294 FLIES AND DISEASE 



into the fly, and finally the adult fly emerges 

 by rupturing the covering. At first the fly is soft 

 and the wings are folded up close to the body, but 

 soon the wings expand and the fly becomes hard 

 and capable of flight. An adult fly never grows, 

 but remains the same size during the whole of its life. 

 The winter is generally passed in the pupal stage. 



The adults feed on excrement and decaying 

 bodies, and visit, when opportunity offers, fruit and 

 human food. In walking over many of the filthy 

 substances to which they are attracted their feet 

 become contaminated with putrefactive and other 

 bacteria, and these may be carried to the human 

 food on which they next settle. This, however, is 

 not the only or the most important manner in 

 which these insects infect food. When a fly sucks 

 up liquid filth the greater part of it passes at first 

 into its crop, a sac-like organ in which the food is 

 stored until the fly is able at leisure to discharge 

 it into its intestine. After a meal the fly usually 

 regurgitates part of the contents of its crop, and 

 this vomited material is left wherever it may happen 

 to settle. Further, if a fly wishes to dissolve such 

 a substance as sugar or milk dried near the mouth 

 of a jug it does so by vomiting the crop contents 

 over it. Since the crop contents are usually greatly 

 contaminated with bacteria, some of which may be 

 disease-producing forms, the infection of human 

 food by means of vomit is often very gross. 



Experimentally it has been shown that flies can 



