236 INSECTS AND MAN 



cow or fowl dung, human excrement, straw, sacking, and 

 other manufactured stuffs soiled by excrement, decaying 

 vegetable matter, and even the contents of slop-pails and 

 spittoons may be selected as suitable nurseries for the larvae. 

 From June to October are the months during which egg 

 laying normally takes place, though in warm kitchens the 

 process may continue during the winter. Each female de- 

 posits five or six batches of eggs, each one containing from 

 one hundred to one hundred and fifty oval, pearly white 

 eggs about -^g- in. long. Under favourable conditions the 

 larvae emerge in from eight to twenty-four hours. After 

 two moults, taking place respectively after about twenty- 

 four and forty-eight hours, the larvae are full grown (fig. 60) 

 and are then about half an inch in length, cream coloured, 

 and legless. The head end of the larva is pointed and 

 provided with a hooked process, which is used in locomotion 

 and in tearing up food ; the posterior end, or the thirteenth 

 segment, is furnished with a pair of eye-like spiracles or 

 breathing pores ; on the under surface of the sixth to the 

 twelfth segments are pads furnished with recurved spines, 

 also used in locomotion. Almost the whole of the larval 

 period is passed literally within the food material on which 

 the maggot feeds ; but, when the time for pupation arrives, 

 a short emigration takes place to some drier situation, the 

 maggot contracts, assumes a cylindrical form, and changes 

 into a dark brown, barrel-shaped pupa, from which the 

 perfect fly emerges in from three to four days. The adult 

 insect is so well known that a description of its appearance 

 ought to be unnecessary; so frequently, however, is it 

 confused with other house-frequenting flies that a brief 

 mention of its more salient characters may not be out of 

 place. The front of the dark head is marked with a black 

 stripe; the prominent compound eyes are wider apart in 

 the females than in the males, as is usual among the 

 Diptera. The grey thorax bears four equidistant black 

 stripes, whilst the yellow abdomen has a medium dark 



