INCUBATION OF FLIES. 125 



several as it rose and fell upon the top of the water. 

 After this last and most important function of their 

 life had been performed, each having deposited its 

 hundreds of eggs, every sign of vitality vanished, and 

 they appeared lifeless, and merely a flimsy form 

 inflated with air. 



By the aid of a powerful microscope we were 

 enabled to minutely investigate, from time to time 

 several points of special interest to us. By careful 

 and oft-repeated researches we ascertained that from 

 36 to 40 days serve to hatch the eggs, when deposited 

 in the summer months, but much longer if in spring ; 

 and almost immediately after this has happened, the 

 larva or grub secretes itself amongst the sediment at 

 the bottom of the water, instinct, doubtless, prompting 

 this as security against the numerous enemies which 

 prey upon it. The form of the larva is shown to be 

 elongated, with six perfect legs, and whisked tail, also 

 armed with a pair of formidable forceps, with which 

 its food is seized. Along the sides there is a range of 

 web-like appendages, which serve as fins ; and by the 

 aid of this propelling power the larva becomes 

 exceedingly active. In the course of five or six 

 months the larva changes into a pupa, or nympha, by 

 breaking through a filament or outward skin. Upon 

 the shoulders there are now two small protuberances, 

 which ultimately develop into wings. When in the 

 larva and pupa states the insect is excessively 

 voracious ; like the small clothes-moth it feeds, not 

 when fully developed, but when " swathed " up in its 

 tough pliable case. The larvae of both moth and fly 



