Insects, six-footed articulated animals, the most beautiful, most active, and most highly organized of the invertebrata, in which, anatomically considered, they bear a remarkable analogy to birds among the vertebrates. Like birds they inhabit the air, earth, and water, have an extensive respiratory apparatus, and consequently a higher calorific and motor power than any other invertebrates. The number of species and of individuals is exceedingly great, and their metamorphoses are among the most interesting phenomena in nature. The class of insects includes all articulates having a distinct head, thorax, and abdomen, with antennae, three pairs of feet, an aerial respiration by means of tracheae, a feebly developed circulating system, almost all being winged and undergoing transformation. The cutaneous envelope forms a kind of external skeleton, generally of a horny consistence, formed principally of chitine; it is made up of a considerable number of pieces more or less movable on each other, and is frequently provided with hairs, which are sometimes sharp and barbed (as in the processionary caterpillars), producing considerable irritation when introduced into the human skin. The limbs, which are appendages of the thorax, are hollow tubes containing the muscles and nerves for their motion.

The first segment constitutes the head, on which are placed the antennae, the eyes, and the oral appendages. The antennae are composed of a variable number of joints, generally resembling delicate and flexible horns, plumed, serrated, clubbed, or foliated, according to genera and families; they are principally organs of touch. The mouth in the chewing insects, like the beetles, cockroaches, and grasshoppers, consists of an upper middle piece or labrum with a mandible on each side, the latter being very hard, often toothed at the extremity; the maxilla or under jaws are softer and of many pieces, with maxillary jointed palpi, and the central piece is the labium or under lip, also supporting jointed palpi. At the base of the under lip is attached the tongue, which in some is abortive and in others long and changed into a suctorial organ. In the sucking insects the under lip is transformed into a tube, enclosing delicate lancet-like filaments or bristles, modifications of the mandibles and maxillae; in the hymenoptera (bees, etc.) the mouth is intermediate between the chewing and the suctorial, having parts belonging to both; in the lepidoptera (butterflies, etc.) the mandibles are very small, but the under jaws are changed each into a semi-canal which may be rolled up spirally.

The eyes are either simple or compound, the first occurring chiefly in the larvae of the metamorphic orders, and the second in perfect insects; some have both kinds in the perfect state, and some adults, larvae, and pupae are blind. The compound organ is made up of many simple eyes, each having its cornea, conical vitreous body, pigment, and nervous filament; the number of these facets is sometimes more than 25,000. The simple eyes (stemmata) consist of a cornea, a lens lodged in an expansion of the optic nerve, and a surrounding pigment layer; they are placed either on the sides of the head, or in small groups on the vertex. The thorax supports the legs and wings, and consists always of three rings, called respectively pro-thorax, mesothorax, and metathorax, each bearing on its ventral arch a pair of legs; the wings arise from the dorsal aspect of the two posterior rings. The limbs consist each of a two-jointed hip, a thigh, a leg, and a kind of finger or tarsus of two to five joints terminated by the claws; in the jumpers, like the grasshoppers, the hind legs are very long and muscular; in the swimmers, like the water beetles, the tarsi are flattened, ciliated, and arranged for oars; in the flies, the feet are provided with pads and hooks by which they are enabled to hang suspended from smooth surfaces; the anterior limbs are often enlarged, as in the mole crickets, which dig in the ground, and armed with spines, as in the mantis, which uses them to seize its prey; in some of the butterflies the anterior limbs are mere rudiments, useless as means of progression.

The wings are membranous expansions, rendered firm by solid nervures; there are never more than two pairs, and one or the other may be wanting; in the butterfly they are covered with a colored dust consisting of microscopic scales; in the beetles the first pair becomes thick and hard, forming the elytra, which cover and protect the second pair; the wings are sometimes half membranous, half corneous, at others divided into barbed plumules, or wanting and replaced by the knob-like balancers. The legs and wings are moved by striated muscles, attached directly to the cutaneous skeleton; those of the wings of the diptera have their fibrillae separable into series of disks, the astonishing rapidity of their movements being dependent on alternate contraction and relaxation. The abdomen is composed of rings movable upon each other, sometimes to the number of nine; they bear in the perfect insect neither legs nor wings, but are provided with various appendages useful in the economy of the animal, as the delicate bristles of the ephemera, the nippers of the earwig, the spring of the podurella, the sting of the bee and wasp, and the ovipositor of the grasshopper and the ichneumons.

