740 MEDICAL ENTOMOLOGY 



of Kala Azar, for instance, will not continue to develop in the bed bug 

 except within a narrow range of temperature. Kinghorn and Yorke have 

 shown that the development of T. rhodesiense in G. morsitans is directly 

 dependent on the temperature to which the flies are subjected. The 

 age of the invertebrate also comes into play ; in a proportion of the 

 individuals infected under natural conditions the length of life 

 remaining to the host may not be enough to allow of the completion 

 of the cycle and the formation of the infective phases ; it is also 

 known that with increase in age many changes take place in the 

 tissues of insects, and these may render them unsuitable for the parasite. 

 Other causes may be looked for in connection with the processes of diges- 

 tion and the nature of the food subsequent to the infecting meal. It has 

 been shown in an earlier chapter that very extensive changes go on in the 

 mid-gut during the secretion and excretion of the digestive fluid, and 

 during the subsequent regeneration of the epithelium. This may affect 

 the chances of a parasite becoming established in many ways. If it is 

 essential that it should become intracellular at one stage of its cycle, 

 its fate would appear to depend on whether it happens to penetrate a 

 cell which will remain in position after the excretion of its secretion, 

 or one which is destined to be cast off and replaced. If the parasite 

 is to pierce the wall of the gut in order to pass into the haematocoele, its 

 chances may depend on whether it happens to reach a part of the wall 

 which is weakened temporarily by the desquamation of its cells, or a 

 part which is still of considerable thickness. As regards the nature 

 of the food, there is some evidence to show that this affects the develop- 

 ment of the parasites very materially. The Sleeping Sickness Commission 

 of the Royal Society found that Trypanosoma gambiense was present 

 in the blood of the water buck, but in such very small numbers that 

 it could not be detected by microscopical examination, and gave rise 

 to no symptoms ; in infected monkeys, on the other hand, the trypano- 

 somes were numerous, and the disease ran a fatal course. One would 

 expect that a much larger proportion of the tsetse flies which were fed on the 

 monkeys would become infected than of those which were fed on the 

 water buck, but the reverse was found to be actually the case. One 

 inference which may be drawn from this remarkable phenomenon is that 

 the blood of the water buck, within the alimentary tract of the fly, 

 constitutes a more favourable medium for the developmental cycle of the 

 parasite than does the blood of the monkey, and that this factor was 

 sufficiently powerful to overweigh the advantage of the greater initial 

 infection. According to Robertson and other observers, the successful 



