330 PHYLUM ARTHROPOD A. 



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-fly 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 upon these 

 muscles. They are chiefly situated in the abdomen, but 

 in some beetles (at least) they are also present in the 

 metathorax. 



The tracheae seem to arise as tubular ingrowths of skin, 

 and, primitively, each segment probably contained a distinct 

 pair ; but their number has been reduced, and they are often 

 in part 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 without marked metamorphosis, and even in some beetles 

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

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

 "holopneustic." 



When the larvae 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) 

 breathe by "tracheal gills" (see Fig. 162) little wing-like outgrowths 

 from the sides of the abdomen, rich in trachea: or by tracheal folds 



