310 T. GAMBIENSE [CH. 



produced by the Uganda strain of T. gamhiense in monkeys 

 was only about one in 40. 



Life history within the invertebrate host, G. palpalis — 

 Exogenous cycle. — In addition to the initia linfectivity of the 

 trypanosome mentioned above, several other conditions have 

 a marked effect upon the development of T. gambiense in its 

 invertebrate host, the tsetse-fly. A certain degree of warmth 

 is essential and the more favourable results obtained on 

 Tanganyika than in Uganda, in the transmission of T. gambiense 

 by G. morsitans, can be explained by the differences in tem- 

 perature between these two localities. 



One of the most important conditions affecting develop- 

 ment is the interval that elapses between the feed of infected 

 blood and the following meal. When a newly-hatched fly is 

 fed, it usually ingests sufficient blood to fill both its crop and 

 gut. The next feed, taken in two or three days later, may 

 either entirely replace the undigested remains of the first meal, 

 or some of the first feed may remain in the crop and the fresh 

 blood be taken on top of it. Occasionally the blood may be 

 taken directly into the gut without even going into the crop. 



If a fly has ingested trypanosomes at its first feed, the 

 result of subsequent clean feeds varies in different individuals. 

 The trypanosomes may be digested and disappear from the 

 ahmentary canal of the Glossina during the first 50 to 72 hours, 

 without the intervention of a second feed. In other cases the 

 flagellates survive in the gut in small numbers but disappear 

 during the early stages of digestion of the new blood. It is 

 evident from these two results that all trypanosomes cannot 

 withstand the digestive processes and, of course, in such cases 

 the fly does not become infected. In some flies the trypano- 

 somes may survive and multiply in the gut in the blood 

 retained from the first feed, although a second feed has been 

 taken in on top. In these cases the trypanosomes may be swept 

 out when the original blood is digested, and thus no infection 

 be produced. The trypanosomes may survive and develop in 

 the crop for as long as 12 days, providing that blood is con- 

 stantly present. The gut of these flies may often shew no 

 signs of flagellates, in which case the insect will not become 



