DEVELOPMENT IN TSETSE FLY 99 



tsetse flies. There is evidence that sleeping sickness, hke surra, 

 a trypanosome disease of horses, may also be transmitted sexu- 

 ally or through abrasions of the skin, but this is certainly not 

 the usual method of transmission. 



The tsetses are blood-sucking flies resembling stable-flies, 

 which inhabit the brushy borders of lakes, streams or swamps, 

 — the so-called " fly-belts." The distinguishing characteristics 

 of the tsetse flies and of the various disease-carrying species are 

 discussed in the chapter on biting flies, p. 490. 



For a long time it was thought that the tsetse flies could trans- 

 mit trypanosomes only in a simple mechanical way, the para- 

 sites adhering to the proboscis, and being subsequently injected 

 into the blood of another person. It is now known that the 

 trypanosomes of sleeping sickness can be transferred in this 

 manner only for a few minutes after an infective feed, but that the 

 fly again becomes infective after a period of three or four weeks. 

 Meanwhile the parasites have undergone a series of changes in 

 the gut of the insect and finally become stored in the salivary 

 glands from which they are poured with the saUvary juices into 

 the blood of a new victim. 



Life Cycle in Fly. — According to observations on Trypanosoma 

 gambiense in Qlossina palpalis by Miss Robertson the critical 

 time for the trypanosomes after they are sucked up by the fly 

 is when the fly feeds the next time, since in many cases they are 

 swept out of the body with the new influx of blood, or digested. 

 Having stood their ground until they have become established in 

 the new influx of blood they multiply so rapidly that permanent 

 infection of the fly is almost certain. The difficulty experienced 

 by the parasites in establishing themselves in the gut of their 

 insect hosts largely accounts for the relatively low percentage 

 (usually less than five per cent) of infections which result from'f ced- 

 ing of tsetse flies on infected blood. When conditions are favor- 

 able for development in the fly the parasites multiply first in 

 the middle intestine, producing long-snouted forms such as shown 

 in Fig. 21B. After the tenth to fifteenth day long slender forms 

 (Fig. 21C) are developed, and these move forward in the digestive 

 tract. These slender trypanosomes have long snouts and differ 

 most strikingly from the earlier forms in the appearance of the 

 nucleus (Fig. 21C). After several days more the trypanosomes 

 make their way to the fly's salivary glands, to the walls of which 



