THE ORGANS OF CIRCULATION AND RESPIRATION. 



149 



Tracheal Tubes. 



The accompanying figures sufficiently explain the chief 

 features of the tracheal system of the Cockroach, so far as it 

 can be explored by simple dissection. Leaving them to tell 

 their own tale, we shall pass on to the minute structure 

 of the air- tubes, the spiracles, and the physiology of Insect 

 respiration. 



The tracheal wall is a folding-in of the integument, and 

 agrees with it in general structure. Its inner lining, the 



O O O ' 



intima, is chitinous, and continuous with the outer cuticle. It 

 is secreted by an epithelium of nucleated, chitinogenous cells, 

 and outside this is a thin and homogeneous basement mem- 

 brane. The integument, the tracheal wall, and the inner 

 layers of nearly the whole alimentary canal are continuous 

 and equivalent structures. The lining of the larger tracheal 

 tubes at least is shed at every moult, like that of the stomodseum 

 and proctodoeum. 



Fig. 83. Tracheal tube with its epithelium and spiral thread. Slightly altered 

 from a figure given by Chun (Rectal-driisen bei den Insekten, pi. iv., fig. 1). 



