THE CHIGOES AND THEIR ALLIES The chigoes and their allies belong to a group of fleas sufficiently remarkable to deserve a somewhat detailed account. The reader may remember that they form a family to which the name of SarcopsyllidÆ has been given. They are the most completely parasitic of any fleas; and the South American chigoe (Dermatophilus penetrans) enjoys the distinction of being the first foreign flea ever described. This pestilent insect, of which the female has the habit of burrowing into the flesh of the host, soon made itself known to the early travellers in the tropics of America. Oviedo, the Spaniard and historiographer of South America, in his Historia General y Natural de las Indias (1551), seems to have been the first European author who mentions it. After this the chigoe is referred to by writers of various nationalities in many works which were published during the sixteenth, seventeenth and eighteenth centuries. It is an insect which appears under a vast number of The chigoes and their allies are of special interest for more than one reason. The females are to a greater or less degree stationary; they fix themselves firmly to their hosts and become veritable parasites. Several of the earlier zoologists believed that the animal was a mite; and it is somewhat remarkable that Oviedo himself should have so promptly detected the relationship of the insect he saw with the fleas. By reason of the parasitic habits of the females, more is known about their appearance and life than in the case of the more active males. In some species the males remain, for the present, quite unknown: and not very much is recorded about the early life-history, eggs, larvÆ and pupÆ of either sex. The parasitic habits of the chigoes and other allied fleas lead one to expect peculiar modifications of form such as are usually to be observed when an animal passes from an active to a stationary life. These modifications are the more easily understood as the various species are not all stationary to the same degree. It is fairly plain that this family of fleas is a development from the less specialised and less parasitic family PulicidÆ. In fact the gradual development of the organs from a generalised to a more specialised stage is strikingly shown in these insects. To follow this in detail would, however, require a very minute and technical knowledge of their form. The chigoe family is so well characterised that The thorax of a chigoe is exceedingly short. Two reasons for this may be suggested. In the first place, the jumping power of these fleas is very small and In most fleas the piercing organs of the mouth (upper lip and mandibles) are directed obliquely downwards. In the chigoes they are directed obliquely forwards. It has been suggested that this forward movement of the mouth is connected with the stationary life which the females assume. Fleas which fasten themselves permanently to the skin of their host, do so in a manner similar to ticks. The mouth-parts are in a line with the longitudinal axis of the body. This attitude, so far as we know, is assumed by the females of all the family. The females of one genus, Dermatophilus, actually go head foremost right into the skin of their host. The shape of the head is also beautifully adapted to enable the insect to fix itself firmly in a tick-like posture. The fore-part is remarkably obtuse, and almost has the appearance of being truncate and abruptly cut off. When the piercing organs have been thrown forward horizontally it must be a great support to the insect, which is fixed by them, if it can press its head down firmly against the skin of the host. The wider the All the chigoe family have eyes; but in one recently discovered species the eye is very small and devoid of pigment. Like those of other fleas, the antennÆ fit into grooves at the side of the head, and the club, which is the sensitive part of the organ, consists of eight segments. In a good many fleas the antennÆ are different in the two sexes, but there is no obvious sexual distinction in this family. The peculiar development of the mouth-parts is one of the most singular features in the structure of the chigoes and their relatives. These important organs are modified in a fashion not to be found in any fleas outside the family. Here, as in other fleas, the mandibles are piercing organs which penetrate the skin of the host, the upper lip serving in conjunction as a sucking tube. In ordinary fleas these organs are retracted when the insect has done its meal; in the present family they remain, in the case of the females, apparently permanently fastened in the skin. The piercing organs are broader and the serrate edges of the mandibles more solid and heavy in this family than in the case of other fleas. The two methods by which fleas keep in touch with their hosts have already been alluded to. The The modification found in the rostrum of the chigoes has already been referred to, and the explanation of this will now be understood. The rostrum is a sheath, on either side of the piercing organs, consisting of an under lip and two labial palpi. When the flea sucks, the labial palpi are pushed asunder, as the piercing organs are driven in, and lie flat on the skin of the host. In this family the rostrum is almost white in colour and soft instead of being horny or chitinised. Where the rostrum is strongly chitinised or very horny the flea has to use a certain amount of force to counteract the spring-like action of the labial palpi. It is conceivable that rigid labial palpi would prove inconvenient to fleas which remain permanently attached to their host by their mouth organs. The swollen abdomen of the female chigoe is a strange and a conspicuous object, which is not, however, found occurring to the same extent in all the members of the family. Having burrowed into the flesh of a man, or other mammal, the pregnant female swells and enlarges until she reaches the gigantic proportions of a small pea. Itching and inflammation ensue unless the whole insect be skilfully removed with a needle. To such an extent does the abdomen swell that the segments and the horny plates are driven asunder and the connecting membrane The belief that the eggs are laid in the flesh of the victim is mistaken. The hind segments of the body and their stigmata are always exposed to the air. The stigma of the eighth abdominal segment is particularly large. As soon as the eggs have been laid, the body of the mother dies, withers, and falls away from the skin of the host. The fact that several females are often found where one has buried herself, led to the notion that these parasites bred in the wound. The truth seems to be that other chigoes are attracted to a spot where inflammation has made it easy to burrow. Chigoes love warmth and drought. The deserted huts of natives swarm with them if they are dry. It is always said that newcomers are more liable to attack than natives; but the explanation of this seems to be that they do not understand the significance of the slight pricks which are felt when the chigoe fixes itself. Once the parasite has got under the skin no pain is felt unless the wound is inflamed by scratching. The tender flesh under toe-nails is a The chigoe (Dermatophilus penetrans) is a native originally of South America. It ranges from Mexico to Northern Argentina. Some time after the middle of the nineteenth century it was, somehow, carried across the Atlantic and introduced into West Africa. From there it has now spread across the Dark Continent to the Great Lakes, and has even reached Madagascar. Such are the modern facilities of transport which parasites are quick to take advantage of. |