42 



LIFE-HISTORY 



Fig. 17. Eggs of house 

 fly, greatly enlarged 

 (After Hewitt, 1908.) 



surface due to the clear, viscous substance with which they are 

 coated." Each egg is about i mm. {^ inch) 

 in length, cylindrically oval, and slightly 

 curved in its long axis, and somewhat 

 broader at one end than at the other 

 (anterior end). The extremities are 

 rounded, and along the concave dorsal side 

 run two distinct, nearly parallel, rib-like 

 thickenings. Under a high power of the 

 microscope the polished surface appears to 

 be covered with minute hexagonal markings, 

 " The number of eggs laid by a single 

 fly averages from 120 to 140. More than 

 one batch may be laid during the life of 

 the fly." Howard (1911, p. 18) says that as many as four batches 

 may be laid. 



" The larvae or maggots hatch out from the eggs in periods 

 varying from eight hours to three or four days ; the average 

 time may be given as twelve hours, but when laid in fermenting 

 materials the incubation period is reduced to a minimum of 

 eight to twelve hours." 



"The time of hatching varies according to the temperature. 

 With a temperature of 25° C. to 35° C. the larvae hatch out in 

 8 — 12 hours after the deposition of the eggs ; at a temperature of 

 15 — 20° C. it takes 24 hours, and if kept as low as lo" C. two or 

 three days elapse before the larvae emerge." 



Hewitt (1908, p. 506) has carefully observed the hatching of 

 the eggs and described the process as follows : " A minute split 

 appeared at the anterior end of the dorsal side to the outside of 

 one of the ribs ; this split was continued posteriorly and the 

 larva crawled out, the walls of the chorion (egg-shell) collapsing 

 after its emergence." 



" The young larva as it issues from the egg is a slender 

 creature tapering from the blunt, round hinder end to the pointed 

 head end. It is glistening white in colour and only about 2 mm. 

 (j^:j inch) in length. It is extremely active and burrows at once 

 into the substance upon which the Qgg from which it has been 

 hatched had been laid, rapidly disappearing from sight. In the 



