PHYSIOLOGY OF INSECTS. 153 



to be owing to its action on, or incorporation 

 with, the blood. 



577. The air tubes of insects (trachece) com- 

 municate 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. 



578. The spiracles (spiracula) are sometimes 

 furnished with moveable lips, which the insects 

 can close at pleasure, and thus exclude all injurious 

 substances : sometimes 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. 



579. 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 evi- 

 dence that the motion of the spiracles is con- 

 nected with respiration. 



580. The spiracles are commonly oblong in 

 their shape, but there are instances of circular, 

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

 spiracles. 



581. The number of spiracles is generally 

 eighteen, nine on each side ; this number is pos- 

 sessed by almost all butterflies, moths, beetles, 

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

 moreover, the number is not uniform. 



582. The names of the spiracles depend wholly 



