TRANSMISSION OF DISEASES BY INSECTS 



247 



m 



Trypanosomes. The organisms under consideration are flagellate 

 protozoans. A typical trypanosome, for example, T. lewisi (Fig. 271) 

 of the rat, is essentially an elongated cell, tapering at each end, serpen- 

 tine in form and with no definite cell-wall. A round or oval nucleus is 

 present, also a peculiar chromatin body situated often near the posterior 

 end of the cell and termed the blepharoplast. Along one side of the cell 

 is a delicate protoplasmic contractile membrane, the undulating mem- 

 brane, along the edge of which is a marginal cord, which arises by growth 

 from the blepharoplast and is continued beyond 

 the anterior end of the cell as a vibratile flagellum. 



Asexual reproduction is by means of a longi- 

 tudinal division of the cell body, preceded by divi- 

 sion of the flagellum, blepharoplast and nucleus, 

 the nucleus dividing amitotically. In regard to the 

 existence of sexual stages, or gametes, the results 

 of investigators seem to be inconclusive as yet. 



In a film of fresh blood under the microscope, 

 any active trypanosomes in the field of view attract 

 attention as centers of commotion among the red 

 blood corpuscles,, which are pushed aside by the 

 lashing, twisting and other movements of the try- 

 panosomes. 



The nutrition is by means of osmosis. Try- 

 panosomes have not been seen to attack erythro- 

 cytes, but according to MacNeal and Novy haemo- 

 globin is useful if not indispensable to them. 



All five classes of vertebrates serve as hosts for 

 trypanosomes, of which more than seventy species 

 have received names. Most of these species are 

 carried from one vertebrate host to another by 

 means as yet unknown, but about twenty per cent, are known or suspected 

 to be transmitted by an intermediate invertebrate host. Thus trypano- 

 somes of frogs are conveyed by leeches; pigeons are infected by mosqui- 

 toes, rats by sucking lice and fleas, and many mammals through the 

 agency of blood-sucking flies of the genus Glossina, and probably also 

 by Stomoxys and certain Tabanidae. 



Tsetse Flies. The name tsetse fly, originally limited to Glossina 

 morsitans (Muscidas) is now used for any of the eight known species of 

 the genus. These flies are a little larger than the common house fly 

 (Musca domestica). Their wings, in the resting position, overlap exactly 



u 



FIG. 271. Trypanoso- 

 ma lewisi. b, blepharo- 

 plast; /, flagellum; m, 

 marginal cord; u, nucleus; 

 w, undulating membrane. 

 Greatly magnified. 



