INSECTS : THEIR BEEATHING OEGANS. 



109 



of more exqiiisite meclianism, whether -we consider the 

 object of the contrivance or the remarlcable beauty of 

 the structure employed. The air-tubes themselves are 

 necessarily extremely thin and delicate ; so that on the 

 slightest pressure their sides would inevitably collapse 

 and thus completely piit a stop to the passage of air 



AIK-PIPE OF PLY. 



through them, producing, of course, the speedy suffoca- 

 tion of the insect, had not some means been adopted to 

 keep them always permeable ; and yet to do so, and at 

 the same time to preserve their softness and perfect 

 flexibility, might seem a problem not easily solved. 

 The plan adopted, however, fully combines both these 

 requisites. Between the two thin layers of membrane 

 which form the walls of every air-tube, a delicate elastic 

 thread (a wire of exquisite tenuity) has been interposed, 

 which, winding round and round in close spirals, forms 

 by its revolutions a cylindrical pipe of sufficient firm- 

 ness to preserve the air-vessels in a permeable condi- 

 tion, whilst at the same time it does not at all interfere 

 with its flexibility ; this fine coil is continued through 

 every division of the trachew, even to their most minute 



