314 T. GAMBIENSE [CH. 



the typical long slender forms and are only invaded about the 

 middle of the developmental cycle. There seems to be some 

 difficulty in the trypanosomes reaching the proventriculus and 

 once arrived there they cannot maintain themselves if the fly 

 is exposed to too long a fast. If there is any considerable 

 interval before another meal the trypanosomes gradually ebb 

 backwards to the posterior part of the anterior intestine and 

 only gradually recover their position again after the next feed. 

 The intestinal infection is thus the focus from which the sub- 

 sequent stages are derived. 



Fig. 76. Transverse section (semi-diagrammatic) of the proboscis of an infected 

 Glossina. The section shews the arrangement and aspect of the trypano- 

 somes attached to the walls of the labrum (L) and the hypopharynx 

 (Hyp.). ( X about 700.) (After Roubaud.) 

 L, labrum ; Hyp. hypopharynx ; M, M' , muscles ; L. inf., under lip. 



After becoming established in the proventriculus the slender 

 forms pass up into the hypopharynx and then along the salivary 

 ducts into the salivary glands. The period at which this happens 

 depends upon the virulence of the trypanosomes, and early 

 infectivity is generally a character found in a strain which 

 produces many positive flies. The trypanosomes settle down 

 in the salivary glands in the cellular part of the lumen immedi- 

 ately above the narrow tubular part of the duct. At first they 

 are slender forms attached to the wall of the gland by their 

 flagella, but later they gradually change into broad crithidial 

 forms. 



