THE LIFE-HISTORY OF THE BLOW-FLY. 5 



tending its respiratory sacs with air, assumes a dark colour, 

 and its integument becomes hard and elastic. Its wings are 

 fully developed, and it rises in the air and takes its first 

 flight. 



By this time the frontal sac is permanently withdrawn into 

 the head, and the external form, characteristic of the mature 

 insect, is attained. There are still, however, traces of im- 

 maturity in the coloration of the integument, and the imperfect 

 hardening of the head and thorax, which is not complete for 

 several days. 



A few hours appear to suffice for the full sexual development 

 of the male, though three or four weeks are needed for that of 

 the female a condition which probably prevents the fertiliza- 

 tion of females by males of the same brood. 



The female may lay several hundred eggs, but these cannot 

 all be deposited at the same time, as the ovaries contain four 

 or five sets in different stages of development. About 180 eggs 

 are matured at one time, three or four times as many remain- 

 ing in a rudimentary condition. 



The female is fecundated but once ; the eggs are usually 

 fertilized as they are deposited ; but two, or possibly three, 

 fertilized eggs may be retained in the oviduct, so that under 

 exceptional circumstances one or more living larvae may be 

 deposited by the female fly. 



Many species of allied genera normally retain the fertilized 

 eggs in a special ovisac until they hatch, and living larvae are 

 deposited instead of eggs. Sarcophaga, Scatophaga, and Tachina 

 are examples of such viviparous flies. 



The female blow-fly usually exercises discrimination in the 

 deposition of her eggs, and the number laid apparently bears 

 a proportion to the mass of carrion. I have repeatedly 

 seen these insects examining a portion of flesh on all sides, 

 and if eggs in sufficient number are already there, rejecting 

 it as unsuitable for their purpose. The males seldom enter 

 houses except in cold weather, and are usually found on 

 flowers ; impregnated females frequent carrion and are found 

 in houses ; but those with ripe eggs are seldom seen in 



