ENTOMOLOGY, the branch of zoology which treats of insects. Insects have jointed bodies and limbs, an enveloping cuticle of chitin, a ventral chain of gan glia, a dorsal brain, and breathe by air tubes or trachea. Contrasted with peri patus and myriopods, they have made two great steps of progress; the body is centralized, with locomotor limbs re duced to three pairs, and all the typical average forms have wings. Insects fre quently have a metamorphosis in their life history. The adult body is divided into (1) a head, with three pairs of ap pendages (—legs), plus a pair of pre oral outgrowths, the antennae or feel ers; (2) a thorax, with three pairs of jointed legs, typically plus two pairs of dorsal, compressed sacs—the wings; (3) an abdomen, without legs, except in so far as these are rudimentarily repre sented in stings, ovipositors, etc. In sects exceed in number all other animals taken together. Over 80,000 species of beetles or coleoptera and about 15,000 moths and butterflies have been re corded; and Speyer estimates the total census at 200,000.
Form.—The body of an insect consists of a distinct undivided head, probably composed of four obscured segments, of a thorax with three divisions (pro, meso, and metathorax), and of an abdomen typically with 11 rings.
Appendages.—The jointed feelers or antenna, which are outgrowths of the head, not strictly comparable to legs, have often numerous nerve-endings and seem to be used in smelling, as organs of touch and guidance or in communicating impressions to friends. Exactly com parable with legs are the three pairs of mouth appendages, projecting downward or forward from the head, to which they are jointed and from which they are worked by muscles. The first pair, the mandibles (biting and chewing organs), more or less reduced in those insects which suck, have but one joint and are without the lateral "palp" present in the crustacean organs of the same name. Next come the first pair of maxilla, which have jointed "palps." The second pair of maxilla, are united at their base, and form the so-called labium, also pro vided with "palps." In the different
orders, and in association with the di verse diet, these three pairs of mouth organs vary greatly.
Wings.—The adult insect usually bears two pairs of dorsal outgrowths or wings on the two posterior rings of the thorax. They are undeveloped in some passive females and are likewise absent from many parasitic forms. In these cases the wings have been lost, while they have never been attained by the lowest insects. When at rest the wings are usually folded in various wasrs, the dragon flies and some others keep them expanded. The two pairs may be al most alike, as in bees and butterflies; those in front may be merely covers for the hind pair, as in beetles, or contorted rudiments in the little bee-parasites (Strepsiptera); the hind pair may be linked to the fore pair, and are rudi mentary "balancers" or "halters" in flies. They are often hairy or scaly, or gorgeous with pigment, or occasion ally odoriferous.
Locomotion.—Insects are emphatically locomotor animals. They walk, run and jump with the quadrupeds; they fly with the birds, they glide with the serpents; and they swim with the fish. Even the limbless larva of many forms move deftly, contracting their bodies, and uti lizing jaws, hairs, and tubercles to help them along. The limbed larva, and es pecially the true caterpillars, often move with great rapidity; a few jump, and many climb; others utilize their silken threads in spiderlike fashion; while the young dragon flies propel themselves along by the forcible expulsion of water.
Skin.—Insects have a firm chitinous cuticle formed from the epidermis or hy podermic. The cuticle bears scales, tu bercles and hairs, of which the last are sometimes olfactory or otherwise sen sory. In spite of the ensheathing arma ture there are often glands in connection with the skin. Before the full size is reached there are often numerous skin castings or moltings.