and is notorious in the bed-bug. It enables the insect to cling more closely to the host, and reduces the risks of being blown or brushed off. The feet are provided with claws for holding fast to hair or feathers. Wings are present in most species, and are sometimes well developed. But there is progressive reduction in the size of the wings until, in the genus Melophagus, parasitic on sheep and chamois, both sexes are entirely devoid of wings and are obliged to crawl.

In the winged species there are peculiarities in the veins and the shape of the wing which seem to indicate incipient atrophy. The veins are crowded together along the front border of the wing, while the membrane is only supported by a few weak veins running obliquely across it. These flies are, however, true parasites, and make little use of their wings except to reach a host, or, in the case of males, to find a mate. There is, in some cases, a strange compromise between wings and winglessness. In the species Lipoptena cervi, which is parasitic on several kinds of deer, both sexes emerge from the pupa-case with wings; but those of the female break off close to the base, leaving two odd-looking stumps, and she becomes entirely sedentary. It was at one time thought that she bit them off; and it may be that she assists by other means in ridding herself of organs of locomotion, not wanted by a parasite which has solved the problem of food, and only wants the visit of a male to solve the problem of continuing the race. The male flies seem to retain their wings longer; but both sexes are found on deer, crawling in a wingless condition among the hairs.

Being parasites of wild animals, it is not unnatural that most of this strange family of flies should be but little known. In this country we have six native species, of which the Forest-fly (Hippobosca equinus), a parasite of the horse, and the sheep-tick (Melophagus ovinus) have the most chance of becoming familiar to man. The Forest-fly is found all over Europe and in other parts of the world to which horses have been imported. The English name is derived from the New Forest, where from May to October masses, in clusters, at times infest the ponies which there run wild. The flies congregate where the skin is thinnest, between the thighs and

beneath the tail; and, though the bite does not seem to be painful and the Forest ponies are indifferent to the fly, strange horses are driven frantic by the mere tickling. The claws of the parasite fastened in the hairs enable the flies to despise the swishing of the horse's tail. When seriously disturbed they move quickly sideways with a crablike motion. Cattle are also sometimes infested by these flies. Their near allies in other countries are parasites of dogs, camels, ostriches, and other animals.

A second British species (Lipoptena cervi) is usually parasitic on roe-deer in this country, but is also obtained here, and in other parts of Europe, from fallow-deer and red-deer. In Scandinavia it is a parasite of the elk, but, strange to say, is not found on the closely allied moose of the New World. We have little detailed knowledge of the life-history of these insects. The pupa-cases are sometimes found in the hair of deer, but it seems to be the habit of these insects, which are known to German foresters as Lausfliegen, to deposit their larvæ on the ground where the herds congregate. Both sexes emerge with wings and lose them when they have found a deer to live on. In the autumn winged males are met with in the woods, which are different from the wingless males that consort with the females in the hair of the deer. They are paler and more slender. The significance and purpose of this second type of male remains an unexplained mystery.

The most advanced stage of parasitism, in this family, is reached in the insect found on domestic sheep. Nothing is known of the forms which doubtless infest the different species of wild sheep; and their study would perhaps elucidate the evolution of these wingless flies. The insect, known to shepherds as a 'ked, is also incorrectly called a sheep-' tick' or 'louse.' It passes the whole of its life on the sheep and moves slowly and quietly through the fleece. The new-born larva is attached to the wool and there passes the pupal stage. Being incapable of flight, the parasites can only get to new hosts when these are in contact or near adjacent. The gregarious habits of sheep may have had an effect on the evolution of the parasite, which bears little resemblance to an ordinary fly. Other symptoms of a parasitic life are seen in the head, which is deeply Vol. 234.-No. 464.

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embedded in the thorax, so that it is protected laterally by the coxal segments of the front legs; in the eyes, which are linear and small instead of being round and prominent; in the absence of outstanding antennæ; in the strong legs ending in clawed feet. Anything in the structure of a normal fly which would impede progress through wool has been lost; anything that would facilitate clinging has been developed. Head and thorax tend to become fused into one. There are no external signs of segments in the leathery abdomen. Nearly allied forms have been obtained in the Caucasus from an ibex, and in Western Asia from an antelope and a chamois.

