Sense-organs.— Excep. 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 larva, 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 tho tactile antenna, 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 System.—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 crop, or bear an ap pended pouch (so-called sucking stom ach), or may be continued into a giz zard with hard grinding plates. The mid-gut is glandular, digestive, and ab sorptive; it often bears secular out growths or glandular caeca, and has, as its origin implies, no chitinous lining. The hind-gut is often coiled, terminally expanded in the rectum, and in that region sometimes associated with glands.
Respiratory System. — Insects when resting often show panting movements in the abdomen, which is swayed by mus cles whose activity is the chief condition of the circulation of air throughout the body. For in all insects the whole body is penetrated by air-tubes or trachea, which send fine branches into all the organs and tissues. These tubes are really ingrowths from the skin, and are lined by chitin, raised in what appear to be spiral thickenings which keep them elastically tense. In most cases these trachea open to the exterior by paired apertures or stigmata on the breast and abdomen, often guarded by hairs and very variously disposed. There are never more than 10 pairs of openings, though primitively there was probably a pair to each segment.
Many insects produce sounds which often express a variety of emotions. In some cases, when not simply automatic, these sounds serve the purpose of love songs; they may also express fear, an ger, or even sorrow, or they may give alarm and convey tidings.
Circulatory System. — As the tissues are riddled with air-tubes, the need for definite blood-vessels is greatly lessened, and so the circulatory system is slight ly developed in comparison with the thorough respiratory arrangements. The blood—which is colorless, yellow, green ish, or even reddish, with amceboid cells —flows for the most part along lacuna without definite walls. The central organ is the dorsal blood-vessel or heart.
Metamorphosis.—From the egg-shell of such insects as butterflies, beetles, flies, and bees, there emerges a larva (maggot, grub, or caterpillar) which lives a life of its own, growing, resting, and molting, often very active in its movements and voracious. Having ac
cumulated a rich store of reserve food in its fat-body, the larva becomes for a longer time more or less quiescent, be comes a pupa, nymph, or chrysalis. In this stage, often within the shelter of a spun cocoon, great transformations oc cur: wings bud out, appendages of the adult pattern appear, reconstruction and centralization of organs are effected; and, finally, out of the pupal husk there emerges an imago or miniature fully formed insect.
The Internal Metamorphosis. — In those forms which have no metamorpho sis, or only an incomplete one, the organs of the larva develop continuously into those of the adult. It is otherwise in the complete metamorphosis of the higher insects. There the internal changes are as marked as the external; there is a gradual reconstruction of organs during the later larval, especially during the pupal stages. Most of the larval organs are absorbed by amceboid cells, and their debris is utilized in building up new structures.
General Life.—(a) While insects are predominantly active animals, we find in contrasting the families abundant illus tration of the antithesis between activity and passivity. (b) In the majority of cases the adult insect is short-lived, and dies within a year; an adult Ephenierid may be literally the fly of a day, but from this there are many graduations leading up to the rare cases of a queen bee five years old, or an aged queen ant of thirteen. (c) Reproduction in a great number of insects of both sexes is shortly followed by death, love being in such cases at once the climax and end of life.
Economic Import.—As far as insects are concerned, the struggle between man and animals is by no means finished. Direct injuries to man's person are familiarly illustrated in the parasitism of fleas, lice, and other more or less inti mate "boarders"; but these are less im portant than the share the mosquito seems to have in the loathsome disease Elephantiasis arabum. Personal injuries are dwarfed when we think of those done to property, and especially to crops and herds, by voracious or by parasitic insects. Clothes-moth and furniture borer, vine-insects and Colorado beetle, the botflies which attack sheep, cattle, and horses are familiar illustrations of formidable pests. It should also be noted how the hostile insects which infest for est trees and vegetation generally may occasion changes which have far-off effects on the fauna, scenery, and even climate of a countryside. The majority of plants are dependent on insects, as the unconscious bearers of the pollen essen tial to the normal cross-fertilization of flowers.
Plants and Insects.—Many insects in jure plants without any compensating benefit, and in this connection there are namerous cases in which plants and in sects (especially ants) form a mutual partnership. Such plants are saved by their bodyguard of ants from unwel come visitors, and the benefit is some times returned by the growth of special shelters, tenanted by the partner-msccts.