152 ENTOMOLOGY FOR MEDICAL OFFICERS 



The breeding-habits of the house-fly have been observed 

 with great care .both in tropical and temperate latitudes. 

 The female lays from 120 to 150 eggs in a batch, in rubbish 

 of any sort provided it is fairly moist. Stable-sweepings are 

 a favourite nidus, but any kind of excrement rich in vegetable 

 debris is good enough, or any kind of house-refuse, even 

 rotting paper, or even, according to Williston, the contents 

 of smokers' spittoons. 



The eggs are white, sticky, and shiny, resembling those 

 of the blow-fly (Fig. 52) on a smaller scale. They hatch 

 within twenty-four hours in hot weather, but may take 

 three or four days to develop if the weather be cool. 



The larva or maggot is whitish and, except that it is 

 smaller, much resembles that of the blow-fly (Fig. 53). It is 

 cylindrical, tapering anteriorly, blunt posteriorly, and consists 

 of 12 segments besides a minute head hardly visible to the 

 naked eye. The head, in fact, is formed chiefly by a pair of 

 papillae (antennae). Immediately behind these papillae lies 

 the mouth, from which the tips of a pair of large, black, 

 chitinous mandibles project ; these are used for tearing and 

 burrowing into the food-material. At the blunt posterior 

 end there is a pair of large, depressed, chitinous plates — the 

 stigmata — on which the main tracheal trunks open by three 

 slits. A second, much smaller, pair of stigmata project from 

 the sides of the 2nd segment in well-grown larvae. The 

 larva feeds on the decomposing refuse in which it is hatched. 

 In the tropics, in the hot season, it becomes full grown in 

 about five days ; in England it may in hot weather attain its 

 full development in a week, but if the summer temperature 

 be low it may take six to eight weeks. 



When the larva is full grown it leaves its food and 

 contracts to an elongate barrel shape ; its skin becomes hard, 

 and gradually changes in colour from dirty white to dark 

 brown or black ; and inside this firm larval ^€A,or pupariunt, 

 the pupa is formed. The pupal stage lasts about three days 

 in the tropics ; but in temperate climates its duration depends 

 entirely on the weather, from a week in warm weather, to a 

 month or more in inclement weather. The manner in which 

 the adult fly emerges .from the pupal case is illustrated in 

 Fig. 9- 



