INSECTS: THEIR BREATHING ORGANS 115 



tion; and perhaps in the whole range of animal mechanics 

 it would be difficult to point out an example of more ex- 

 quisite mechanism, whether we consider the object of the 

 contrivance or the remarkable beauty of the structure em- 

 ployed. The air-tubes themselves are necessarily extremely 

 thin and delicate; so that on the slightest pressure their 

 sides would inevitably collapse and thus completely put 

 a stop to the passage of air through them, producing, of 

 course, the speedy suffocation of the insect, had not some 



AIR-PIPE OF FLY. 



means been adopted to keep them always permeable; and 

 yet to do so, and at the same time to preserve their soft- 

 ness 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 mem- 

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

 elastic thread (a wire of exquisite tenuity) has been inter- 

 posed, which, winding round and round in close spirals, 

 forms by its revolutions a cylindrical pipe of sufficient 

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

 tion, while at the same time it does not at all interfere with 

 its flexibility; this fine coil is continued through every 



