314 PERirATUS, MYRIOPODS, AND INSECTS. 



Respiratory System. 



The body of an insect is traversed by a system of air 

 tubes (tracheae), which open laterally by special apertures 

 (stigmata), and by means of numerous branches conduct 

 the air to all the recesses of the tissues. In animals which 

 breathe by gills or lungs the blood is carried to the air ; in 

 insects the air permeates the whole body. But how does 

 the air pass in and out ? In part, no doubt, there is a slow 

 diffusion ; in part, the movements of the wings and legs will 

 help ; but there are also special expiratory muscles. We 

 see their action when we watch a drone fiy panting on a 

 flower. Inspiration is passive, as in birds, and depends on 

 the elasticity of the skin and of the tracheal walls; expiration 

 is active, and depends on special muscles. These are 

 chiefly situated in the abdomen, but in some beetles (at 

 least) they are also present in the metathorax. 



The tracheas seem to arise as tubular ingrowths of skin, 

 and, primitively, each segment probably contained a distinct 

 pair. But their number has been reduced, and many are 

 often connected into a system. With the doubtful excep- 

 tion of one of the primitive Collembola, and the certain 

 exception of caterpillars, no insects have any tracheal 

 openings in the head region. There are rarely more than 

 two pairs in the thorax, there are often six to eight pairs in 

 the abdomen, the maximum total is ten pairs. Each trachea 

 is kept tense throughout the greater part of its course by 

 internal chitinous thickenings, which apparently have a 

 spiral course. The branches of the tracheae penetrate 

 into all the organs of the body, carrying oxygen to every 

 part. The very efficient respiration of insects must be 

 kept in mind in an appreciation of the general activity of 

 their life. 



As the conditions of larval life are often different from those of the 

 adult insects, the mode of respiration may also differ in details. 



In insects witliout marked metamorphosis, and even in some beetles 

 in which the metamorphosis is complete, the young insect and the adult 

 both breathe by trachen; with open stigmata. Both are said to be 

 " holopneustic." 



When the larva; live in water, the tracheal system is closed, other- 

 wise the creatures would drown. This closed condition is termed 

 "apneustic." These larva; (of dragon flies, May flies, and some others) 



