
















18 THE INSECT WORLD. 
ted and dilated alternately by the play of the 
s of which the skeleton is composed, and which 
a manner that they can be drawn into each 
other to a greater or less extent. When the insect contracts its 
essed and the air driven out. But 
body the tracheze are compr 
when, on the other hand, the visceral cavity which contains the 
trachese assumes its normal size or dilates, these channels become 
larger, and the air with which they are filled being rarefied by 
in equilibrium with the outer air with 
hrough the medium of the spiracles. 
he interior of the respira- 
of being contrac 
different segment 
are placed in such 
this expansion, is no longer 
which it is in communication t 
The exterior air is then impelled into t 
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 trachese 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 communication between the respiratory apparatus and the sur- 
rounding 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 1s necessary for respiration, 
above the water, certain insects employ their elytra* as a sort of re- 
servoir ; others make use of their antenne, 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 amore 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 sur- 
face are provided with gills; organs which, though variable in 
form, generally consist of foliaceous 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. 
—~Eip. 
























