1 6 THE INSECT WORLD. 



1,336 transverse branches ; in short, that the body of this creature is 

 traversed in all directions by 1,572 aeriferous tubes which are visible 

 to the eye by the aid of a magnifying glass, without taking into 

 account those which may be imperceptible. 



The complicated system of the breathing apparatus which we are 

 describing is sometimes composed of an assemblage of tubes and 

 membranous pouches, besides the elastic tubes which we have already 

 mentioned. These pouches vary in size, and are very elastic, 

 expanding when the air enters, and contracting when it leaves them, 

 as they are altogether without the species of framework formed by 

 the spiral thread of the tubular tracheae, of which they are only 

 enlargements. 



Fig. 13 is explanatory of these organs of respiration. 



The respiratory mechanism of an insect is easily understood. 

 " The abdominal cavity," says M. Milne-Edwards, " in which is 

 placed the greater part of the respiratory apparatus, is susceptible 

 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 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 irt 

 equilibrium with 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 respiratory 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 commu- 

 nication between the respiratory apparatus and the surrounding 

 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, 



