MUSCIDiE. 
643 
temperature. The number of eggs varies very much in the different 
species. Musca corvina lays only twenty or thirty, M. domestica a hundred 
Fig. 422— Eggs of House-fly x about 20. 
(After Newstecui.) 
to a hundred and fifty, while some of the blue-bottles ( Lucilia and 
Calliphora , etc.), may lay five or six hundred. The eggs rarely take longer 
than a day to hatch, and the young are sometimes deposited as larvae, 
as in many Sarcophagidce and some Tachinidce. The eggs are as a rule 
white (those of Hcematobia are black) and mostly resemble those of the 
house-fly (fig. 418) though some have a process from one end running 
longitudinally down one side of the egg. The great majority of the 
larvae are scavengers and live in dung or in decaying animal or vegetable 
matter. They are typical “ maggots,” the body thick behind and taper¬ 
ing in front to a rudimentary head provided with a pair of dark-coloured 
chitinous mouth-hooks. The maggots of different species are often 
extremely alike, and practically the only way of distinguishing them is 
by slight variations in the structure of the spiracles. The pupae are 
cylindrical, rounded at both ends, and chestnut brown in colour, and 
the larvae nearly always pupate in the earth, generally one or two inches 
below the surface, sometimes travelling some little distance before they 
bury themselves. A few are found in wounds, and the larvae of 
Pycnosoma (PI. LXIX, fig. 2) appear not infrequently to cause Myiasis 
by their presence in the nostrils of human beings and camels. With 
these exceptions, however, the larvae are distinctly beneficial as 
scavengers. The fly figured on PI. LXIX, fig. 4 ( Ochromyia ), is of some 
