INSECTS, Insecta, one of the classes of articulata (q.v.), or articulated animals, of the division having articulated members. All the articulate having articulated members were included by Linfiteus in the class of insects; hut the crustacea and arachuida were soon separated from it, and afterwards the myriapoda. See these heads, This application of the term insect corresponds more nearly with its popular use, and so well accords with its derivation, that it may be regarded as one of the most appropriate names employed in natural history. It is from a Latin word, signifying cut into; a derivation exactly answering to that of the Greek entoma, from which the science having insects for its subject receives the name of entomology. Insects, a natural and extremely well defined class of organized beings, are remarkable, in their mature or perfect state, for the division of their bodies into three very distinct portions—the head, thorax, and abdomen; the divisions being often so deep, that the slenderness to which the body is there reduced cannot be contemplated without admiration.
The body of an insect, as of all the other articulate, is composed of a certain number of rings. One of these forms the head; or, if the head ought to be regarded as really composed of several rings, modified and condensed together, as the skull of vertebrate animals is formed of modified vertebrae, yet no distinction of rings appears. The eyes, the antenna, and the 'organs of the mouth, are the most conspicuous organs connected with the head.
The thorax is formed of three rings, closely combined, but easily distinguishable. The first is the prof/Lorca; the second, the mesothorax; the third, the Metathorax (Gr. pro, before; mesos, middle; and meta, after). Of these rings, one or another is often remark ably developed. The legs and wings are attached to the thorax. Insects have six legs, and generally four or two wings, never any other number; but some are wingless, and this is the case not only in all the insects of certain groups, lint also in particular species of groups ordinarily winged, and is sometimes even a distinction of sex, as in the glow worm. The first pair of legs are attached to the prothorax; the second, to the meso
thorax; and the third, to the metathorax. The first pair of wings are attached to the mesothorax; the second, to the metathorax. In dipterous (two-winged) insects, the place of the second pair of wings is occupied by two small organs—little threads, terminated by a knob—called balancers (halteres), the use of which is not well known.
The abdomen consists of nine rings, or of fewer; as some are often obliterated, or modified, to form various appendages. It contains the principal viscera. In it, the sexual organs are situated. The rings of the abdomen are much more separate and mov able than those of the thorax. The terminal rings of the females of sonic groups form an oviduct or ovipositor, which is sometimes capable of being employed as a borer, to make a place for the eggs in the animal or vegetable organism destined to receive them, and which in wasps and bees is replaced by a sling.
The nervous system of insects, in all their stages of existence, exhibits the general characters noticed as belonging to the articulata (q.v.). There is a brain, or ganglion of the head, from which arise the nerves of the eyes, antennae, and mouth.
The rings of which the body of an insect is composed appear most in the external covering. This is in most parts hard, but more or less flexible, of a horn-like substance, chiefly composed of chitin (q.v.). The external covering of insects is the prin cipal framework of their bodies, and to it the muscles are at,aehed. The external cover ing of each ring is more or less distinctly divided into two parts—a dorsal and a ventral —the connection at the sides being effected by a softer and more flexible membrane, a still softer membrane connecting the rings of the abdomen, so as tc, allow considerable freedom of motion; whilst between the rings are minute pores called stigmata or spiracles, by which air is admitted to the trachea or air-tubes (q.v.), the organs of respiration.