THE INSECTS 241 



to tiny knobbed hairs called balancers. Their function is seemingly 

 that of equilibrium. 



The head is freely movable, the compound eyes being extremely 

 large. Seemingly the fly has fairly acute vision. Home experi- 

 ments can be easily made which prove its keenness of scent and 

 caste.. It is well equipped to care for itself in its artificial environ- 

 ment in the house. 



The mouth parts of the fly are prolonged to form a proboscis, 

 which is tonguelike, the animal obtaining its food by lapping and 

 sucking. It is the rubbing of this file- 

 like organ over the surface of the skin 

 that causes the painful bite of the horsefly. 



If possible, we should examine the 

 foot of a fly under the compound mi- 

 croscope. The foot shows a wonderful 

 adaptation for clinging to smooth sur- 

 faces. 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. Hooks are also pres- 

 ent which doubtless aid in locomotion Foot of a fl ^ showmg the 



, . . , . hooks, hairs, and pads. 



in this position. 



Development. 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 

 about stables, ash heaps, garbage cans, and fermenting vegetable 

 refuse form the best breeding places for flies. In warm weather, 

 within a day after the eggs are laid, the young maggots, as the larvae 

 are called, hatch. After about one week of active feeding, these 

 wormlike maggots become quiet and go into the pupal stage, 

 whence under favorable conditions they emerge within less than 

 another 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. Fortunately few flies survive the 

 winter. 



Other Diptera. Other examples of this group are the mos- 

 quitoes, of which more will be said hereafter; the Hessian fly, the 

 HUNT. ES. BIO. 16 



