246 



Miss M. Robertson. Development of [Feb. 26, 



if the figures are so handled. It becomes clear that although a trypan osome- 

 infection is in a continual state of flux the percentage of plus flies produced 

 over infective periods gives a measure of the virulence of the strain to fly and 

 forms a basis of comparison between different strains. 



Another set of experiments bears on this point. Monkey 199 was infected 

 by Dr. Duke by direct injection of blood from a bush-buck which had been 

 infected with T. gambiense by laboratory infected flies. The bush-buck had 

 harboured T. gambiense for 15 months. This monkey showed infective and 

 non-infective periods in exactly the same way as other infections, but the 

 infective periods gave quite an unusual number of flies harbouring trypano- 

 somes. Thus — 



Experiment 71, 13/9/11, 54 flies fed, gave 4 positive, = 7'4 per cent. 

 70, 14/9/11, 50 •„ „ 7 = 14-0 



74, 15/9/11, 46 „ „ 5 „ = 10 8 



Considering all the figures together, out of 150 flies, 16 showed trypano- 

 somes, that is a percentage of 10 - 6 per cent. 



This relatively very high percentage was also borne out in experiments of 

 Dr. Duke's in which he made use of this monkey and which he kindly permits 

 me to quote. Thus of 188 flies from two experiments the conditions of 

 which admit of comparison with those of Experiments 71, 70, and 74 just 

 cited, 11 were infected, that is 5 - 8 per cent. 



Taking this set of figures with those quoted above, of 338 flies, 27 were 

 infected, which is equal to a percentage of 8 per cent. This is more than 

 double the normal percentage of infected flies produced by the Uganda strain of 

 T. gambiense in monkeys. All the conditions being considered, it is impossible 

 to attribute this difference to anything but the strain of trypanosomes. 



Besides having a virulent character as regards the production of infected 

 flies as a whole, an individual strain has often a recognisable type or method 

 of development in the glossina. For instance, all the flies fed on Monkey 1 99 

 gave very numerous and rapidly developing infections ; the trypanosomes 

 reached the pi oventriculus earlier in the cycle than is usual, and were 

 established in the salivary glands much more promptly than in the case of 

 ordinary cycles. One cage was infective on the 24th day. This difference of 

 character appeared in the flies from Dr. Duke's experiments, as well as in 

 those cited above, and I am indebted to him for the opportunity of examining 

 them. The monkeys infected by fly fed on Monkey 199 showed good 

 infections in the blood (Monkeys 330, 390, and 391), but flies fed on 

 these monkeys gave only an average number of infected flies, i.e. 3 per 

 cent., which, however, showed rather sluggish and very slowly developino- 



