276 RESPIRATION OF INSECTS AND SPIDERS. 
the extraordinary activity of their respiration. There are 
no animals which consume so much oxygen, in proportion 
to their size, as Insects do when they are in motion (§ 308); 
but when they are at rest, their respiration falls to the 
low standard of the tribes to which they bear 
the greatest general resemblance. Although, as 
we have seen, the . respiration of aquatic larvae is 
sometimes accomplished by means of gills, yet 
many aquatic larvae breathe air by means of 
tracheae; and such are consequently obliged, like 
Whales and other aquatic Mammals, to come 
occasionally to the surface for the purpose of 
gaining a fresh supply of air. The larva of the 
Gnat, which breathes in this manner, has one of 
the spiracles of its tail-segment prolonged into a 
tube ; and it may often be seen suspended, as it 
were, in the water, with its head downwards, the 
end of this tube (t, fig. 160) being at the surface. 
322. In the greater number of perfect Insects, we find the 
tracheae dilated at certain parts into large air-sacs (fig. 159); 
these are usually largest in Insects that sustain the longest 
and most powerful flight; in some of which, as in the 
common Bee, they occupy a greater portion of the trunk than 
they do in the insect whose system of air-tubes has been just 
represented,—this insect, the Nepa or water-scorpion, being 
of aquatic habits, and seldom using its wings for flight. 
There can be little doubt that one use of these cavities is to 
diminish the specific gravity of the Insect, and thus to render 
it more buoyant in the atmosphere; but it would not seem 
improbable that they are intended to contain a store of air 
for its use while on the wing, as at that time a part of 
the spiracles are closed. We shall find in Birds, the Insects 
of the Yertebrated division, a structure bearing remarkable 
analogy to this (§ 326). 
323. In some of the Arachnida, such as the Cheese-mite, 
the respiration is accomplished by tracheae, as in Insects ; but 
in the Spiders it is performed by a different kind of apparatus. 
Instead of opening into a system of prolonged tubes, each 
spiracle leads to a little chamber, the lining membrane of 
which is arranged in a number of folds that lie together like 
the leaves of a book; and thus a large surface is exposed 
t 
Fig. 160. 
Larva of 
Gnat. 
