516 FAMILY: TRYPANOSOMID^ 



developmental forms of various trypanosomes in the fly, have reached 

 the conclusion that it is possible to identify the trypanosomes from their 

 morphology alone. 



It is known that T. brucei and T, gambiense develop in the stomach 

 into long thin trypanosomes, which then make their way to the proboscis, 

 enter the hypopharynx, and travel to the salivary glands, where crithidia 

 forms and eventually metacyclic trypanosomes are produced (Figs. 217 and 

 224). They pass down the hypopharynx with the salivary secretion, and 

 are injected into the vertebrate when the fly feeds. It is evident that in 

 the case of these trypanosomes they may occur in the stomach, proboscis, 

 and salivary glands. Those which may be found in the proboscis are 

 merely travelling forms, either on their way to the salivary glands from 

 the stomach, or from the salivary gland to the vertebrate host. At all 

 stages the trypanosomes have fiagella except the metacyclic forms, which 

 resemble the short stumpy trypanosomes occurring in the blood. 



In the case of T. congolense, development takes place in the stomach, 

 with the production of long slender trypanosomes, which migrate to the 

 proboscis (Figs. 218 and 228). In the labial cavity crithidia forms are pro- 

 duced, and these make their way into the hypopharynx, where the crithidia 

 forms give rise to metacyclic trypanosomes which resemble the blood 

 forms. As in the vertebrate host, all these stages are devoid of fiagella, 

 so that they can be distinguished by this character from most of the 

 stages of T. brucei and T. gatnbiense. The metacyclic trypanosomes of 

 T. brucei and T. gambiense, though devoid of fiagella, differ from the 

 metacyclic trypanosomes of T. congolense in size and other respects. 



The development of trypanosomes of the T. vivax group in tsetse flies 

 is limited to the proboscis (Figs. 219 and 233). Trypanosomes are taken 

 into the stomach, but these quickly degenerate. Before they do so they 

 can be distinguished from other trypanosomes by their characteristic 

 swollen posterior ends and fiagella. The trypanosomes in the proboscis 

 become quickly transformed in the labial cavity into crithidia forms with 

 flagella. These pass into the hypopharynx, where metacyclic trypano- 

 somes of the blood type are evolved. All these stages have fiagella. 



As regards the trypanosomes which may occur in the stomach, a 

 difficulty is introduced in that another trypanosome, T. grayi, commonly 

 occurs in this region (Figs. 220 and 173). It probably represents develop- 

 mental stages of the trypanosome of the crocodile or the monitor. The 

 characteristic type is a broad crithidia form. Trypanosome forms also 

 occur, but these have very short flagella, and differ in other respects from 

 T. brucei and T. gambiense. It will thus be seen that the following forms 

 can be recognized in the different regions of the body of tsetse flies in 

 which development occurs: 



