There are at least seven species of the genus which received its name as long ago as 1830, when Wiedemann first described it. Perhaps the best known species is Glossina morsitans, which was named by Westwood.
The members of the genus Glossina are unattractive insects, a little larger than our common house-fly, with a sober brownish or brownish-grey coloration. When at rest the two wings are completely super-imposed, like the blades of a shut pair of scissors; and this feature readily serves to distinguish the genus from that of all other blood-sucking flies, and is of great use in discriminating between the tsetse and the somewhat nearly allied Stomoxys and Hæmatopota.
The tsetse flies rapidly and directly to the objects it seeks, and must have a keen sense of smell or sight, or both, making straight for its prey, and being most persistent in its attacks. The buzzing which it produces when flying is peculiar, and easily recognized again when once heard. After feeding, the fly emits a higher note, a fact recalling the observation of Dr. Nuttall and the present writer on the note of Anopheles, in which animal they observed that, ‘the larger the meal, the higher the note.’ The tsetse does not settle lightly and imperceptibly on the sufferer as the Culicidæ do, nor does it alight slowly and circumspectly after the manner of the house-flies, but it comes down with a bump, square on its legs. Like the mosquito, the tsetse is greedy, and sucks voraciously. The abdomen becomes almost spherical, and of a crimson red, and in the course of a few seconds the fly has exchanged the meagre proportions of a Don Quixote for the ampler circumference of a Sancho Panza. There is a good deal of discrepancy between the reports of the various sufferers as to the pain of the bite. No doubt different persons are very differently affected, and suffer to very varying degrees. Unlike so many of the blood-sucking Diptera, in which the habit is confined to the females, both sexes of Glossina attack warm-blooded creatures.
The fly always seems to choose a very inaccessible portion of the body to operate on—between the shoulders in man, or on the back and belly in cattle and horses; even inside the nostrils in the latter, or on the forehead in dogs. According to Lieutenant-Colonel D. Bruce, R.A.M.C., to whom we owe so much of our knowledge of this fly and its evil work, the female does not lay eggs, but is viviparous, and produces a large active yellow larva, which immediately crawls away to some secluded crevice, and straightway turns into a hard, black pupa, from which the imago emerges in some six weeks. Thus two stages, the egg and the larva, both peculiarly liable to destruction in the Culicidæ, are practically skipped in the tsetse—at any rate, in some species. On the other hand, this advantage is probably to a great extent counterbalanced by the smallness of the number of the larvæ produced, compared with the number of the eggs laid by the oviparous Diptera.
The genera of the Culicidæ which we have considered are found practically all over the world, but the genus Glossina, except that it just reaches Arabia, is fortunately confined to Africa. From the admirable map of the geographical distribution of the fly compiled by Mr. Austen we gather that its northern limit corresponds with a line drawn from the Gambia, through Lake Chad to Somaliland, somewhere about the 13th parallel of north latitude. Its southern limit is about on a level with the northern limit of Zululand. The tsetse, of course, is not found everywhere within this area, and, though it has probably escaped observation in many districts, it seems clear that it is very sporadically distributed. Mr. Austen further thinks that it may occur outside the boundary above laid down, and suggests that the great mortality amongst the horses in the Abyssinian campaign against King Theodore may have been caused by it.
Even where the tsetse is found it is not uniformly distributed, but occurs in certain localities only. These form the much dreaded ‘fly-belts.’ The normal prey of the fly is undoubtedly the big game of Africa, including crocodiles, but they are not the only factor in its distribution; the nature of the land also plays a part. There are the usual discrepancies in the accounts of travellers, especially of African travellers, as to the exact localities the Glossina affects; but most writers agree that the tsetse is not found in the open veld. It must have cover. Warm, moist, steamy hollows, containing water and clothed with forest growth, are the haunts chosen. Even within the fly-belt there are oases, due, perhaps, to an absence of shrubs or trees, where no flies are.
The tsetse fly belongs to the family Muscidæ, the true flies, a very large family, which also includes our house-fly, blue-bottle fly, etc. These flies, unlike Anopheles and Culex, are day-flies, and begin to disappear at or about sunset, a fact noted centuries ago by Dante:
‘Nel tempo che colui, che il mondo schiara,
La faccia sua a noi tien meno ascosa,
Come la mosca cede alla zanzara.’[7]
The practical disappearance as the temperature drops has enabled the South African traveller to traverse the fly-belts with impunity during the cooler hours of the night. At nightfall the tsetse seems to retire to rest amongst the shrubs and undergrowth, but, if the weather be warm, it may sit up late; and some experienced travellers refrain from entering a fly-belt, especially on a summer’s night, until the temperature has considerably fallen.
The sickness and death of the cattle bitten by the tsetse were formerly attributed to some specific poison secreted by the fly, and injected during the process of biting. It is now, largely owing to the researches of Colonel Bruce, known to be due to the inoculation of the beasts with a minute parasitic organism conveyed from host to host by the fly. The disease is known as ‘nagana,’ and the organism that causes it is a species of Trypanosoma, a flagellate Protozoon or unicellular organism, which moves by means of the lashing of a minute, whip-like process. Since Bruce’s researches a number of Trypanosomas have been found causing diseases in various parts of the world. Thus T. evansii causes the ‘surra’ disease of cattle, horses, and camels in India. T. equinum produces the ‘mal de caderas’ of the horse-ranches of South America, and T. equiperdum is responsible for the North African disease called by the French the ‘dourine.’ T. theileri causes the gall-sickness, and there are others. These parasites were first seen by Gruby, who named them in 1843, in the blood of a frog; they live, not as does the malaria parasite, in the blood-cells, but in the fluid of the blood. The particular species of Trypanosoma which causes nagana is Trypanosoma brucei, and it does not attack man, and some goats and donkeys seem also immune; but, with these exceptions, all domesticated animals suffer, and in a great percentage of cases the disease terminates in death. Just as the native children in Africa form the source of the supply of the malarial parasite without appearing to suffer much, so the big game of the country abound in Trypanosoma without appearing to be any the worse. They are, in Lankester’s phrase, ‘tolerant’ of the parasite, and a harmony between them and the parasite has been established, so that both live together without hurting one another. Under a more natural condition of things than at present obtains in South Africa, the big game formed the natural prey of the tsetse; and, indeed, so dependent is the fly on the antelopes, etc., that, in places where the game has been exterminated, the fly has also disappeared. It is from the big game that the disease has spread. In their bodies the harmful effect of the parasite has through countless generations become attenuated, but it leaps into full activity again as soon as the Trypanosoma wins its way into the body of any introduced cattle, horse, or domesticated animal. Whether the Trypanosoma does any harm to the fly, or whether it passes through any stages of its life-history in the body of the fly, is still a debatable point. Possibly it does not, and the proboscis of the fly acts then simply as an inoculating needle.