ATMOSPHERIC RESPIRATION. 



311 



commence, are called spiracles, or stigmata, and 

 are usually situated in rows on each side of 



the body, as is shown in Fig. 370, which repre- 

 sents the lower or abdominal surface of the Dy- 

 tiscus marginalis. They are seen very distinctly 

 in the caterpillar, which has generally ten on 

 each side, corresponding to the number of abdo- 

 minal segments. In many insects we find them 

 guarded by bristles, or tufts of hair, and some- 

 times by valves, placed at the orifice, to prevent 

 the entrance of extraneous bodies. The spira- 

 cles are opened and closed by muscles provided 

 for that purpose. Fig. 371 is a magnified view 

 of spiracles of this description, from the Ceram- 

 hyx heros. (Fab.) They are the beginnings of 

 short tubes, which open into large trunks (as 

 shown in Fig. 372), extending longitudinally 



