e spiders (Fig. 1), however, there are no
antennae, and the second maxillae, or labium, is wanting. Moreover, there
are four pairs of legs. The centipedes (Fig. 2, a Myriopod) also differ
from the rest of the insects in having an indefinite number of abdominal
rings, each bearing a pair of legs.

[Illustration: 1. Spider (Tegenaria).]

On examining the arrangement of the parts within, we find the nervous
cord, consisting of two chains of swellings, or nerve-knots, resting
upon the floor or under side of the body; and the heart, or dorsal
vessel, situated just under the skin of the back; and in looking at
living caterpillars, such as the cut-worm, and many thin-skinned aquatic
larvae, we can see this long tubular heart pulsating about as often as
our own heart, and when the insect is held against its will, or is
agitated, the rapidity of the pulsations increases just as with us.

[Illustration: 2. Centipede.]

Insects do not breathe as in the higher animals by taking the air into
the mouth and filling the lungs, but there are a series of holes or
pores along the side of the body, as seen in the grub of the humble bee,
through which the air enters and is conveyed to every part of the body
by an immense number of air tubes. (Fig. 3, air tubes, or tracheae, in
the caudal appendage of the larva of a dragon fly). These air tubes are
everywhere bathed by the blood, by which the latter becomes oxygenated.

[Illustration: 3. Caudal appendage of larva of Agrion.]

Indeed the structure of an insect is entirely different from that of man
or the quadrupeds, or any other vertebrate animal, and what we call
head, thorax, abdomen, gills, stomach, skin, or lungs, or jaws, are
called so simply for convenience, and not that they are made in the same
way as those parts in the higher animals.

An insect differs from a horse, for example, as much as a modern
printing press differs from the press Franklin used. Both machines are
made of iron, steel, wood, etc., and both print; but the plan of their
structure differs throughout, and some parts are wanting in the simpler
press which are present and absolutely essential in the other. So with
the two sorts of animals; they are built up originally out of
protoplasm, or the original jelly-like germinal matter, which fills the
cells composing their tissues, and nearly the same chemical elements
occur in both, but the mode in which these are combined, the arrangement
of their products: the mus