204 



HARDWICKE'S SCIENCE-GOSSIP. 



to the minute structure of the air-tubes 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 intiina, is chitinous, and continuous with 

 the outer cuticle. It is secreted by an epithelium of 



In the finest tracheal tubes ('cxjoi in. and under) 

 tlie intima is to all appearance homogeneous. In 

 wider tubes it is strengthened by a spiral thread, 

 which is denser, more refractive, and more flexible 

 than the intervening membrane. The thread pro- 

 jects slightly into the lumen of the tube, and is often 



Fig. 117. — Tracheal System of Cockroach. The 

 dorsal integument removed and the viscera in 

 place. X 5. 



Fig. 118.— Tracheal System of Cockroach. The viscera 

 removed to show ventral tracheal communications. 

 X 5- 



nucleated, chitinogenous cells, and outside this is a 

 thin and homogeneous basement membrane. 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 stomodreum and proctodaeum. 



branched. It is interrupted frequently, each length 

 making but a few turns round the tube, and ending 

 in a point. The thread of a branch is never cnntinued 

 into a main trunk. Both the thread and the 

 intervening membrane become invisible or faint when 

 the tissue is soaked with a transparent fluid, so as to 

 expel the air. Both, but especially the thread. 



