HOW INSECTS BREATHE. 130 



serve, as favourable to this hypothesis, that they are par- 

 ticularly developed in certain flies ( Volucella) and other 

 insects which fly much, and on the other hand that they are 

 absent in all caterpillars and maggots. 



Turning now to the mechanism by which the renewal of 

 the air is effected, it will be sufficient to state with regard to 

 the expiration that it appears to be caused by the action of 

 certain muscles which compress the general cavity of the 

 body. This must be materially assisted by the presence of 

 the air- sacks mentioned above, which accordingly are found 

 in those insects which, from their great powers of movement, 

 require a large supply of oxygen. 



On the other hand, the inspiration is no doubt caused by 

 the general elasticity of the hard insect skin, assisted by that 

 of the tracheae. If these organs were formed like other tubes 

 of the animal body, of a simple membranous skin, it would 

 be difficult to imagine in what way when once emptied of air 

 they could ever be refilled. The same plan is however 

 adopted here as in our throats ; namely, the introduction of 

 cartilaginous elastic substance, which in mankind is de- 

 veloped in the form of rings, as we must all be aware, but in 

 insects is a thread wound round and round inside the outer 

 membranous tube of the tracheae. 



If we compare this type of the respiratory organs, which 

 is found so far as we know the same in essential points in all 

 perfect insects, except Pteronarcys, which has branchiae as 

 well as spiracles, and in most larvae, with that which prevails 

 in the Vertebrata, or animals with bones, we are at once 

 struck with the fact that in insects the air never enters the 

 body through the mouth, although in a few cases (the larvae of 

 Libellula) the opposite end of the intestinal canal is used for 

 that purpose ; and secondly, that the air vessels are not col- 



