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. 12. Linnæus has divided the animals of this class into seven ORDERS. _ORDERS OF INSECTS._ I. COLEOPTEROUS Have elytra or crustaceous cases covering the wings; and which, when closed, form a longitudinal division along the middle of the back, as the _chafer_. II. HEMIPTEROUS Have four wings, the upper ones partly crustaceous, and partly membranous; not divided straight down the middle of the back, but crossed, or incumbent on each other, as the _cock-roach_. III. LEPIDOPTEROUS Have four wings covered with fine scales, almost like powder, as the _butterflies_ and _moths_.