HEXAPODA, 75 



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 Syrphida, 

 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 terminations or 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 will 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 blood, 

 which is purified by being brought in contact with the air 

 in the water, while the tracheal gill is supplied with tracheae fig.' 88.— 

 containing air to be purified. giTi^^o^ 



Tracheal gills are usually borne by the abdomen, some- Agrion. 

 times by the thorax, and in case of one genus of Stone-flies by the 



