THE ECONOMIC IMPORTANCE OF ANIMALS 223 



about stables, privy vaults, ash heaps, uncared-for garbage cans, 

 and fermenting vegetable refuse form the best breeding places for 

 flies. In warm weather, the eggs hatch a 

 day or so after they are laid and become 

 larvae, called maggots. After about one 

 week of active feeding, these wormlike 

 maggots become quiet and go into the 

 pupal stage, whence under favorable con- 

 ditions they emerge within less than an- 

 other week as adult flies. The adults 

 breed at once, and in a short summer there 

 may be over ten generations of flies. This 

 accounts for the great number. Fortu- 

 nately relatively few flies survive the 

 winter. The membranous wings of the The foot of a fly, showing the 



- , , - , . . hoots, hairs, and pads 



adult fly appear to be two in number, a 



which collect and carry 

 bacteria. The fly doesn't 

 wipe his feet. 



second pair being reduced to tiny knobbed 

 hairs called balancers. The head is freely 

 movable, with large compound eyes. The mouth parts form a 

 proboscis, which is tonguelike, the animal obtaining its food by 

 lapping and sucking. The foot shows a wonderful adaptation for 



clinging to smooth surfaces. 

 Two or three pads, each of 

 which bears tubelike hairs that 

 secrete a sticky fluid, are found 

 on its under surface. It is by 

 this means that the fly is able 

 to walk upside down, and carry 

 bacteria on its feet. 



The Typhoid Fly a Pest. 

 The common fly is recognized 

 as a pest the world over. Flies 

 have long been known to spoil 

 food through their filthy habits, 

 but it is more recently that the 



very serious charge of spread of 



fly was allowed to walk. diseases, caused by bacteria, has 



