PUBLIC HEALTH 27 



these flies are included, is restricted to the African continent, 

 but is there represented by a number of species, several of 

 which have been shown to act as carriers for trypanosome 

 diseases in animals. One only, Glossina palpalis, carries the 

 common trypanosome of human sleeping sickness, Trypano- 

 soma gambiense. The disease appears- to have been originally 

 endemic only in West Africa, but was found in eastern equa- 

 torial Africa something over fifteen years ago, and it is in 

 this latter region that its ravages have been so pronounced. 

 Owing to certain peculiarities in the habits of the tsetse-flies, 

 the distribution of sleeping sickness is limited to very definite 

 areas in the region where it occurs. The fly, which has a 

 sharp needle-like beak for sucking blood, resembles our own 

 stable-fly (Stomoxys calcitrans) in general appearance but is 

 considerably larger, measuring about half an inch in length. 

 It is found only in the dense brush which grows along the 

 edges of streams, ponds and lakes. In such places persons 

 and animals may be bitten by the flies and it is. exclusively 

 through such bites that these insects may obtain virus of 

 sleeping sickness from the blood of a person or animal suffer- 

 ing with the disease. Should the fly obtain a meal of blood 

 containing trypanosomes, these may multiply in the body of 

 the fly, although not always, for only about one in twenty of 

 such flies becomes infectious. A considerable period must 

 now elapse before the infected fly is in condition to inoculate 

 a new patient, usually thirty or forty days, but after this for 

 at least seventy-five days it remains infectious, and may in- 

 troduce the trypanosomes into the blood of any animal upon 

 which it feeds during this period. 



The tsetse-flies develop in a very different way from most 

 insects. The female does not deposit her eggs, but a single 

 one develops to the fully grown larval condition before being 

 deposited. This larva soon pupates in the shade beneath the 

 brush bordering the water where it has been dropped by the 

 parent fly, and later emerges in the winged adult condition. 

 The pupae require such moist shade, and it is apparently for 

 this reason alone that the flies never occur away from the 



