42 ANIMAL BIOLOGY 



cle of food, house flies are a particularly dangerous means 

 of spreading disease. The old contention that they are 

 useful insects because they act as scavengers is a mis- 

 chievous doctrine; as a matter of fact they possess no 

 redeeming virtues. Since they wade about in all sorts 

 of filth, they get germs of various diseases on their feet 

 and may carry them to food. In many cases typhoid 

 fever is definitely known to have been carried by flies, 

 and it is quite probable that other diseases are carried in 



FIG. 32. Common house fly, Musca domestica. Puparium at left; 

 adult next; larva and enlarged parts at right enlarged, a, antenna. 

 (After Howard.) 



the same way. To a certain extent flies may be gotten 

 rid of by traps, fly poison and sticky paper, but as they 

 are continually being bred in such enormous numbers it is 

 best to check them by keeping covered the manure and 

 other refuse in which they breed. 



Closely related to the house fly are the blow flies and flesh 

 flies whose larvae live upon decaying flesh. The fly whose 

 larva is known as the screw worm sometimes lays its eggs 

 in wounds and in the nostrils of men and animals; the larvae 

 may devour the nasal membrane , and get into deeper 



