AND FI.IES
its being the carrier of the parasite of Affican 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 Trypauosoma that lires 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 round as'parasites in many vertebrate animais, 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 tropic.al parts of Africa, the other ?s 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 dra? along for years. The sl'eepin? sick- ness and nagana trypanosomes are entirely dependent in nature on the tsetse files for their means of transport from one person or from one animal to another. The tsetse tir (Fig. ?Sç) is a
FIG. 18 S. A tsetse fly, Glossina ?al?al?s, male (about rive times natural size)
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 1
INSECTS