Besides the antennae, the palpi about the mouth, the end of the suctorial tube, the ovipositor, and the feet in some instances, are delicate organs of touch; the tongue, when present, as in bees and flies, is undoubtedly the seat of an acute sense of taste. Though insects apparently perceive by the sense of smell what food is proper for themselves or their young, the seat of this sense has not been satisfactorily determined; Dumeril and Cuvier, reasoning from analogy, concluded that it was placed at the openings of the respiratory tracheae; Hu-ber, from his experiments on bees, placed it in the mouth, Kirby in the anterior portion of the head or the nose, and others in the antennae and palpi. Hearing is acute in many insects; the shrilling of the locust, the tick of the deathwatch, the song of the cricket, etc, would be useless unless they could be heard by their companions; in the orthoptera especially an auditory apparatus is connected with the stigmata of the thorax and the anterior legs; the sense has also been placed inward at the base of the antennae.

The sounds of insects are produced by the friction of one part of the external skeleton on another, by the vibration of special organs, or by a particular soniferous apparatus, always due to the action of voluntary muscles and unconnected with the respiratory system; the buzzing of flies seems to depend on the rapid vibrations of the thorax during flight and on the passage of air through the thoracic stigmata, perhaps intensified by the motions of the wings themselves; some beetles produce a sharp sound by rubbing the last abdominal segments against the curved points of the wing covers, or the thoracic rings against each other; the sounds of butterflies and of the death's-head moth are referred to friction of the hips together, and to various causes not at all satisfactory. The nervous system consists of a brain and spinal cord; the former is constituted by the ganglia which embrace the oesophagus, and is situated in the first segment; the spinal cord is made up generally of a double series of ganglia united by longitudinal cords, in number corresponding to that of the segments of the body; the three thoracic ganglia are much the largest, and from them are given off the nerves to the legs and wings.

The alimentary canal is generally complicated and more or less convoluted; it consists of a pharynx, oesophagus, first stomach or crop, second or gizzard with muscular walls for trituration, third or chylific ventricle of soft and delicate texture, a small intestine, caecum, and rectum; as in the higher animals, it is shortest in the carnivorous families, and very long in the vegetable feeders; it is kept in place by numerous fine trachea) which envelop its whole extent; in the sucking insects there is only a sucking stomach opening from the oesophagus, into which the fluid food is first taken, as in the first stomach of ruminants. The anus opens on the last segment, except in some non-feeding pupae, in which both it and the mouth are wanting; the salivary glands are well developed, opening into the pharynx; the villosities of the third stomach seem to secrete a gastric juice, the biliary secretion being poured into this cavity; the office of a liver is performed by caecal appendages lying upon the ventricle; similar organs on the small intestine sometimes perform the office of a pancreas.

An adipose tissue is found in all insects, especially toward the end of the larva state, gradually disappearing in the perfect condition, freely traversed by trachean branches; the fatty contents are intimately connected with the functions of nutrition. The circulatory system consists of a contractile chambered dorsal vessel which serves as a heart, and a cephalic aorta which conducts the blood into the body; the blood moves from behind forward, and passes from the aorta all over the system, forming regular currents without vascular walls, and returning as venous blood to the lateral vessels; the blood is usually a colorless liquid, containing a few small oval corpuscles. Respiration is carried on by a system of tracheae spread through the entire body, which open externally by stigmata, and admit air either directly or by means of lamelliform or tubular prolongations which have been compared to branchiae; they divide into branches, gradually becoming smaller, ending caecally, so that the air passes out by the same way that it enters. The branchial tracheae are found in certain aquatic larvae and pupae, and never in the perfect insect; they do not communicate externally, but the air is received by en-dosmosis and exosmosis.

The stigmata of the pulmonary tracheae are usually bordered with a fringe of hairs, and can be opened and shut by internal muscles, whose action gives to the abdomen of many insects well marked movements of respiration; there is generally a pair on the upper portion of the interstices between the rings, being wanting between the head and prothorax and the last two abdominal segments; the tracheae are often dilated into large reservoirs of air. Respiration is very active in insects, and performed by the movements of the abdominal segments; they require a great deal of air, and are very quickly asphyxiated by deprivation of oxygen; though not producing much animal heat ordinarily, sometimes, as in the bees when hived, the respiration is accelerated and their temperature perceptibly elevated. The Malpighian vessels, which were formerly supposed to be biliary, are now ascertained to be urinary organs, secreting uric acid products; they are small convoluted tubes, yellowish or brownish, and open into the posterior extremity of the stomach.

