HEXAPODA. 
7 5 
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 we can only make a few generalizations. 
The various modes of respiration of aquatic insects may be classi¬ 
fied under two heads: first, those in which the insects obtain air 
from above the surface of the water; second, those in which the 
insects breathe 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 tip of its wings, forms a cavity be¬ 
neath 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 
surface, forces out the air from beneath its wings, and takes in a new 
supply. Water-beetles and aquatic bugs afford familiar examples of 
this mode of respiration. 
Some insects are provided with long tubes connected with their 
spiracles, by means of which they can draw their supply of air from 
above the surface of the water while they crawl upon the bottom of 
shallow ponds. Our most common illustrations of this are bugs of 
the family Nepidce\ but the most remarkable development of this 
kind is exhibited by certain Dipterous larvae of the family Syrphidce , 
known as Rat-tailed Maggots. 
Although there are many insects that live in the water and draw 
their supply of air from above it, the greater number of aquatic 
insects breathe, as do fishes, the air that is mixed with the water. 
This is accomplished by organs known as tracheal gills . These are 
hair-like or more or less plate-like expansions of the body-wall, 
abundantly supplied with tracheae (Fig. 88). These tracheae 
divide and subdivide, and their terminationsor fine branches 
are separated from the water that bathes the organ only by 
its thin walls. In this way the air contained in the tracheae 
is separated from the air in the water only by a delicate 
membrane, which admits of the transfer of gases between 
them. It win be observed that the difference between a 
tracheal gill and a true gill (as of fishes, Crustacea, etc.) is 
that the true gill is supplied with vessels containing bloody 
which is purified by being brought in contact with the air 
in the water, while the tracheal gill is supplied with tracheae 
containing air to be purified. 
Tracheal gills are usually borne by the abdomen, some¬ 
times by the thorax, and in case of one genus of Stone-flies by the 
Fig. 88 .— 
Tracheal 
pill of 
Agrion. 
