16 _ THE INSECT WORLD. 
things is maintained by the existence, throughout the whole length 
of the trachez, of a thread of half horny consistency, rolled up in 
a spiral, and covered externally by a very delicate membranous 
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 
away 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 tracheze in the body of an insect is very great. 
That patient anatomist, Lyonet, 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. iyonet, 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 air-tubes 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 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 frame- 
work formed by the spiral thread of the tubular trachez, of which 
they are only enlargements. These, which are called vesicular 
trachez, 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, &e. 
