2 THE LIFE-HISTORY OF THE BLOW-FLY. 



During the remainder of the embryonic stage all the changes 

 which occur are apparently retrogressive. The head is in great 

 part withdrawn into the interior of the embryo, so that when 

 the maggot emerges from the ^^^, the parts destined to form 

 the head in the perfect insect are found deeply invaginated, 

 and lie far back in the thoracic region, in front of the highly 

 concentrated nervous system. 



The newly-hatched larva buries itself in the carrion on which 

 the eggs are deposited, and feeds continuously for fourteen 

 days, by which time it has attained its full growth. I believe 

 in warm weather this period is considerably shortened, and 

 that the full growth of the larva is attained in eight to ten days, 

 but the time necessary evidently varies with the temperature 

 and with the condition and nature of the food, and it has been 

 differently estimated by several writers. It is shorter in Musca 

 caesar and Sarcophaga carnaria. The newly-hatched larva 

 measures nearly 2 mm., oryV in., in length when fully extended. 

 It sheds its cuticular integument within two hours after its escape 

 from the ^^^, and I have actually observed this first moult in 

 larvse hatched in a watch-glass. A change occurs in the form 

 of the mouth armature and the structure of the posterior 

 spiracles at the first moult. As a pair of anterior spiracles are 

 subsequently formed, and further changes occur in the mouth 

 organs and posterior spiracles, it must be inferred that other 

 moults occur, but their number is unknown, as the newly- 

 hatched maggot immediately buries itself in its food. Weis- 

 mann concluded that the larva undergoes at least three. 

 Burmeister [8] erroneously supposed that no moults occur in 

 the larvae of the Muscidse. 



The full-grown larva when it ceases to feed measures f of an 

 inch in length when fully extended, and weighs 10 to 12 centi- 

 grammes, or about 1*5 grains. It leaves the carrion in which 

 it has been nourished and buries itself in the earth. It is not im- 

 mediately transformed into a pupa, but becomes a resting larva. 



The Resting Larva. — The duration of the resting period varies 

 greatly with the temperature : it may not exceed two days, and 

 may extend to several weeks or even months. 



