Tsetse-fly - Encyclopedia

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TSETSE-FLY (Tsetse, an English rendering of the Bantu nsi-nsi, a fly), a name applied indiscriminately to any one of the eight species of Glossina, a genus of African blood-sucking Diptera (two-winged flies, see Diptera), of the family Muscidae. Tsetse-flies are of great economic and pathological importance as the disseminators of tsetse-fly disease (nagana) and sleeping sickness. These maladies are caused by minute unicellula animal parasites (haematozoa) of the genus Trypanosoma (see Trypanosomes); and recent investigations have shown that, under normal conditions, the particular species of Trypanosoma concerned (T. brucei, in the case of nagana, and T. gambiense in that of sleeping sickness) are introduced into the blood of susceptible animals or man only by the bite of one or other of the species of tsetse. (See Parasitic Diseases). The names of the recognized species of tsetse-flies are as follows: Glossina palpalis (see fig.); G. pallicera; G. morsitans; G. tachinoides; G. pallidipes; G. longipalpis; G. fusca; and G. longipennis. A ninth so-called species, described in 1905 from specimens from Angola, is not really distinct from G. palpalis but appears to be identical with the sub-species G. palpalis wellmani. In appearance tsetse are somewhat narrow-bodied flies, with a prominent proboscis, which projects horizontally in front of the head, and with the wings in the resting position closed flat one over the other like the blades of a pair of scissors (see fig., B). The latter characteristic affords an infallible means for the recognition of these insects, since it at once serves to distinguish them from any blood-sucking flies with which they might otherwise be confused. The coloration of tsetse-flies is sombre and inconspicuous; the brownish or greyish-brown thorax usually exhibits darker longitudinal markings, and when the insect is at rest the abdomen or hinder half of the body is entirely concealed by the brownish wings. In some species the abdomen is of a paler colour and marked with sharply defined, dark brown bands, which are interrupted on the middle line. The length of the body, exclusive of the proboscis, which measures about a line to a line and a half, varies according to the species from 6 or 8 millimetres in the case of G. tachinoides, to about i i z millimetres in that of G. fusca or longipennis; the closed wings, however, project beyond the body and thus increase its apparent length. G. palpalis, the disseminator of sleeping sickness (see fig.), is about 92 millimetres in length and is the darkest of all the tsetse-flies, though the dark brown abdomen has pale lateral triangular markings and usually at least an indication of a pale longitudinal median stripe. In all tsetse-flies the proboscis in the living insect is entirely concealed by the palpi, which are grooved in their inner sides and form a closely fitting sheath for the piercing organ; the base of the proboscis is expanded beneath into a large onion-shaped bulb, which is filled with muscles. The head of the insect contains a muscular pharynx by means of which the blood from the wound inflicted by the proboscis (labium) is pumped into the alimentary canal and the so-called sucking-stomach. The tip of the proboscis is armed with a complicated series of chitinous teeth and rasps, by means of which the fly is enabled to pierce the skin of its victim; as usual in Diptera the organ is closed on the upper side by the labrum, or upper lip, and contains the hypopharynx or common outlet of the paired salivary glands, which are situated in the abdomen. The proboscis of tsetse-flies is without the paired piercing stilets (mandibles and maxillae) possessed by other bloodsucking Diptera, such as the female horse-flies and mosquitoes.

For the anatomy of the tsetse see E. A. Minchin, Proc. Roy. Soc. lxxvi. 531-547.

Tsetse-flies are restricted to Africa, where they occur in suitable localities throughout the greater portion of the tropical region, although not found either in the Sahara or in the veld country of the extreme south. For practical purposes the northern limit of Glossina, as at present known, may be shown on the map by drawing a line from Cape Verde to the Nile a little to the south-east of El Obeid, and thence to the coast of Somaliland at 4° N; while the southern boundary of the genus may similarly be represented by the Cunene river, in the south of Angola, and a line thence to the north-eastern end of St Lucia lake, in Zululand. Within the area thus defined tsetse-flies are not found continuously, however, but occur only in small tracts called" belts " or " patches," which, since cover and shade are necessities of life to these insects, are always situated in forest, bush or banana plantations, or among other shady vegetation. In South and Central Africa, at any rate, " fly-belts " are usually met with in damp, hot, low-lying spots on the margins of water-courses, rivers and lakes, and seldom far from water of some kind. It appears, however, that in this respect the habits of the different species show a certain amount of variation; thus, while G. palpalis exhibits an especial fondness for water and haunts more or less dense cover at the water's edge, recent observations in German East Africa show that G. fusca is in no way connected with water, but is much more frequently encountered at a distance from it. Similarly, the oft-repeated assertion that there is a definite connexion between tsetse-flies and big game, especially the buffalo (Bubalus caffer), in that the former are dependent upon the latter for their continued existence, is certainly not true as regards G. palpalis, although in South Africa there can be no question that the extermination of big game has been followed or accompanied by the disappearance of tsetse from many localities in which they formerly abounded.

As a rule tsetse-flies are most active during the warmer hours of the day, but they frequently bite at night, especially by moonlight. The blood-sucking habit is common to both sexes, and the abdomen, being capable of great expansion, is adapted for the periodical ingestion of an abundant food-supply. The act of feeding, in which the proboscis is buried in the skin of the victim nearly up to the bulb, is remarkably quick, and in thirty seconds or less the abdomen of the fly, previously flat, becomes swollen out with blood like a berry. Stuhlmann's experiments with G. fusca show that the insect is able to ingest considerably more than (sometimes more than twice) its own weight of blood, which would appear to be the only food, and must be drawn from the tissues of a victim. Specimens of G. fusca, even though fasting and kept for days in absolutely dry air, could never be induced to imbibe water, sugar-cane juice or extra vasated blood. The reproduction of tsetse-flies is highly remarkable; instead of laying eggs or being ovovivi parous the females deposit at intervals of about a fortnight or three weeks a single full-grown larva, which forthwith buries itself in the ground to a depth of several centi metres, and assumes the pupal state. The practical importance of this peculiar life-history is very great, since larvae thus protected cannot easily be destroyed.

It is important to note that although sleeping sickness (of which the chief foci are at present the Congo Free State and Uganda) has hitherto been associated with one particular species of Glossina, it has been shown experi mentally both that other tsetse-flies are able to transmit the parasite of the disease, and that G. palpalis can convey kindred parasites which are fatal to domestic animals.

Since, moreover, it is believed that at least five species of Glossina are carriers of nagana, it may well be that all tsetse-flies can disseminate both nagana and sleeping sickness. (E. E. A.)

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