2 INTERNAL ANATOMY OF INSECTS. 
ticular apparatus ; nutrition is effected through a stomach 
and intestines ; the analogue of the blood prepared by 
these organs pervades every part of the body, and from 
it are secreted various peculiar substances ; generation 
takes place, and an intercourse between the sexes, by 
means of appropriate organs ,• and lastly, motion is the 
result of the action of muscles. Some of these functions 
are, however, exercised in a mode apparently so dissimi- 
lar from what obtains in the higher animals, that upon 
a first view we are inclined to pronounce them the effect 
of processes altogether peculiar. Thus, though insects 
respire air, they do not receive it by the mouth, but 
through litttle orifices in the sides of the body ; and in- 
stead of lungs, they are furnished with a system of air- 
vessels, ramified ad infinitum, and penetrating to every 
part and organ of their frame ; and though they are nou- 
rished by a fluid prepared from the food received into 
the stomach, this fluid, unlike the blood of vertebrate 
animals, is 'white, and the mode in which it is distributed 
to the different parts of the system, except in the case of 
the true Arachnida, in which a circulation in the ordi- 
nary way has been detected, is altogether obscure. 
In order that you may more clearly understand the 
variations that occur in insects, and in what respects 
they differ amongst themselves, and from the higher ani- 
mals, in the vital functions and their organs, I shall con- 
sider them as to their organs of sensation, respiration, 
circulation, nutrition, generation, secretion, and muscular 
motion. 
Organs of Sensation. — The nervous system of animals 
is one of the most wonderful and mysterious works of 
