1910.] The Development of Trypanosomes in Tsetse Flies. 373 



Thus it is seen that the infected flies found in three of the positive 

 experiments when injected into healthy monkeys gave negative results, while 

 those found in three others gave positive results. The infected fly found in 

 the seventh successful experiment was not injected. 



It is difficult to understand why the results of injecting infected flies 

 into healthy animals are so irregular. The only theory brought forward is 

 that the trypanosomes introduced under the skin along with the tissues of the 

 fly may give rise to a reaction at that point, which will so damage the 

 parasites that they fail to infect. 



In these experiments, 1840 flies were used, and of these 16 became 

 infected, or, at least, were found to have flagellates in their gut. This 

 works out at less than 1 per cent. The smallness of the percentage is due to 

 the fact that less care was taken to dissect the flies which died during the 

 course of the experiments. 



B. The Development of Trypanosoma gambiense in Laboratory-Bred 



Glossina palpalis. 



The pupte of the fly were found on the Lake-shore, and hatched out in the 

 laboratory. For a long time the Commission failed to find any pupae, 

 although days were spent in turning over soil and decaying vegetable matter 

 in those places where the fly most abounded. At last, Lieutenant A. D. 

 Fraser, Eoyal Army Medical Corps, found them in numbers in patches of 

 sand on the edge of the Lake in the Sesse Islands. After the Sesse Islands 

 were emptied of their inhabitants, Fraser's native collectors came into the 

 service of the Commission, and from that time there was no lack of pupae. 

 These natives found them in large numbers. One day they brought up as 

 many as 7000. These pupae proved to be much healthier than those obtained 

 from flies in captivity. The flies bred from larvae born in the laboratory 

 rarely showed any marked vitality. Many of the larvae were immature, and 

 those which hatched out were rarely a success as experimental flies. On the 

 other hand, the flies hatched out from pupae found on the Lake-shore were 

 fairly strong and vigorous, and lived in captivity for a couple of months or more. 

 It was, however, difficult to get them to feed at first, and very few became 

 infective, as the following table shows. The flies were fed chiefly on infected 

 monkeys. In one negative experiment (l-^^l) they were fed on a case of 

 Sleeping Sickness in man, and in five — two positive (1566 and 1602) and 

 three negative (1269, 1452, and 1672) — on oxen. oSTumerous observations 

 went to show that there is no hereditary transmission of trypanosomes in 

 Glossina palpalis ; and no evidence was gained that the flies became infected 

 with any flagellate by contact with other flies or fouled cages. Any trypano- 



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