222 BIOLOGY FOR BEGINNERS 



kind in various ways, but their real harm has only recently 

 been realized. 



They live in and feed upon manure and filth, then come and 

 crawl over our food and faces, or wash themselves in the cream 

 pitcher. When we realize that typhoid, cholera, and dysentery are 

 intestinal diseases, that the germs are carried off by the excrement 

 in which flies thrive, it is no wonder that they infect our food when 



FIG. ?4. Larvae and pupae of housefly, Musca domestica, in manure. Natural 

 size. From Kellogg and Doane. 



they crawl upon and share it with us. The fly is not only a filthy 

 but a very harmful insect and one to be avoided and destroyed. 



A fly eats its own weight of food every day. Its food is largely 

 manure, sputum, and other filth, though it also samples our food 

 at table. Disease germs pass through the fly's intestine unharmed 

 and remain active in the familiar " fly specks " which are deposited 

 at intervals of five minutes. Thus the fly carries filth and disease 



