630 



CLASS IV. INSECTA. 



not believed to give rise to them, and their exact function is by 

 no means settled. 



The Respiratory System. 

 Insects breathe by means of 

 tracheae or branching, elastic 

 tubes, which as a rule com- 

 municate with the atmo- 

 sphere by means of pores 

 called stigmata or spiracles at 

 their outer ends, and at their 

 inner are distributed to all 

 the fixed cells in the body. 

 Since each cell has a direct 

 air- supply the respiratory 

 function of the blood is re- 

 duced to a minimum. 



The tracheae from the fact 

 that they contain air appear 

 of a silvery brightness. Each 



consists of a chitinous lining continuous with the chitinous 

 exoskeleton ; this is secreted by a single layer of well-marked 

 cells continuous with the epidermis. The chitinous layer is 



FIG. 385. Tracheal branch with finer twigs 

 (after Leydig). Z cellular external wall ; 

 Sp cuticular lining (spiral fibre). 



FKJ. 386. Portion of the abdominal part of tracheal system of a Locust (Oedipoda). a 

 spiracular orifices ; 5 tracheal tubes ; c vesicular dilatations ; d tracheal twigs or capil- 

 laries (after L. Dufour). 



strengthened by spiral threads sometimes called taenidae which 

 project on its inner surface and serve to prevent the tube 

 from collapsing. In insects which fly actively the tracheae in 



