18 THE INSECT WORLD. 
of being contracted and dilated alternately by the play of the 
different segments of which the skeleton is composed, and which 
are placed in such a manner that they can be drawn into each 
other to a greater or less extent. When the insect contracts its 
body, the trachez are compressed and the air driven out. But 
when, on the other hand, the visceral cavity which contains the 
tracheze assumes its normal size, or dilates, these channels become 
larger, and the air with which they are filled being rarefied by 
this expansion, is no longer in equilibrium with the outer air with 
which it is in communication through the medium of the spiracles. 
The exterior air 1s then impelled into the interior of the respira- 
tory tubes, and the inspiration is effected.” 
The respiratory movements can be accelerated or diminished, 
according to the wants of the animal; in general, there are from 
thirty to fifty to the minute. in a state of repose the spiracles 
are open, and all the tracheze are free to receive air whenever 
the visceral cavity is dilated, but those orifices may be closed, 
and the insect thus possesses the faculty of stopping all communi- 
cation between the respiratory apparatus and the surrounding 
atmosphere. 
Some insects live in the water; they are therefore obliged to 
come to the surface to take the air they are in need of, or else to 
possess themselves of the small amount contained in the water. 
Both these methods of respiration exist under different forms in 
aquatic insects. | 
To inhale atmospheric air, which is necessary for respiration, 
above the water, certain insects employ their elytra* as a sort of 
reservoir ; others make use of their antennz, the hairs of which 
retain the globules of air. In this case it is brought under the 
thorax, whence a groove carries it to the spiracles. Sometimes 
the same result is obtained by a more complicated arrangement, 
consisting of respiratory tubes which can be thrust into the air, 
which it is their function to introduce into the organisation. 
Insects which breathe in the water without rising to the surface 
are provided with gills; organs which, though variable in form, 
generally consist of foiiaceous or fringed expansions, in the midst 
of which the trachez ramify in considerable numbers. These 
* The horny upper wings with which some insects are provided are called elytra. 
—Ep. 
