fly. Hence it is much less likely to carry typhoid and other germs which may be found in such places."

Questions of the possible agency of Stomoxys calcitrans in the transmission of infantile paralysis and of pellagra, we shall consider later.Other arthropods which may serve as simple carriers of pathogenic organisms—It should be again emphasized that any insect which has access to, and comes in contact with, pathogenic organisms and then passes to the food, or drink, or the body of man, may serve as a simple carrier of disease. In addition to the more obvious illustrations, an interesting one is the previously cited case of the transfer of Dermatobia cyaniventris by a mosquito (fig.81-84). Darling (1913) has shown that in the tropics, the omnipresent ants may be important factors in the spread of disease.