284 THE RELATIONS OF ANIMALS TO DISEASE 



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 



/ADULTS^ 



Four stages in the complete metamorphosis of the house fly. 



great number. Fortunately, relatively few flies survive the winter. 

 The membranous wings of the adult fly appear to be two in number, 

 ^ 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 House Fly a Disease 

 Carrier. — The common fly 

 is recognized everjr^here as 

 a pest. Flies have long been 

 known to spoil food through 

 their filthy habits, but it is 

 only recently that they 

 have been blamed for 

 spreading several diseases 

 caused by bacteria. It 

 has been found that a single fly might carry on its feet anywhere 

 from 500 to 6,600,000 bacteria, the average number being over 

 1,200,000. Not all of these germs are harmful, but they might 



Foot of a house fly, highly magnified. 



