MODES OF EMERGING FROM PUP.E. 321 



breathing, by admitting a current of fresh water, effect- 

 ed, no doubt, by the spiracles of the pupa; and he 

 actually saw the grate-work in alternate motion from 

 convex to concave, as the water passed out and in. 



Our motive, however, for introducing a notice of 

 these structures is for the purpose of explaining the 

 contrivance by which the pupa makes its exit through 

 the grating. To effect this, it is provided with a pair 

 of curved mandibles, which appear to be applied to no 

 other use, for they are thrown off upon its transform- 

 ation into a fly, as was observed by Vallisnieri. These 

 facts may be verified by searching for caddis worms 

 during the early spring months, as most of them are 

 transformed in the first part of the summer. 



c, Pupa of a case-fly (Phryganea). c <Z, grate-works of the 

 same at the openings of the cases, greatly magnified. 6, the - 

 perfect insect. 



In the common blow-fly (Musca carnaria*), and 

 many of the same family, the exit of the perfect insect 

 from the pupa case is effected by a very different, but 

 no less admirable contrivance. The head of the per- 

 fect fly, it may be remarked, is hard and unyielding; 

 but in the pupa it is soft, and capable of great disten- 



