16 THE INSECT WOELD. 



things is maintained by the existence, throughout the whole length 

 of the trachea, of a thread of half horny consistency, rolled up in 

 a spiral, and covered externally by a very delicate membraneous 

 sheath. The external membrane is thin, smooth, and generally 

 colourless, or of a pearly white. The cartilaginous spiral is some- 

 times cylindrical, sometimes flat, and also resembles mother-of- 

 pearl. It only adheres slightly to the external membrane, but is, 

 on the other hand, closely united to the internal one. This spiral 

 thread is only continuous in the same trunk ; it breaks off when 

 it branches, and each branch then possesses its own thread, in such 

 a way that it is not joined to the thread of the trunk from which it 

 issued, except by continuity, just as the branch of a tree is attached 

 to the stem which supports it. This thread is prolonged, without 

 interruption, to the extreme points of the finest ramifications. 



The number of tracheae in the body of an insect is very great. 

 That patient anatomist, Lyonnet, has proved to us, in his great 

 work on the goat-moth caterpillar, Cossus ligniperda, that the 

 insect has much affinity as regards its muscles with animals of a 

 superior class. Lyonnet, who congratulated himself on having 

 finished his long labours without having had to destroy more than 

 eight or nine of the species he wished to describe, had the patience 

 to count the different air4ubes in that caterpillar. He found that 

 there were 256 longitudinal and 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 

 membraneous 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 frame- 

 work formed by the spiral thread of the tubular tracheee, of which 

 they are only enlargements. These, which are called vesicular 

 tracheae, more especially belong to those species whose flight is 

 frequent and sustained, such as the grasshopper, the humble-bee, 

 the bee, the fly, the butterfly, &c. 



