4O AN INTRO1 



\ the wall of the trachea, the intima, like the 



cuti : is sheil from the t \\ ith the cuticle 



There is a peculiarity in the structure of the 



chitinoi; .ill of the trachea that has attracted much 



:i. It is furnished with thickenings which extend spirally. 

 These give the trachea,- their characteristic 

 transversely striated appearance. If a 

 piece of one of the larger trachea- be 

 pulled apart the intima will tear bet 

 the folds of the spiral thickenings, and 

 the latter will uncoil from within the tra- 

 chea like a thread. In some 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 

 - 4 . -section of trachea and larger tracheae of the larva of Corydalus. 



body-wall, c. cuticle; hy, hypoder- T"V^ crvirol thtv^a^c or* n-inf irrr in tVo 

 mis; ;*i. basal membrane; jr/, spiral L n< - ^P^al ttireadS are Wanting 111 the 



smaller terminal portions of the trach 



In many adult insects, especially those that have great powers 



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



acs. These doubtless tend to lessen the specific gravity of the 



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



air-sacs. 



'.lough 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 p: to us many wonderful modifications of structure. Some 



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

 book; in this place I can only make a few geiierali/ 

 The various modes of respiration of aquatic insects may be 

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

 'rom ab< urface of the water- hecond, those in which the 



t breath< if that is mechanically mixed with tli< 



h many aquatic i -h the \ 



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

 ater and lifting the tips of its wings forms a cavity 

 beneath them into which th< The In then swim 



through the water c 'his air with it in a position where it 



be respired. When the j impure, the insect rises to the 



