242 



Miss M. Robertson. Development of 



[Feb. 26, 



between the 6th and 12th day, that is to say 21*3 per cent, of the flies 

 harboured trypanosomes. Six flies of the total 22 showed trypanosomes only 

 in the sucking stomach, or crop, as this organ should perhaps be more 

 appropriately called, 13 showed a well-established infection in the gut and 

 three showed trypanosomes in both situations. It is perhaps advisable to 

 neglect the six flies in which the parasite was only present in the crop, 

 although a certain amount of development may go on in this organ ; the 

 percentage thus obtained is still very high, namely 15"5. 



Monkeys infected with T. gambiense, and probably most other animals 

 with trypanosomes in their blood, have negative periods, that is to say, 

 periods during which they do not infect flies. A number of the experiments 

 have shown that trypanosomes may be found by microscopic examination, 

 although the blood is not infective to flies. It is interesting to note that 

 such negative periods appear to be negative in starved as well as in fed flies. 

 This relation is, however, not yet sutficiently worked out. In the starvation 

 experiments the microscopic appearances do not, so far as I have yet seen, 

 show any distinction from those to be observed in established infections in 

 fed flies of the corresponding ages. 



The crucial moment in the cycle appears to be the first feed of clean blood 

 subsequent to the infecting feed. It is not evident if this clearing out of the 

 trypanosomes by the clean feed is a purely mechanical action due to the 

 flooding of the gut or is a result of the general change of condition thus 

 brought about. 



In any case the number of flies containing trypanosomes obtained in 

 starvation experiments during periods when the vertebrate is in the infective 

 condition would give the maximum register of the potential infectivity of 

 that individual strain to fly. The actual number of flies containing 

 trypanosomes from parallel experiments, which were, however, subsequently 

 fed, would give an indication of the additional inhibiting power of the fly 

 under ordinary conditions whatever the cause to which the inhibition may be 

 due. Experiments of this type were undertaken but gave no result, as the 

 whole series proved negative, the monkey not being in the infective condition. 



The total number of flies used in the whole group of experiments under 

 consideration in this paper is 1411 males and 1322 females, of which 42 males 

 and 39 females show trypanosomes. Irrespective of sex, the total number is 

 2733, of which 81 gave a positive result. 



From this total must be deducted the starvation experiments and a small 

 group which are not strictly comparable, owing to the feeding having 

 included toad's blood, and also those flies dissected before the 5th day. 

 Concisely the figures stand thus : — 



