Diseases of Domestic Animals in Nyasaland. 65 



hypopharynx with the saliva and are injected with it under the skin of the 

 fly's victim. 



Further examination of flies by inducing them to salivate on cover-glasses 

 revealed the fact that sometimes the long, narrow intestinal forms of 

 trypanosomes are ejected in large numbers on to the cover-glass (Plate 8, 

 figs. 1 to 11). There is no doubt, therefore, that an infected fly has the power 

 of regurgitating the contents of its proventriculus and intestines forward into 

 the labial cavity and probably into the blood stream of the bitten animal. It 

 is conceivable that in this way the proboscis first becomes infected by the 

 intestinal forms of trypanosomes, which attach themselves to the inner 

 surface of the labrum and enter the lumen of the hypopharynx, which they 

 invade, however, only as far as the entrance of the two salivary ducts. Here 

 in the chitinous hypopharynx they establish themselves and, bathed in the 

 salivary secretion, finally complete their development into the infective blood 

 form of the parasite. 



It is a curious fact that neither the salivary glands nor even the salivary 

 ducts beyond the hypopharynx have ever been found infected with T. simice. 



It was proved by the Commission in Uganda that the blood forms of 

 T. gambiense developing in the salivary tract were the virulent forms of the 

 parasite, and it now seems also proved that the developmental forms of 

 T. simim found in the hypopharynx represent the last and infective stage of 

 development of this species of trypanosome in the " fly." 



It may be noted here that in the negative Experiment 1477, in which an 

 infected fly was found (Table V), the labial cavity was infected with the long 

 forms of the parasite attached to the labrum, but most careful search failed 

 to reveal infection of the hypopharynx with blood forms. In this fly the 

 parasite had not attained the final and essential stage of its development — 

 the reversion to the blood type — and so the fly was harmless. 



The Trypanosomes found in the Alimentary Canal. 



The intestines of infected flies were generally packed full of trypanosomes 

 from the proventriculus to the mid-gut. Sometimes the infection extended to 

 the hind-gut, but never beyond. 



Little need be said in regard to the developmental forms found in the 

 intestines. One curious fact, however, emerges and that is, that it is 

 impossible to differentiate one species of trypanosome from another by the 

 study of these intestinal forms. Whether it is T. brucei or T, gambiense, 

 T, pecoritm or T. simice, they present the same appearance. Perhaps on 

 further work some differences may become apparent, but at present no 

 difference has been found to exist. The most numerous forms are long, 



VOL. LXXXVII. — B. F 



