THE FLY AND ITS ALLIES 63 



stables to avoid the wet, and otherwise show so great a 

 sensitiveness to moisture that we predict rain by their 

 stickiness and general increased ability to annoy. Both 

 the house-fly and the stable-fly are abundant about stables, 

 where their eggs are laid, and also about household pro- 

 visions. 



The different species of flies require different kinds of 

 food, and the food of the larvae is usually different from 

 that of the adult. Thus the larva of the house-fly develops 

 in horse manure and various other kinds of filth, while the 

 adult feeds chiefly upon fluids or substances which can 

 be dissolved by their saliva and then sucked up. The 

 larva of the stable-fly lives in and feeds upon horse manure. 

 The adult sucks blood. The larvae of the blow-fly develop 

 in meat, cheese, or nitrogenous vegetable material. 



The development of flies is rapid. One or two hundred 

 eggs may be laid by a single individual. These, in warm 

 weather, hatch in a few hours into larvae, commonly called 

 " maggots." The larvae are wholly footless, and even the 

 head is only a slightly developed structure. The larvae 

 acquire full size in about a week ; pupate and hatch about 

 a week later. The process of pupation is a complicated 

 one, for all the larval organs, excepting certain patches of 

 tissue, are destroyed. By the growth of these patches the 

 individual is formed anew. These changes are all deep 

 lying, and nothing seems more passive than the brown 

 pupal case. Finally the case breaks at one end, and the 

 fully formed fly emerges. The metamorphosis which the 

 fly has just undergone is a complete one. 



The larvae as well as the adult flies breathe, like other 

 insects, by means of a system of air-tubes, which begin 

 with slits in the body wall, stigmata, and pass inward to 



