RESPIRATION IN INSECTS. 



281 



as is shown in Fig. 370, which represents the lower 

 or abdominal surface of the Di/tiscus margiualis. 



They are seen very distinctly in the caterpillar, 

 which has generally ten on each side, correspond- 

 ing to the number of abdominal segments. In 

 many insects we find them guarded by bristles, or 

 tufts of hair, and sometimes by valves, placed at 

 the orifice, to prevent the entrance of extraneous 

 bodies. The spiracles are opened and closed by 

 muscles provided for that purpose. Fig. 371 is a 

 magnified view of spiracles of this description, from 

 the Cerambyx lieros. (Fab.) They are the begin- 

 nings of short tubes, which open into large trunks 

 (as shown in Fig. 372), extending longitudinally 

 on each side, and sending off radiating branches 

 from the parts which are opposite to the spiracles ; 

 and these branches are farther subdivided, in the 

 same manner as the arteries of the larger animals, 

 so that their minute ramifications pervade every 

 organ in the body. This ramified distribution has 