Many insects have secretory follicles just under the skin, whose ducts open between the segments or between the joints of the limbs, or by the side of the anus; the fluid secreted is generally of a disagreeable odor, and sometimes, as in the bugs, very fetid. The females in many of the hymenoptera, as the bees and wasps, have a glandular apparatus in the anal region, which secretes an irritating poison introduced into the tissues of their enemies by their hollow stings. Most insects undergoing a complete metamorphosis have in their larva state silk organs, whose secretion they use in the formation of their cocoons and webs; they consist of two long, flexuous tubes on the side of the body, continuous in front with two small excretory ducts opening on the under lip; in a few the silk is spun from a spinneret projecting from the anus; the wax-secreting apparatus has been described under Bee. The sexes are distinct, and the females often differ greatly from the males, as in the glow-worm; among the bees and ants the females are much less numerous than the males, and certain individuals of neither sex, or neuters, do the work and protect the colony.

Most insects lay eggs, though a few, like the aphides, are viviparous; by means of an ovipositor many introduce their eggs into a deep-seated nidus, in or near which the young can find the food suited for them, almost always different from that required by the parents. There are generally two symmetrical ovaries and testes, situated in the abdominal cavity, and two oviducts uniting into a single one at the posterior end of the body. In their progress to maturity insects change their skins many times, and many of them undergo transformations as singular as those already mentioned in the frogs; on coming from the egg they are very different from their parents and from their pupa forms. Before arriving at their perfect state they usually pass through the larva and pupa form, which may be entirely different, or vary chiefly in the development of wings, according as the metamorphosis is complete or not. Insects with complete metamorphosis when they leave the egg or are in the larva state are more or less worm-like, with an elongated soft body divided into movable rings, normally 13 in number, sometimes with and sometimes without feet; in no respect do they resemble the parents; the eyes are generally simple, and occasionally absent; the mouth is almost always armed with jaws for chewing, even in insects which are sucking in the perfect state; these larvae are called caterpillars or maggots, according to their size, form, and habitat.

After remaining in this state, either in the water, in the air, or under ground, a certain length of time, varying according to the species, and undergoing several moults, rudimentary wings form under the skin, and they change into nymphs, chrysalids, or pupae; the larval condition persists sometimes for several months, as from the autumn to the following summer, and in the case of the harvest fly for a much longer period. Larvae are generally voracious and active, hut nymphs are as generally motionless and do not eat; sometimes the larval skin hardens into a shell-like covering for the nymph; at others a thin investing pellicle applied to the body permits the animal to be seen through it. Before undergoing this change the larva often prepares a shelter, making a cocoon of silk secreted by itself; the nymph may be suspended from a twig by silken filaments or concealed in some crevice. In the nymph state growth takes place rapidly, and the form of the future insect is gradually assumed. The metamorphoses are easily studied in the common caterpillars, the bee, the mosquito, the fly, and the silkworm. The life of the perfect insect is short, enduring at most for the summer months, until the work of reproduction is completed; in the ephemera the adult state continues for a few hours only.

As instances of incomplete metamorphosis may be mentioned the cockroach, the cricket, the grasshopper, and other orthoptera, in which the larva differs from the perfect insect principally in the absence of wings. For further details on larvae and pupae, the reader is referred to Caterpillar, Chrysalis, and the various insects in their respective order. - As insects furnish food for a great variety of vertebrate and invertebrate animals, their extermination would ensue were it not for their astonishing fecundity, paralleled only in the case of fishes; a female termes (ant) has been estimated to lay about 90,000 eggs in a day; the queen bee deposits between 5,000 and 6,000, the common ant about 1,000 less, the wasp about 3,000; a posterity of 1,000 in one generation is common; in the silkworm the average is 500; the beetles are far less prolific. Reaumur observed 350 young one3 developed from the numerous eggs of a moth (phalaena), many of which died as caterpillars, so that only 65 females reached the perfect state; these were calculated to produce the following year 22,750, which in the next would produce 1,500,000. A single plant louse (aphis), which brings forth a numerous progeny, but only one at a time, according to the above author's calculation, would produce in the fifth generation about 6,000,000,000, the great-great-grandmother laying eggs when the ninth member of her descendants is capable of reproduction, without contact with the male. - The muscular activity of insects is very great, whether in leaping, swimming, flying, digging, or carrying weights; no mammal can leap in proportion so high or so far as the flea, to a distance more than 200 times the length of its own body; no bird has a facility of motion, and a rapidity and endurance of flight, comparable to those of insects.

The wings of the butterfly have been found to display the structure ascertained by civil engineers to combine the greatest lightness with the greatest strength; in the nervure of the wing, as in the strongest beam, the utmost possible material is thrown into the flanges, and the upright support is as thin as practicable; in the hollow nervures we have two flanges connected by the thin membrane of the wing, and the strongest nervure at or near the anterior edge. The apparatus by which many insects walk upon perpendicular surfaces is described in the article Fly. The larva of the ant lion digs its sand pit, and the fossorial wasp a hole for its eggs, in a very short time; a few ants are strong enough to drag from their hill a large caterpillar; a few burying beetles will place a mole under the earth in an hour, a feat equivalent to as many men burying a large whale in the same space of time; the gadfly is faster than the fleetest horse; a humblebee has been known to distance a locomotive going at the rate of 20 miles an hour, and a dragon fly to lead a swallow a weary chase of an hour, and at last escape. The instincts of insects, which sometimes closely border upon intelligence, are very remarkable, and calculated to excite the admiration of the most superficial observer.

