148 HOW INSECTS BREATHE. 



have wings, or organs homologous with wings, on the ab- 

 domen. The wings of insects, therefore, are branchiae which 

 have lost their primary function, that of respiration, and 

 become organs of motion. This change may appear 

 extraordinary, but we must remember that we find, con- 

 versely, among Crustacea, numerous cases, as, for instance, 

 the Daphnice, in which the legs have ceased to serve as 

 organs of locomotion, and on the contrary act as branchiae. 

 These views have been ably advocated by Prof. Owen, in 

 his Lectures on the Invertebrate Animals, 1st edit. p. 198, 

 but they can hardly be said to be yet generally received by 

 Entomologists. 



The larvae of the Phryganece, the well-known caddis- 

 worms, live in streams and ponds ; they are slow in their 

 movements, but even if it were otherwise, to say nothing of 

 other enemies that usually live in the same places, they 

 would have but little chance of escaping from the fish. 



As we notice throughout the animal kingdom that any 

 loss in one respect is usually compensated by a gain in another, 

 so we find that these slow, delicate creatures are endowed 

 with an instinct which teaches them to make for themselves 

 tubes or cases of sand, bits of stick, or any other material 

 that comes to hand, in which they can live secure. 



This contrivance, however, is evidently an obstacle to 

 agility, and the caddisworm is therefore provided with 

 branchiae, in the form of hairs or processes of the skin, into 

 which the tracheae penetrate. 



We at present know of no Lepidopterous larvae which can 

 be said to move rapidly. That of Parapoynx Stratiotalis, 

 whose aquatic habits have been already alluded to, is no 

 exception to the rule. Living as it does in ponds the same 

 mode of protection is resorted to as in the similar case of the 

 Phryganece. It forms for itself a tube, and is provided 



