STRUCTURE OF LARV^. 



187 



body is often very different from the preceding in its form, 

 and its under-side in some hexapod larvae is furnished with 

 a retractile tubercle, which the insect sometimes employs 

 as a seventh leg. 



In addition to the organs above described, are to be no- 

 ticed the spiracles, or apertures by which the larvae take in 

 a supply of air. These spiracles are not, as in the higher 

 animals, situated in the head, but are arranged in a series on 

 each side of the body, opening into two longitudinal tubular 

 internal vessels called tracheae, from which an immense num- 

 ber of ramifications diverge to every part of the bod}^ of the 

 insect. These spiracles are placed in tlie first, fom'th, and 

 following segments (except the last), and consist of a small 

 elongated opening, surrounded by a callous margin (fig. 25 

 represents a spiracle open, and 26 a closed spiracle). In 

 aquatic larvae, however, a variation in the organization of 

 the animal necessarily takes place, as it is not exposed to the 

 ordinary action of the air. In the larva of the Dyticidce the 

 only spiracles which the insect possesses are placed at the 

 extremity of the body, so that it is obliged to rise to the 

 sm-face of the water, or at least to elevate the extremity of 



the perfect Dyticus marginalis. 



the body till it reaches the surface. The larva of the dragon- 

 flies is provided with an anal pouch, in which it receives from 



