PARASITISM 331 



Thus, in each of these cases the parasite lives alter- 

 nately in two hosts, one of which, the insect, acts as the 

 distributor. 



These discoveries gave a great impetus to the investi- 

 gation of the means by which the blood parasites were 

 transmitted, and it is now known that the trypanosomes 

 of rats are transmitted by their insect parasites, the 

 trypanosomes of Nagana by the "tsetse fly," Glossina 

 morsitans, and the trypanosome of African lethargy by 



FIQ. 126. Glossina palpalis (X 3 3/4), the carrier of the trypanosome of 

 sleeping sickness. (After Adami.) 



another tsetse fly, Glossina palpalis. It has also been 

 discovered that the trypanosomes of Surra and of mal 

 de caderas are transmitted by a biting fly fairly well 

 identified as Stomoxys calcitrans; and that the spiro- 

 chsete of African relapsing fever is transmitted by a tick, 

 the Ornithodorus moubata. 



The mention of the tick introduces another matter 

 of interest, for it is not the tick that sucks the blood that 

 transmits the disease, but its progeny which have been . 

 infected as eggs in the body of the mother. 



