AN INTRODUCTION TO ENTOMOLOGY. 



body-wall, c, cuticle; hy, hypoder- 

 mis; \bm, basal membrane; sf, spiral 



body. This inner layer of the wall of the trachea, the intima, like the 

 cuticle is chitinous, and is shed from the tracheae with the cuticle 

 when the insect moults. There is a peculiarity in the structure of the 

 chitinous layer of the wall of the tracheae that has attracted much 

 attention. It is furnished with thickenings which extend spirally. 



These give the tracheae their characteristic 

 transversely striated appearance. If a 

 piece of one of the larger tracheae be 

 pulled apart the intima will tear between 

 the folds of the spiral thickenings, and 

 the latter will uncoil from within the tra- 

 chea like a thread. In some insects there 

 are several parallel thickenings of the in- 

 tima ; so that when an attempt is made 

 to uncoil the thread a ribbon-like band 

 is produced, composed of several parallel 

 threads. I have often observed this in the 

 FIG. 54 . -section of trachea and larger tracheae of the larva Q{' Corydalus. 



'"TUp, cniral thr^aHc arf* wantincr in fVif> 

 - 1 ne Spiral UireaaS are Wanting in tne 



smaller terminal portions of the tracheae. 



In many adult insects, especially those that have great powers 

 of flight, the tracheae are enlarged in many places so as to form 

 air-sacs. These doubtless tend to lessen the specific gravity of the 

 insect. The spiral thickenings are not developed in the intima of 

 these air-sacs. 



Although insects are, strictly speaking, air-breathing animals, 

 many of them, as is well known, live in the water. The study of 

 the ways in which aquatic insects breathe is a very interesting one ; 

 it presents to us many wonderful modifications of structure. Some 

 of the more common of these are described in subsequent pages of 

 this book ; in this place I can only make a few generalizations. 



The various modes of respiration of aquatic insects may be clas- 

 sified under two heads : first, those in which the insect obtains its 

 air from above the surface of the water ; second, those in which the 

 insect breathes the air that is mechanically mixed with the water. 



With many aquatic insects the spiracles open beneath the wings, 

 which are folded upon the abdomen. The insect by coming to the 

 surface of the water and lifting the tips of its wings forms a cavity 

 beneath them into which the air rushes. The insect can then swim 

 through the water carrying this air with it in a position where it can 

 be respired. When the air becomes impure, the insect rises to the 



