18 THE INSECT WORLD. 



of being contracted and dilated alternately by the play of the 

 different segments of which the skeleton is composed, and which 

 are placed in such a manner that they can be drawn into each 

 other to a greater or less extent. When the insect contracts its 

 body the tracheae are compressed and the air driven out. But 

 when, on the other hand, the visceral cavity which contains the 

 tracheae assumes its normal size or dilates, these channels become 

 larger, and the air with which they are filled being rarefied by 

 this expansion, is no longer in equilibrium w*ith the outer air with 

 which it is in communication through the medium of the spiracles. 

 The exterior air is then impelled into the interior of the respira- 

 tory tubes, and the inspiration is effected." 



The respiratory movements can be accelerated or diminished, 

 according to the wants of the animal ; in general, there are 

 from thirty to fifty to the minute. In a state of repose the 

 spiracles are open, and all the tracheae are free to receive air 

 whenever the visceral cavity is dilated, but those orifices may 

 be closed, and the insect thus possesses the faculty of stopping 

 all communication between the respiratory apparatus and the sur- 

 rounding atmosphere. 



Some insects live in the water; they are therefore obliged 

 to come to the surface to take the air they are in need of, or else to 

 possess themselves of the small amount contained in the water. 

 Both these methods of respiration exist under different forms in 

 aquatic insects. 



To inhale atmospheric air, which is necessary for respiration, 

 above the water, certain insects employ their elytra* as a sort of re- 

 servoir ; others make use of their antennae, the hairs of which retain 

 the globules of air. In this case it is brought under the thorax, 

 whence a groove carries it to the spiracles. Sometimes the same 

 result is obtained by a more complicated arrangement, consisting of 

 respiratory tubes which can be thrust into the air, which it is their 

 function to introduce into the organisation. 



Insects which breathe in the water without rising to the sur- 

 face are provided with gills ; organs which, though variable in 

 form, generally consist of foliaceous or fringed expansions, in the 

 midst of which the tracheae ramify in considerable numbers. These 



* The horny upper wings with which some insects are provided are called elytra. 

 ED. 



