VI. CHONDROPTERYGEOUS Have cartilaginous fins, as the _sturgeons_,
_sharks_, and _skate_.
11. The fifth class of animals comprises the INSECTS. These are so
denominated from the greater number of them having a separation in the
middle of their bodies, by which they are, as it were, cut into two parts.
The science which treats of them is called ENTOMOLOGY.
Insects have, in general, six or more _legs_, which are, for the most part,
nearly of equal length and thickness. Sometimes, however (as in the
mole-cricket), the forelegs are very thick and strong, for burrowing into
the ground; sometimes the hind thighs are long and thick, for leaping; or
flattened, fringed with hairs, and situated nearly in an horizontal
position, to serve as oars for swimming.
Most of the insect tribes are furnished with _wings_. Some, as the beetles,
have two membranous wings, covered and protected by hard and crustaceous
cases, called elytra; some, as the wasps and bees, have four wings without
elytra; others, as the common houseflies, have two wings; and others, as
the spiders, are entirely destitute of these members.
They are furnished with _antennæ_, which are usually jointed, and moveable
organs, formed of a horny substance, and situated on the front and upper
part of the head. These serve as instruments of touch, or of some sense
which is to us unknown. The _eyes_ of insects are formed of a transparent
substance, so hard as to require no coverings to protect them. Their
_mouth_ is generally situated somewhat beneath the front part of the head,
and in a few of the tribes is below the breast; and the jaws are
transverse, and move in lateral directions. These are furnished with
feelers, and other organs, of various arrangement and structure, which
constitute the foundation of arrangement in some of the systems of
entomology. All insects breathe, not through their mouth, but through pores
or holes along the sides of their bodies; or, as in the crabs and lobsters,
by means of gills. The skin of insects is, in general, of hard or bony
consistence, divided into plates or joints which admit of some degree of
motion, and is generally clad with very short hairs.
Nearly all insects go through certain great _changes_ at different periods
of their existence. From the _egg_ is hatched the _larva_, grub, or
caterpillar, which is destitute of wings; this afterwards changes to a
_pupa_, or _crysalis_, wholly covered with a hard shell, or strong skin,
from which the _perfect_ or _winged insect_, bursts forth. Spiders, and
some other wingless insects, issue from the egg nearly in a perfect state.