INTRODUCTION. 15 



views were long contested. Now, however, one can easily recognise 

 the apparatus by the aid of which the respiration of the insect is 

 effected. 



The respiratory apparatus is essentially composed of membranous 

 ducts of great tenuity, their ramifications spread everywhere in 

 incalculable numbers, and bury themselves in the different organs, 

 much in the same way as the fibrous roots of plants bury themselves 

 in the soil. These vessels are called tracheae. Their communications 

 with the air are established externally in different ways, according to 

 the character of the medium in which the insect lives. 



It is well known that a vast number of insects live in the air. The 

 air penetrates into the tracheae by a number of orifices placed at the 

 sides of the body, which are termed spiracles. On close examination 

 these may be seen in the shape of button-holes in a number of 

 different species. Let us dwell for a moment on the breathing 

 apparatus of the insect, that is to say, on the tracheae. 



This apparatus is sometimes composed of elastic tubes only, some- 

 times of a collection of tubes and membranous pouches. We will 

 first treat of the former. 



The coats of these breathing tubes are very elastic, and always 

 preserve a cylindrical form, even when not distended. This state of 

 things is maintained by the existence, throughout the whole length ot 

 the tracheae, 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 sometimes cylindrical 

 and sometimes flat. It only adheres slightly to the external mem- 

 brane, 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, Lyonet, has proved this in his great work on 

 the Goat-moth Caterpillar, Cossi/s ligniperda. Lyonet, who con- 

 gratulated himself with 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 



