XVIII] LIFE-CYCLE 311 



infected, as there is never a permanent infection of the crop. 

 When trypanosomes persist in greater or less numbers in the 

 gut and crop of the same fly, the issue is somewhat doubtful as 

 the flagellates may be swept out by subsequent clean feeds. 



A permanent infection of the fly is obtained when the whole 

 material of the first feed has been displaced from the gut by the 

 second feed, and the trypanosomes still persist. Once parasites 

 are well established in the new blood the rate of multiplication 

 is such that there is little chance of their being destroyed at the 

 next influx of blood. 



There is no doubt that the critical time for the parasite is 

 this influx of fresh blood after an infected feed. This is shewn 

 by the very much larger percentage of infected flies found 

 amongst those individuals that had only one feed and this 

 the infecting one, than among those that had been subsequently 

 fed every two or three days in the usual way. Out of 103 

 starved flies trypanosomes were found in 22 between the 6th 

 and i2th day. 16 or 15-5 per cent, of these starved flies shewed 

 a well developed infection of the gut, whereas under ordinary 

 feeding conditions only about 3 per cent, of these flies became 

 infected. 



In the early days of the cycle in the alimentary tract of the 

 fly many forms of parasites may be observed, depending on the 

 various conditions. The trypanosomes may persist without 

 multiplying, under which circumstances they degenerate and 

 disappear within three to four days. In other cases the parasites 

 persist in small numbers and begin to multiply, but when the 

 adverse conditions of a new r feed come upon them, they are 

 unable to withstand them, and dividing and degenerating speci- 

 mens may thus be found side by side. Persistence and quite 

 normal development may occur in the crop and continue till 

 the loth or I2th day, so that it is evident that the stimulus to 

 development in the fly is not dependent upon the digestive 

 action of the gut fluid upon the blood. 



The very large number of cases where the attempted multi- 

 plication fails to establish an infection, indicates the presence 

 of a general inhibiting property in the Glossina and is a fairly 

 constant factor in experiments with freshly hatched flies. 



