192 PHYSIOLOGY OF INSECT". 



the mouth and nostrils : the trachea and lungs are the 

 organs of respiration. We find that in insects there is no 

 part appropriated solely to the organs of respiration, but 

 that they permeate every part of the body, mixing with 

 the blood vessels, and, as we might say, seeking the blood, 

 in order to aerate it, instead of the blood seeking the organs 

 of respiration by rushing into the lungs, as it does in ver- 

 tebrated animals. The organs of respiration in insects 

 consist of simple tubes, which communicate externally 

 with the air, and internally with the blood : the utility to 

 all animals of imbibing air appears to be owing to its action 

 on, or incorporation with, the blood. 



The air tubes of insects or trachece communicate with 

 the air by means of apertures which occur at intervals 

 along the sides of insects ; these apertures, which are 

 called spiracles, are very various in their form, size, and 

 number. The spiracles or spiracula are sometimes fur- 

 nished with moveable lips, which the insects can close at 

 pleasure, and thus exclude all injurious substances : some- 

 times they are furnished with a fringe of delicate hair, 

 which answers the same purpose ; and sometimes they 

 are constantly open, and without any kind of protection. 

 The spiracles in those instances in which they are furnished 

 with moveable lips, open and close with great regularity, 

 the body at the same time expanding and contracting ; a 

 certain evidence that the motion of the spiracles is con- 

 nected with respiration. The spiracles are commonly ob- 

 long in their shape, but there are instances of circular, 

 linear, quadrate, ear-shaped, and crescent-shaped spiracles. 



The number of spiracles is generally eighteen, nine on 

 each side ; this number is possessed by almost all butter- 

 flies, moths, beetles, &c. ; in flies there are not so many ; 

 in these, moreover, the number is not uniform. The names 

 of the spiracles depend wholly on the segment in which 



