The foregoing description of the nature and mode of the infection of sleeping sickness will not cause any astonishment to the layman of the present day who knows anything of recent medical science. We are all familiar with the danger of fly-bites, even in this country, where deadly bacteria are occasionally carried by biting flies, such as the horse-flies, into the human subject; and nowadays every one is more or less familiar with the discovery of the minute blood-parasite which causes malaria or ague and is carried by a particular kind of gnat in the interior of which it multiplies by a process of sexual conjugation. At the same time the reader who is interested in sleeping sickness will probably desire to know more about the nature of the tsetze flies and some further details as to the parasite spoken of as trypanosome. [Illustration: FIG. 48. Tsetze flies--_Glossina morsitans_--magnified two diameters. This is the “fly” of the Nagana or horse and cattle disease of South Africa. The _Glossina palpalis_, which carries the _Trypanosoma Gambiense_ causing sleeping sickness, is very closely similar to it in appearance.] The tsetze flies form a genus called by Wiedemann (in 1830) “Glossina.” They are only found in Africa; and some seven species in all are known. They are little bigger than a common house-fly, and much like it in colour (fig. 48). They differ in appearance from the house-fly in the fact that the wings, when the insect is at rest, are parallel to one another, and slightly over-lap in the middle line, instead being to a small extent divergent at their free extremities. The bite, like that of all flies, is rather a stab than a bite, and is effected by a beak-like process of the head, the blood of the animal pricked in this way being drawn into the fly’s mouth by a sucking action of the gullet. The tsetze flies appear to be especially greedy and are said to gorge themselves to such an extent that the blood taken in from one animal overflows the gullet, and so contaminates the wound inflicted by the fly on the next animal it visits. It is at the present moment assumed very generally that this is the way in which infection is produced. But it is not at all improbable that the trypanosome undergoes some kind of multiplication and change of form when sucked into the tsetze fly as happens in the case of the malaria parasite when swallowed by the _Anopheles_ gnat. No such change has yet been discovered in regard to the trypanosome of sleeping sickness: but it cannot be said that the matter has been exhaustively studied, or that a negative conclusion is justified.[25] [Illustration: FIG. 49. The earliest discovered Trypanosome, described by Gruby in 1843 as “_Trypanosoma sanguinis_” and found by him in the blood of the common esculent Frog.