Insects apparently acquire knowledge from experience, possess the faculty of memory, and are able to communicate their purposes to their fellows; they evince great sagacity in their methods of procuring food and in defending themselves against their enemies; their devices for entrapping prey are very ingenious: to escape their enemies, some feign death, and others conceal themselves, fight bravely with their jaws and stings, and emit a nauseous odor or corrosive juices. As examples of insect instincts we need only mention those of the bee, wasp, and ant in constructing their habitations, of the silkworm, of the caterpillars (like tortrix and the clothes moth) which roll up leaves or woolly materials for their protection, of insects which unite in communities for mutual protection and support, and of those which lay their eggs on substances most proper for their young, which they will never see, and which feed on matters entirely different from the food of their parents (as the wasps). In their adaptation of these instincts to accidental circumstances, they approach very near to intelligent acts.

Insects have many passive means of avoiding their enemies in the form and structure of their bodies, and in their resemblance in color to the objects on which they live, whether ground or tree, as in beetles, grasshoppers, the mantis, and many bugs living on bark; the larvae of tortoise beetles are spiny, others are hairy, and consequently avoided by insectivorous birds; hardness of integument and tenacity of life are also important means of preservation. The continuance of the species is secured by the strong sexual impulse, and by the care of the female in depositing her eggs in places where the future welfare of the young will be insured; the life of the insect generally ceases soon after the period of sexual activity; among the social insects, the young are fed by the neuters and females. For details, see Kirby and Spence's "Introduction to Entomology." - The relations of insects to the rest of organic nature are very interesting and important. Most insects derive their food from the vegetable kingdom, to which they are both injurious and beneficial; by their simple agency not only is a limit set to the increase of plants, but their preservation is due in many instances to insect operations.

Myriads of larvae feed upon the roots, leaves, flowers, fruits, wood, and seeds of plants, not sparing the grains and vegetables most useful to man; the work of Dr. Harris on the "Insects Injurious to Vegetation " gives ample details on this point as far as the northern portion of the United States is concerned, and many of his observations are given in this work in the articles relating to these destructive creatures. On the other hand, fecundation in plants is often promoted by insects; butterflies, bees, wasps, flies, and beetles convey the pollen to the female organs, and thus impregnation is effected in many cases where it would otherwise be unlikely to occur. Insects afford food for each other, for spiders, for many fresh-water fishes, amphibians, reptiles, birds, and mammals; and the last two, with man himself, are infested with many parasitic insects. (See Epizoa.) The direct advantages derived from insects by man are not a few; many larva3 of beetles, grasshoppers, and locusts, South American ants, etc, are occasionally used as food by various savage tribes; the bee supplies honey and wax, the coccus manna and cochineal, the Spanish fly a well known blistering drug, the gall insects a valuable astringent, the silkworm a most valuable and beautiful material for clothing, etc.; and the larvae of flies and many beetles are useful in removing decomposing animal matters. - Insects are found everywhere, even on the surface of the ocean (hydrometradae), but they are essentially animals of the air; though a few may be seen in winter, most are active only in the other seasons; the winter is passed in a state of hibernation, either as eggs, larvae, pupae, or in a few instances as perfect insects; those of tropical regions are the largest, most numerous, and most gorgeously arrayed; they have been found within eight degrees of the north pole, but their geographical distribution has not received the attention it deserves; some are restricted within narrow limits, while others exist almost everywhere.

Insects of a former geological age are found in amber, a fossil resin, in most cases coming very near existing forms, and sometimes of living genera; the number of species thus found is considerable, and, though pertaining only to such as dwelt in woods or on trees, it may reasonably be concluded that then, as now, the insect world was well filled; the beetles are well represented, the hymenoptera very abundant, the lepidoptera exceedingly rare, the diptera and neuroptera very numerous, and the orthoptera and hemiptera not common. Insect impressions have been described in the calcareous formations, especially such as might have been made by aquatic larvae and insects; Dr. Hitchcock describes footmarks in the sandstones of the Connecticut valley as having been made probably by several genera of insects; and Prof. C. F. Hartt has discovered near St. John, N. B., fossil remains of insects in the upper Devonian formation, which he considers the oldest known. - For the systematic classification of insects, and the history of the science, see Entomology.