222 THE VITAL FUNCTIONS. 



are usually situated in rows on each side of the body, as -is 

 shown in Fig. 370, which represents the lower abdominal 



370 ( ^-2a-v 373 



surface of the Dytiscus marginalis. They are seen very 

 distinctly in the caterpillar, which has generally ten on each 

 side, corresponding 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. 

 Pig. 371 is a magnified view of spiracles of this description, 

 from the Cerambyx heros. (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 op- 

 posite to the spiracles; and these branches are farther subdi- 

 vided, in the same manner as the arteries of the larger ani- 

 mals, so that their minute ramifications pervade every organ 

 in the body. This ramified distribution has frequently oc- 

 casioned their being mistaken for blood vessels. In the 

 wings of insects, the nervures, which have the appearance 

 of veins, are only large air-tubes. Jurine asserts that it is 

 by forcing air into these tubes that the insect is enabled sud- 

 denly to expand the wings in preparing them for flight, 



