INSECTS (Lat, insectum), a class of Arthropoda characterized by the body being divided into three regions, that is, a head, thorax and hind-body or abdomen, and by the presence, in all but the more primitive and certain degraded forms, of wings, and of three pairs of thoracic legs. The body of insects consist of 21 segments (somites) of which six are used together to form the head, while there are three thoracic and from 10 to 12 abdominal segments. To the head are appended five pairs of jointed appendages, that is, the antenna, mandibles and two pairs of maxilla, while in the embryo of certain insects and in the adult CamPodea, there has been detected a pair of vestigial appendages. Besides these appendages, there are two compound eyes, one on each side, and usually three simple eyes (ocelli) situated in the middle of the bead, While the antenna are undivided the first maxilla are subdivided into three branches, an inner (lacinia), a middle (galea), and outer (palpifer), bearing the pal pus. The second maxilla are fused together, forming the under lip or labium; each second maxilla is composed of a lacinia, the palpus, while vestiges of the gales occur in certain forms. In bees certain accessory appendages called para. glossa are present. Besides the maxilla, the so called tongue or hypopharynx is present, being highly developed in bees; it lies on the under side of the, mouth, just above the labiuM; in caterpillars it receives the end of the salivary duct, and is called the spinneret. Attention should also be called to the upper lip or labium, on the under side of which is the epipharynx, which bears minute taste-pits. The thorax consists of three segments, which can be easily distinguished in the primitive wingless forms (Canspodea) and in the cockroach and locust, but in the more specialized forms as beetles, moths, bees and flies, the segments are more or less fused together and, owing to the movements of the wing muscles, are subdivided into many separate pieces. In the wasps and bees the basal abdominal segment becomes toward the pupa state transferred to the thorax. The legs as a rule end in five jointed tarsi, the last joint bearing a pair of claws with a cushion (pulvillus) between them. Insects are enabled to walk on glass, etc., by means of a sticky fluid exuded from the ends of hollow hairs fringing the cushion. They climb by means of their claws.
Insects differ from all other animals except birds and bats in possessing wings, and their presence, especially that of the muscles of flight, have greatly modified the shape and structure of the thorax. The front pair of wings is at tached to the middle thoracic segment (meso thorax) and the hind wings to the metathorax. In the two-winged flies (Diptera) the second pair of wings are reduced and modified to form the balancers (halteres). The wings are flat sac
like outgrowths of the skin, and are strength ened by the "veins' which form hollow rods. These veins contain a trachea, so that there is a space between the air-tube and the outer wall. When the insect emerges from the nymph or the pupa, the vein is filled with blood. The spaces enclosed by the veins and their cross branches are called cells, and their shape often affords valuable generic and specific characters, In the more primitive insects there are numer ous cross-veins, and such wings as in locusts, etc., are said to be net-veined. In the Lepidop tera there are few cross-veins. In the Diptera and Hymenoptera the number of veins is lim ited, the cells also being few. The skin of insects is hard, dense and elastic, due to the deposition of chitin.
Internal Anatomy.—One of the distinctive characteristics of insects is their mode of res piration. This is effected by an intricate sys tem of internal air-tubes (trachea), which are filled with air by openings (spiracles) in the sides of the body; of these spiracles there are from one to two pairs in the thorax, and eight pairs in the abdomen. The trachea are kept permanently open by a series of threads (tmni dium) each of which makes from three to five turns around the thin tube; in this way the entire tracheal branch is provided with what at first was supposed to be a continuous spiral thread. The slit-like openings of the spiracles are guarded by a grate of stiff hairs to prevent the ingress of dust, etc. It should be borne in mind that no insect breathes through its mouth, but through the spiracles. Hence the efficacy Of all oily or greasy substances in destroying every kind of insect in whatever stage of growth; wherever the oil touches the body a thin film spreads over it, covering the lugs so that the insect soon dies by asphyxiation. Though insects have .a delicate pulsating tu• bider heart, they have no arteries and veins, since the air in the tracheae seeks the blood in the remotest parts of the body. The blood is thin and colorless. The aquatic larvae and a very few perfect insects breathe b• external tracheal gills, the spiracles being in such cases often absent The genital opening is always situated near the end of the body, in front of the vent on the under side. Besides a com plicated digestive canal, insects' have urinary tubes opening into the end of the intestine. • The nervous system consists, besides the brain, of a chain of ganglia the greatest number of which is 13, but which become more or less fused in the more specialized groups, especially in the flies. The brain is remarkably Is:maples, in accordance with the varied and complicated movements of the segmented body and jointed appendages, all capable of different kinds of motions.