The Insecta are defined as articulate animals in which the head, thorax, and abdomen are distinct; there are three pairs of legs borne on the thorax; the abdomen is destitute of legs ; a single pair of antennae is present; mostly, there are two pairs of wings on the thorax. Respiration is effected by tracheae.
In the Insecta the body is divided into a variable number of definite segments, or somites, some of which are furnished with jointed appendages, and the nervous and circulatory systems are constructed upon essentially the same plan as in the Crustacea, ' Arachni-da, and Myriapoda. The head, thorax, and abdomen are distinct (figs. 174, 175), and the total number of somites in the body never exceeds twenty. "Of these, five certainly, and six probably" (according to some authorities, four only), "constitute the head, which possesses a pair of antennae, a pair of mandibles, and two pairs of maxillae, the hinder pair of which are coalescent, and form the 'labium.' Three, or perhaps, in some cases, more, somites unite and become specially modified to form the thorax, to which the three pairs of locomotive limbs, characteristic of perfect Insects, are attached. Two additional pairs of locomotive organs, the wings, are developed, in most insects, from the tergal walls of the second and third thoracic somites. No locomotive limbs are ever developed from the abdomen of the adult insect; but the ventral portions of the abdominal somites, from the eighth backwards, are often metamorphosed into apparatuses ancillary to the generative function" (Huxley).
Fig. 174. - Diagram of the external anatomy of an insect. a Head carrying the eyes (o) and antennae (an); b First segment of the thorax, with the first pair of legs; c Second segment of the thorax, with the second pair of legs and the first pair of wings ; d Third segment of the thorax, with the third pair of legs and the second pair of wings; e Abdomen, without limbs, but carrying terminal appendages concerned in reproduction ; f Femur ; t Tibia ; ta Tarsus.
Fig. 175. - A, One of the Dragon-flies (AEshna grandis), slightly dissected : h Head, carrying the eyes, antennae, and organs of the mouth ; t t' t" First, second, and third segments of the thorax slightly separated from one another, each carrying a pair of legs, and the two last carrying each a pair of wings ; a Tail or abdomen. B, Young form, or "larva," of the same. C, Second stage, or " pupa." D, Head of a Dragonfly (Libellula depressa), showing the feelers or antennae (an), the eyes (e e), the hinder pair of jaws (m), and the upper lip (f).
The integument of the Insecta, in the mature condition, is more or less hardened by the deposition of chitine, and usually forms a resisting exoskeleton, to which the muscles are attached. The segments of the head are amalgamated into a single piece, which bears a pair of jointed feelers or antennae, a pair of eyes, usually compound, and the appendages of the mouth. The segments of the thorax are also amalgamated into a single piece; but this, nevertheless, admits of separation into its constituent three somites (figs. 174, 175). These are termed respectively, from before backwards, the "protho-rax," "mesothorax," and "metathorax," and each bears a pair of jointed legs. In the great majority of Insects, the dorsal arches of the mesothorax and metathorax give origin each to a pair of wings.
Each leg consists of from six to nine joints (see fig. 178). The first of these, which is attached to the sternal surface of the thorax, is called the "coxa," and is succeeded by a short joint, termed the "trochanter." The trochanter is followed by a joint, often of large size, called the "femur," succeeded by the so-called "tibia," and this has articulated to it the "tarsus," which may be composed of from one to five joints.
The wings of Insects are expansions of the sides of the meso- and meta-thorax, these expansions being supported by slender but firm tubes, known as the "nervures." Each nerv-ure consists of a central trachea or air-tube, running in the centre of a larger blood-tube; so that the wings not only act as organs of flight, but at the same time assist in the process of respiration. Normally, two pairs of wings are present, but one or other may be wanting. In the Coleoptera (Beetles) the anterior pair of wings become hardened by the deposition of chitine, so as to form two protective cases for the hinder membranous wings. In this condition the anterior wings are known as the "elytra," or "wing-cases." In some of the Hemiptera this change only affects the inner portions of the anterior wings, the apices of which remain membranous, and to these the term "hemelytra" is applied. In the Diptera the posterior pair of wings are rudimentary, and are converted into two capitate filaments, called "halteres" or "balancers." In the Strepsiptera the anterior pair of wings are rudimentary, and are converted into twisted filaments.
The typical number of somites in the abdomen of the In-secta is eleven, and this number can often be recognised in the Neuroptera and in some other forms. In the Hymenoptera and Lepidoptera not more than ten can be recognised, and in other cases even fewer can be made out. The abdominal somites are usually more or less freely movable upon one another, and never carry locomotive limbs. The extremity of the abdomen is, however, not infrequently furnished with appendages, which are connected with the generative function, and not infrequently serve as offensive and defensive weapons.