222 THE ECONOMIC IMPORTANCE OF ANIMALS 



Another fly carries a species of trypanosome to the natives of 

 Central Africa, which causes '' the dreaded and incurable sleep- 

 ing sickness." This disease carries off more than fifty thousand 

 natives yearly, and many Europeans have succumbed to it. Its 

 ravages are now largely confined to an area near the large Central 

 African lakes and the Upper Nile, for the fly which carries the 

 disease lives near water, seldom going more than 150 feet from 

 the banks of streams or lakes. The British government is now 

 trying to control the disease in Uganda by moving all the villages 

 at least two miles from the lakes and rivers. Among other 

 diseases that may be due to protozoans is kala-agar, a fever in hot 

 Asiatic countries which is probably carried by the bedbug, and 

 African tick fever, probably carried by a small insect called the 

 tick. Bubonic plague, one of the most dreaded of all bacterial 

 diseases, is carried to man by fleas from rats. In this country 

 many fatal diseases of cattle, as '^ tick," or Texas cattle fever, are 

 probably caused by protozoans. 



The Fly a Disease Carrier. — We have already seen that mos- 

 quitoes of different species carry malaria and yellow fever. An- 

 other rather recent addition to the black list is the house fly or 

 typhoid fly. We shall see later with what reason this name 

 is given. The development of the typhoid fly is extremely 

 rapid. A female may lay from one hundred to two hundred 

 eggs. These are usually deposited in filth or manure. Dung heaps 



Life history of house flies, showing from left to right the eggs, larvse, 

 pupse, and adult flies. (Photograph, about natural size, by Overton.) 



