INSECTS 177 



Some fifty years ago it was noticed that the bite of tsetse-flies caused 

 a fatal disease in domesticated animals, and it was thought to be due to some 

 peculiar poison injected by the insect while sucking blood. In 1895 it was shown 

 that the disease called nagana in domesticated animals was caused by the presence 

 of vast numbers of a minute blood-parasite (Trypanosoma brucei), which was 

 carried to the animals by tsetse. This gave importance to the stud}- of the habits 

 and distribution of tsetses. It has, however, been discovered that the parasite 

 causing the nagana disease can occur in blood-sucking flies other than tsetse ; 

 Trypanosoma brucei having been found in a blood-sucking fly of the genus 

 Stomoxys, which belongs to another group. 



It was formerly believed that tsetse laid eggs in the excreta of the buffalo, 

 but this is erroneous. As already mentioned, female tsetses do not lay eggs, but 

 produce fully grown yellow grubs. These are born active, and on finding a 

 suitable hiding place, such as a small hole or crevice, change in a few hours into 

 black quiescent pupae. The fly emerges from the pupa in about five or six weeks. 

 The tsetses exhibit a great dislike to animal excreta, and natives have been known 

 to plaster their animals with such as a protection against the flies. Tsetses of 

 different species are widely distributed through many parts of Africa. They 

 prefer low-lying bush country, and as a rule, although not invariably, near water. 

 They are seldom found in open grass country. Tsetses are exceedingly local in 

 occurrence, and are frequently found to be confined to comparatively narrow 

 strips of country of considerable length, known as fly-belts. Sometimes these 

 belts are very sharply defined, so that within a few hundred yards of apparently 

 uniform country the traveller can pass from an area swarming with flies to one 

 in which they are absent. 



The cause of this singular distribution is still unknown, but it is certainly 

 not dependent on the presence or absence of game, as a narrow fly-belt may 

 extend through a wide area with abundance of game, or through a region where 

 game is exceedingly scarce or altogether absent. There are certain observations 

 which show that the tsetses sometimes suck vegetable juices, and it is conceivable 

 that the occurrence of fty-belts is partly dependent on the presence of certain 

 plants, which might be confined to belts dependent on geological out-crops. 



Trypanosomas of many kinds have been found in a great variety of fishes, 

 amphibians, reptiles, birds, and mammals. In the majority of cases it appears 

 that these parasites have no obviously prejudicial effect on the host. The 

 trypanosome (T brucei) which causes nagana, in domesticated animals has been 

 found in the blood of jackals and antelopes, and there is little doubt that it 

 occurs in many different kinds of animals. Tsetses while sucking blood convey 

 the parasite from the first host, where it is innocuous, to the second host, in 

 which it causes malignant disease. The question whether the local destruction 

 of game animals would have any permanent effect in lessening the fly-scourge and 

 reducing the occurrence of nagana has received careful consideration, but it is 

 exceedingly doubtful whether any permanent beneficial effect would result from 

 such action. 



In addition to tsetses, Africa has many other biting flies, of which the 

 following, according to a report by Mr. S. A. Neave, are the most important. 

 vol. 111. — 12 