In considering the geographical distribution of a parasite, two questions must be kept in view. First the distribution of the host; and secondly, whether the parasite occurs throughout the range of the host and, if not, in what geographical regions. The study of animal distribution aims at explaining the present range of animals in the light of the past configuration of the earth's surface. Thus the severance of one tract of land from another by the sea may check the migration of land animals and explain present distribution which, but for this, appears inexplicable. In dealing with parasites, however, the first consideration is the distribution of the host and the factors controlling it. A sharp distinction must be drawn between parasites of mammals and those of birds, since the factors controlling the distribution of those two classes of host are fundamentally different.

Bearing these general considerations in mind, one may turn to the Hippoboscids and see what is to be learnt from the distribution of this single family of peculiar parasitic flies. Taking first those which are parasites of mammals, one may distribute the types of host into the following groups: marsupials or pouched mammals; ungulates or hoofed mammals; carnivora ; lemurs; and bats.

First, marsupials. That the genus (Ortholfersia), which is composed of insects parasitic on kangaroos, must be confined to the Australian region is self-evident, since kangaroos are found nowhere else. Secondly, ungulates, whose parasites are comprised in four genera (Hippobosca, Lipoptena, Echestypus, and Melophagus).

Hoofed animals or ungulates are now found all over the world, but in some places they owe their presence to man's agency. There were originally no cattle or deer in Madagascar. There were no horses in America when the New World was discovered. Deer are absent from the African region. The American deer, moreover, fall into two clearly divided groups. The forms which inhabit the northern part of the New World are closely allied to those of the northern parts of Europe and Asia. On the other hand, the deer found in the more southern part, including Central and South America, form a second group clearly marked off by anatomical differences:

It is an interesting matter for inquiry how far the distribution of these parasitic flies fits in with the facts just set out as to the distribution of their hosts. The genus Hippobosca, with its eight different species, studied by itself is most instructive. For the moment one form (H. fossulata) may be left out of consideration. A few specimens, secured in the South American Republic of Colombia, have been examined and (as they are hardly distinguishable from an Indian form) introduction may be suspected. Assuming that to be case, the most obvious and noteworthy fact is that this genus of parasites is confined to the Old World. The second fact, less obvious but also noteworthy, is that some species are restricted to a small portion of the earth's surface, while others have an enormously wide range. For instance, one species (H. rufipes) is confined to South Africa, while another (H. equinus) ranges from England to Australia. The reason becomes obvious when one appreciates that those forms have the widest range which are parasites of animals which man has tamed for his domestic use. They owe their introduction into new countries to man alone. There is abundant direct evidence of this in the present case.

One form, which is a cattle parasite in India, has become well established in Madagascar, to which country Indian cattle have been introduced. During the Boer war this fly got transported to South Africa, and it is now established on the mainland. Little did Mr Chamberlain think, when he espoused the cause of the Uitlanders, that one of the ultimate results of his policy would

be to widen the territory infested by Hippoboscid flies. Another form, which is a horse-parasite, has reached Australia, where there were no horses when the first settlers landed. The journey to Australia taken by these parasites was circuitous; but it has been traced with all the care with which intelligence officers worked out the route of an enemy division from the Eastern to the Western front. The flies started from France and travelled with horses to Algiers; from there they were brought to New Caledonia; and from that place they were subsequently landed in New South Wales. It is a pleasant thought that here they will find distant and long-lost relatives who are parasites of the native kangaroos. There is a third form which is a dogparasite, and is found on the shores of the Mediterranean, down the East African coast, and through Southern Asia up to Japan. Here again man's interference may reasonably be suspected.

Turning now to the origin of this genus, there is reason to believe, in the light of what we know of structure and geographical distribution, that the ancestral form originated in Central Asia. Here the flies became parasitic on horses and cattle. Those which are to-day parasitic on camels are more recent developments, as their structure shows. They deserted their old hosts for camels at what geologists would consider a comparatively recent date. The fact that the llamas, or camels of the New World, are not troubled by Hippoboscid flies confirms this. The transference from the camel to the ostrich, which inhabits the same sandy portions of the world, is natural enough, and came later. This also is confirmed by the significant fact that the two species (H. struthionis and H. rufipes) which are parasitic on the African ostrich are the most widely divergent in structure from what one supposes to be the ancestral form.

The next genus (Lipoptena) presents a different problem. Seven species are found in the Old World and three markedly distinct species in the New World. They are, in the main, parasites of deer, who were probably their original hosts. It is noteworthy that insects of this genus are absent from the African and Australian regions in which, it is hardly necessary to remind the reader, there are no deer. But in the south-eastern