THE FLY'S MOUTH AND LEGS 33 



the fly carries disease. Once the fly has " arrived " no 

 time is wasted. 



The domestic fly is a strong, stout, bristly, hairy, 

 two-winged, six-legged, flying insect. Some flies are 

 larger than others. This is because, probably, the food 

 which the larva received was better and in more 

 quantity in some cases than in others. Or perhaps 

 individual size is a family matter, as it is frequently in 

 human beings. But probably the size of each fly 

 depends upon the size it attained during its larval stage ; 

 the chrysalis and the flying insect do not grow. Small 

 flies never become large flies ; as they are born so they 

 remain. It is thus with many insects, but the females 

 are commonly larger than the males, though occasionally 

 the reverse is the case, and sometimes one sees large 

 males in coitu with small females, although this is 

 unusual. 



From their disease-carrying potentials the mouth 

 and legs and intestines of flies are important parts. 

 The six legs are bristly and strong. Each leg has two 

 claws, and between the claws there are soft sticky pads 

 called pulvilli, with which the fly clings to seemingly 

 impossible slippery surfaces ; for there are hairs around 

 the pads which secrete a sticky fluid. The mouth con- 

 sists of a proboscis which ends in two flabby pads, which 

 can be protruded and applied to the food. There are 

 no teeth, but each mouth-pad has some hard ridges 

 which can be used as rasps or saws for breaking up 

 small hard objects in the food ; the flies' saliva does the 

 rest of the mastication. The saliva is poured out on to 

 the sugar, and a thick paste is made. The mouth-pads 

 are then applied, and the paste is sucked up and 

 swallowed. Then the fly moves on and repeats the 

 process. Some of the paste adheres to the pads and 



