Page:Collier's New Encyclopedia v. 04.djvu/49

LEFT ENTOMOLOGY 31 ENTOMOLOGY 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 antennae, 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 ENTOMOLOGY — SWALLOW-TAIL BUTTERFLY A. Larva B. Pupa C. Butterfly 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 maxillae, which have jointed "palps." The second pair of maxillae, 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 wine:s are usually folded in various ways, 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 larvae of many forms move deftly, contracting their bodies, and uti- lizing jaws, hairs, and tubercles to help them along. The limbed larvae, 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- podermis. 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. Sense-organs. — Except in fleas, lice, and the lowly Collembola, adult insects have compound eyes. These are often associated with simple eyes or ocelli, which are all that ever appear in larvae, or fleas, and such insects. Blind insects also Occur with other blind animals in the darkness of caves. Auditory organs are represented in almost all orders by peculiar nerve-endings superficially dis- posed on various parts of the body. On the tactile antennae, and probably also on the maxillary palps of various insects, there are specially innervated skin cells and hairs believed to be olfactory in func- tion; while others more within the mouth are credited with gustatory sensi- tiveness. The intelligence is greatest in the social insects — especially the ants and bees, where it is associated with complex though very small brains. Alimentary Sijstetn. — The alimentary canal always consists of fore-, mid-, and hind-gut. But the length and structure vary in different insects, to some ex- tent in association with the differences of diet. The fore-gut includes mouth, phar- ynx, and gullet, of which the gullet may be swollen into a ci'op, or bear an ap- pended pouch (so-called sucking stom-