MOSQUITOES AND FLIES 



its being the carrier of the parasite of African sleeping 

 sickness of man, and that of the related disease called 

 nagana in horses and cattle. 



African sleeping sickness is caused by a protozoan para- 

 site of the genus Trypanosoma that lives in the blood and 

 other body liquids. Trypanosomes are active, one-celled 

 organisms having one end of the body prolonged into a 

 tail, or flagellum. They are found as parasites in many 

 vertebrate animals, but most of them do not produce dis- 

 ease conditions. There are at least three African species, 

 however, whose presence in the blood of their hosts means 

 almost certain death. Two cause the sleeping sickness in 

 man, and the other produces nagana in horses, mules, and 

 cattle. The two human species have different distribu- 

 tions and produce each a distinct variety of the disease. 

 One is confined to the tropical 

 parts of Africa, the other is 

 more southern. The southern 

 form of the disease is said to be 

 much more severe than the 

 tropical form, claiming its vic- 

 tims in a matter of months, 

 while the other may drag along 

 for years. The sleeping sick- 

 ness and nagana trypanosomes 

 are entirely dependent in nature 

 on the tsetse flies for their 

 means of transport from one 

 person or from one animal to 

 another. 



The tsetse fly (Fig. 185) is a 

 larger relation of the horn fly 

 and the stable fly, having the same type of beak and an 

 insatiable appetite for blood. The tsetse fly genus is 

 Glossina. There are two species particularly concerned 

 with the transportation of sleeping sickness, corresponding 

 with the two species of trypanosomes that cause the two 



[349] 



Fig. 185. A tsetse fly, Glossina 



palpalis, male (about five times 



natural size) 



