298 MOTIONS OF INSECTS. 



made when a caterpillar ; when its anterior part having 

 emerged, it stops short, and so escapes a fall that might 

 destroy it. After some repose, in consequence of very 

 violent efforts, the puparium opens, and it escapes from 

 its prison a . 



The insects of the Trichoptera order, or case-worm 

 flies are quiescent when they first assume the pupa, but 

 become locomotive towards the close of their existence 

 in that state. Since they inhabit the water when they 

 become pupae. Providence has furnished them with the 

 means of quitting that fluid without injury, when they 

 are to exchange it for the air; which in their winged 

 state is their proper sphere of action. I have before 

 described to you the grates which shut up their cases 

 when they became quiescent b ; if they had no means 

 of piercing these grates, they would perish in the wa- 

 ters. The head of these pupa? is provided at first with 

 a particular instrument, which enables them to effect 

 this purpose ; its anterior part is armed with a pair of 

 hooks in form resembling the beak of a bird ; and 

 with this, previously to their last change, they make an 

 opening in the grate which, though it once defended, 

 now confines them. But at this moment, perhaps, the 

 insect has a considerable space of water to rise through 

 before she can reach the surface. This is all wisely 

 provided for ; before she leaves the envelope which co- 

 vers her body, she emerges from the water, and fixes 

 herself upon some plant or other object, the summit of 

 which is not overflowed. But you will here, perhaps, 

 ask How can a pupa in her envelope, with all her 

 limbs set fast, do this ? This affords another instance 

 3 Lyomt, Trail. Anal. 15 b See above, p. 264. 



