Page:The Journal of Tropical Medicine, volume 6.djvu/239

July 1, 1903.]

‘protozoa, but by a very general law which limits the eontinuancy of growth by asexual reproduction, and, indeed, under unfavourable conditions the. trypanosomes are known to assume peculiar forms which may be the tatives of a sexual or resting stage. gm |bs:| | |iir-| In studying the naturai history of the tsetse ties, I was struck by certain facts which I think may help us in elucidating the etiology and epidemiology of trypano- ‘somiasis. Major Bruce noticed that “the tsetse fly does not lay eggs as do the majority of the Diptera, but extrudes a yellow-coloured larva nearly as large as the abdomen of the mother. This larva is furnished with a black hood at one pole and two minute spikes at the other. It is annulated and consists of ten segments. Imme- diately on being born this larva creeps about with a good deal of activity, evidently searching for some cover or hole in which to hide. Having found a resting place, it immediately begins to change colour, aad after a few hours has turned into a jet-black hard pupa or nympha.” This description clearly shows that the method of reproduction in the tsetse flies is like that of the Hippoboscide, the eggs hatching, and the larve developing and moulting within the body of the parent, so that when extruded they have practi- cally reached the pupa stage. In fact, the extruded _ larva becomes almost immediately a pupa, the larva skin becoming a hard pupariwm.

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Fie, 4. The nourishment of the growing larva requires a | large amount of blood, and the avidity with which it is | imbibed by the gravid female is really astonishing. | his peculiar mode of development suggests, I think, | the possibility that the transmission of nagana may ike place not directly by the fly that imbibes the | blood of an infected animal, but by its progeny. Severai acts are in favour of this hypothesis. Like mosquitoes, he tsetse flies usually suck blood only once every two ‘or three days. In the transmission of cattle hemo- obinuric fever (Texas fever) by Ripicephalus annuatus, id in that of the malignant jaundice of dogs by Haema- ihysalis Leachi, we have examples of the transmission protozoal parasites by the young of ticks fed on ected animals—the parasites in these two cases obably reach the young when the latter are in the [egg-stage, with the food reserve which is derived from the infected blood imbibed by the parent tick. It has been stated by a number of travellers and ortsmen, and it is also the opinion of the natives, F % the tsetse fly follows the big game, and that’ the l wvalence of nagana in a district is proportionate to the number of wild animals. Indeed, it has been asserted that both the fly and the disease disappear as

he game is killed off or driven away, and some have

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gone so far as to urge the destruction of big game, and more especially of the buffalo, in‘order to protect im- ported domestic animals.: Major Bruce has’ clearly: proved that Trypanosoma Brucet affects in a compara- tively harmless way the indigenous wild animals. ~ Of thirty-five wild animals examined, ten contained ‘the parasite. These comprised one buffalo, three’ wilde»: - beests, three koodoos, one bush-buck, and one hyena.” It is also quite likely that the fly follows the herds-- within certain areas, and that, like the Hippbosca of the*’ horse, it may settle for a. time on its host. Major” Bruce one day killed a wildebeeste and found a large number of tsetse flies on the dead animal. And again on a dead buffalo he counted as many as forty flies. However, we should not misunderstand the bearing of these facts, because the distribution of the tsetse fly is in no way connected with that of big game. The tsetse fly is not found in open plains, although such plains may be literally swarming with game, and on the other hand, places devoid of game may be very dangerous “ fly zones.”” The association between buffalo and tsetse is more apparent than real, both inhabiting densely-wooded, damp, secluded districts ;--and, indeed, tsetse flies occur in districts where there are no buffalo, and in these places, in which both formerly co-existed, the almost total destruction of buffalo by rinderpest has made no appreciable difference in the number of flies. The distribution of the tsetse fly shows marked peculiarities which exclude, altogether the big game theory. The most important features are the invariable limitation to the borders of marshes or river-banks, and the formation of small, scattered stations, only a few hundred yards in extent, called “fly belts’’ or “fly zones.” Sometimes the tsetse is confined to one side of a river in thousands, whilst on the opposite bank not one can be found. ‘The constant association of the tsetse with streams, rivers, or other bodies of water, is, undoubtedly, a very important feature in the natural history of this fly, and one which needs investigation. The only explanation so far advanced to account for it is that the fly waits near water to feed on the animals that come down to drink! In Freetown, Mr. Austen” noticed that Glossina palpalis always occurred along the beds of streams, and frequently. settled .on, stonefid projecting from the water. On one occasion: he saws: quite a number of these tsetse flies. congregated, on: a, +; fisherman’s canoe. Uoy Biase The association between mosquitoes and water. ,1s) - very obviously explained by the fact that these Diptera; have aquatic larve. In the pupiparous genus Glossina, the association with water can only. be related, to food:,,.: habits, and I venture to suggest that it might be related _ to the habit of feeding on fishes. This suggestion will probably appear extraordinary to, some, but, I wills » endeavour to show that it is reasonable. . In; the first, . place, we know that certain blood-sucking flies, can, and, ' do feed on fishes. Mr. C. H. Murray” saw mosquitoes — ; alight and immediately transfix the fry of trout by: -., inserting their proboscis into the» head.of, the young; fish as they came to the surface of the water. Ld Was, 5 so interested in this before unheard, of ,destruction, of fish,” says Mr. Murray, “ that I. watched the depreda- tions of these mosquitoes for more than half an hour,

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