Air-tubes 59 



tinuous with that of the outer skin, of which the 

 air-tubes may be regarded as inpushings. In the 

 smallest tubes the chitinous lining is of even thick- 

 ness throughout, but in the trunks and larger 

 branches it is strengthened by thread-like thickenings 

 on its inner surface. In the large tubes close to 

 the spiracles, these thickenings form polygonal areas 

 of network, but throughout the greater part of the 

 tracheal system, the threads are arranged spirally — 

 not forming a continuous spiral, but interrupted after 

 a few turns around the tube. 



In insects of active flight, such as Dragonflies, some 

 Beetles, Bees, Moths and Flies, the tracheal system 

 consists not only of tubes, but of swollen air-sacs, 

 whose walls contain no spiral thickening. In the 

 Cockchafer, the branching tubes, in all parts of the 

 body, swell into numerous pear-shaped air-sacs. In 

 the Bee, there are two great sacs, extending, one on 

 either side, along the greater part of the hind-body. 

 The purpose of these air-sacs is to increase the 

 breathing capacity of such insects of active flight as 

 require a larger supply of oxygen than is sufficient for 

 a ground insect like the Cockroach (2). 



Spiracles. — The spiracles on the thorax of the 

 Cockroach are comparatively large openings, with 

 valves attached to the lower margin. By the action 

 of special muscles, these valves can be made to close 

 the outer opening. The spiracles of the abdomen, on 

 the other hand, have an outer elliptical hole always 

 open, leading into a shallow cavity, whose inner wall 

 is pierced by an elongate slit giving access to the 

 air-tube. This slit divides the cavity of the spiracle 

 into two unequal portions, the larger swelling at one 

 part into a pouch, the smaller being strengthened by a 

 bent rod — the bow. The muscle for closing the slit 

 of the spiracle is stretched between the pouch and 



