ENT.OMOLGY. The science which treats of insects, as to ,their structure, habits, and varieties. The body of an insect consists of four principal parts, namely, the head, the trunk, the abdomen, and the limbs or ex tremities: The head is furnished in most in• sects with eyes, antennas or horns, and a mouth. The eyes are various, both in colour, shape, and number, in different kinds, some being of a different colour from that of the head, and some of the same colour, some placed close together, or almost touching each other, some having the pupil glassy and trans parent, others having it scarcely distinguish able. Many insects have, besides the large eyes, also three small spherical bodies placed triangularly on the crown of the head, called ocelli, or stemmata. The antennas are two articulated moveable processes, placed on the head, which are also subject to great variety in their form and structure, being setaceous, or bristle shaped, filiform, or thread-shaped, ,ike. The mouth in most insects is situated in the lower part of the bead, and consists of the lips, upper and lower ; the mandibles, or horny substances, one on each side of the mouth; the maxillw, or jaws, two membrana ceous substances, differing in figure from the mandibles, under which they are situated ; the tongue, an involuted, tubular organ, which constitutes the whole mouth in some insects, AS the sphinx ; the rostrum, beak or snout, a moveable articulated member in the grasshop per, the aphis, &c, ; the proboscis, or trunk, which serves as a mouth in the house fly, bee, and some other insects ; the feelers, small moveable filiform organs, placed mostly on each side the jaw, and resembling the antenna;, but much smaller ; these vary in number from two to six in different insects.
The trunk, which is the second general division of which an insect consists, com prehends that portion situated between the head and the abdomen. This consists of the thorax, or upper part of the body, to which the first pair of legs is attached ; the breast, or under part of the thorax, to which the four posterior feet are attached ; the breast bone, a ridge running under the breast, which is conspicuous in some insects; and the scu tellum, or escutcheon, a lobe-like process, situated at the posterior part of the thorax.
The abdomen, or third principal portion of an insect's body, is composed of annular joints, or segments, which vary in form and number in different insects ; this is distin guished into the back, or upper part, and the belly, or under part. The motion of the ab domen is most visible in the fly and bee tribes. To this division belong also the tail and the sting. The tail sometimes spreads like a leaf; as in the cockroach ; and in other insects is bristle-shaped. The sting, which is peculiar to insects of the bee tribe and some few others, is sometimes simple, having but one dart, and sometimes compound, having two darts. In bees and wasps the sting is retrac tile, that is, capable of being drawn in; but i in other insects it is almost always hid in the body, or seldom thrust out. In some tribes of insects it exists in the males, in others in the females only, but seldom in both sexes. The members or extremities of insects are the legs and the wings. Insects have some times six legs, but never more, except what are observable in the larvae, which are termed spirious feet. The feet vary in their form and use, being formed either for running, swimming, or leaping, with or without claws or spines, &c. The wings are mostly two, but sometimes four in number; mostly placed on each side the insect, so as that each pair should correspond in situation, form, &c. ; but where there is more than one pair, the first are mostly larger than those behind. The wings are greatly diversified as to form, figure, texture, construction, &o. To the wings beimg also the elytra, or wing eases, and the halteres, or poisers. The elytra are two coriaceous wings, which are expanded in flight, but when at rest serve to cover the ab domen and enclose theirmembranaceous wings, as in insects of the beetle tribe ; the poisers are two globular bodies placed on slender stalks behind the wings in the tribe 9f winged insects, so called because they are supposed to keep the insect steady in its flight.