316 PTILOTA : INTERNAL ANATOMY. 



of a tree. These tracheae are of two sorts, tubular or vesi- 

 cular : the former are tubes composed of three distinct mem- 

 branes, the external and internal layers being of a cellular 

 texture, and the central being composed of a cartilaginous 

 thread, rolled in a spiral direction, and very elastic. The 

 vesicular tracheae are destitute of this spiral cartilage, and 

 are small bags of a cellular texture, which, when not dis- 

 tended with air, naturally become flaccid. The trachea; 

 often communicate with each other, and penetrate all the 

 organs, such as the legs, wings, &c. ; they are also distri- 

 buted throughout the intestinal parts of the body, so that 

 the air circulates freely in the most obscure parts. 



The Dyticida, Culicidee, Chironomus, Ephemera, and other 

 insects, which are chiefly aquatic, exhibit various modifi- 

 cations in their respiratory apparatus, and in which we 

 perceive two principal modifications : 1st, A series of large 

 lateral organs, which act upon the water, and abstract from 

 it the oxygen ; and, 2ndly, a concentration of the respiratory 

 functions by means of an apparatus placed at the extremity 

 of the body, whereby the insect is enabled to obtain a 

 supply of air, by protruding this part of the body above 

 the surface of the water, whilst it still remains with its 

 greater part submerged. M. Audouin has also recently 

 noticed another remarkable modification of the respiratory 

 apparatus in insects, which, although terrestrial, are occa- 

 sionally covered for a great length of time with water. (See 

 his Memoir upon Aepus fulvescens in the " Nouvelles An- 

 nales du Museum.") 



E. The Motive, or Muscular System. 

 The varied structures described in the preceding pages 

 relative to the external anatomy of insects, are but the 

 visible expression of certain principles which constitute 

 the economy of the various species, and which are re- 

 gulated in the performance of their various functions 



