insect, invertebrate animal of the class Insecta of the phylum Arthropoda Arthropoda (ärthrŏp`ədə) [Gr.,=jointed feet], largest and most diverse animal phylum.
..... Click the link for more information. . Like other arthropods, an insect has a hard outer covering, or exoskeleton, a segmented body, and jointed legs. Adult insects typically have wings and are the only flying invertebrates.

The body of the typical adult insect is divided into three distinct parts, the head, thorax, and abdomen. The head bears three pairs of mouthparts, one pair of compound eyes, three simple eyes (ocelli), and one pair of jointed sensory antennae. The thorax is divided into three segments, each with a pair of jointed legs, and bears two pairs of wings. The abdomen has posterior appendages associated with reproduction. The exoskeleton is composed of a horny substance called chitin.

Insects breathe through a complex network of air tubes (tracheae) that open to the outside through a series of small valved apertures (spiracles) along the sides of the body. In chewing insects the digestive system includes a muscular gizzard that is lacking in sucking insects. The simple circulatory system is composed of a tubular heart that pumps blood forward into the head, from which it diffuses through the tissues and back into the heart. The aquatic larvae of many insects breathe by means of external gills; some very primitive species breathe directly through the body wall.

Insect Species

There are about 900,000 known insect species, three times as many as all other animal species together, and thousands of new ones are described each year. They are commonly grouped in 27 to 32 orders, depending upon the classification used. The largest order is that of the beetles beetle, common name for insects of the order Coleoptera, which, with more than 300,000 described species, is the largest of the insect orders. Beetles have chewing mouthparts and well-developed antennae.
..... Click the link for more information. (Coleoptera). Next, in order of size, are the moths moth, any of the large and varied group of insects which, along with the butterflies , make up the order Lepidoptera. The moths comprise the great majority of the 100,000 species of the order, and about 70 of its 80 families.
..... Click the link for more information. and butterflies butterfly, any of a large group of insects found throughout most of the world; with the moths , they comprise the order Lepidoptera. There are about 12 families of butterflies. Most adult moths and butterflies feed on nectar sucked from flowers.
..... Click the link for more information. (Lepidoptera); the wasps wasp, name applied to many winged insects of the order Hymenoptera, which also includes ants and bees. Most wasps are carnivorous, feeding on insects, grubs, or spiders. They have biting mouthparts, and the females have stings with which they paralyze their prey.
..... Click the link for more information. , ants ant, any of the 2,500 insect species constituting the family Formicidae of the order Hymenoptera, to which the bee and the wasp also belong. Like most members of the order, ants have a "wasp waist," that is, the front part of the abdomen forms a narrow stalk, called
..... Click the link for more information. , and bees bee, name for flying insects of the superfamily Apoidea, in the same order as the ants and the wasps. Bees are characterized by their enlarged hind feet, typically equipped with pollen baskets of stiff hairs for gathering pollen.
..... Click the link for more information. (Hymenoptera); and the flies fly, name commonly used for any of a variety of winged insects , but properly restricted to members of the order Diptera, the true flies, which includes the housefly , gnat , midge , mosquito , and tsetse fly .
..... Click the link for more information. and mosquitoes mosquito (məskē`tō), small, long-legged insect of the order Diptera, the true flies .
..... Click the link for more information. (Diptera). Other major orders are the true bugs bug, common name correctly applied to insects belonging to the order Hemiptera, although members of the order Homoptera (e.g., mealybug ) are sometimes referred to as bugs, as are other insects in general.
..... Click the link for more information. (Hemiptera); the cicadas cicada (sĭkā`də), large, noise-producing insect of the order Homoptera, with a stout body, a wide, blunt head, protruding eyes,
..... Click the link for more information. , aphids aphid or plant louse, tiny, usually green, soft-bodied, pear-shaped insect injurious to vegetation. It is also called greenfly and blight. Aphids are mostly under 1-4 in. (6 mm) long.
..... Click the link for more information. , and scale insects scale insect, common name for members of a highly modified group of insects belonging to several families of the superfamily Coccoidea. Scales possess antennae and are characterized by reduced legs. Only the males have wings; females are always wingless.
..... Click the link for more information. (Homoptera); the grasshoppers grasshopper, name applied to almost 9,000 different species of singing, jumping insects in two families of the order Orthoptera. Grasshoppers are long, slender, winged insects with powerful hind legs and strong mandibles, or mouthparts, adapted for chewing.
..... Click the link for more information. and crickets cricket, common name of the slender, chirping, hopping insects forming the family Gryllidae in the order Orthoptera. Most crickets have long antennae, muscular hind legs for jumping, and two pairs of fully developed wings.
..... Click the link for more information. (Orthoptera); the cockroaches cockroach or roach, name applied to approximately 3,500 species of flat-bodied, oval insects forming the order Blattodea. Cockroaches have long antennae, long legs adapted to running, and a flat extension of the upper body wall that conceals the
..... Click the link for more information. (Blattodea); and the mantids mantid or mantis, name applied to the large, slender, slow-moving, winged insects of the family Mantidae in the order Mantodea. Predatory insects, mantids have strong, elongate, spiny front legs, used for grasping prey.
..... Click the link for more information. (Mantodea).

Insects are found throughout the world except near the poles and pervade every habitat except the sea (although there is one marine species of water strider). Fossil records indicate that many species exist today in much the same form as they did 200 million years ago. Their enormous biological success is attributed to their small size, their high reproductive rate, and the remarkable adaptive abilities of the group as a whole, shown by the enormous variety in body structure and way of life. The mouthparts may be adapted to chewing, sucking, piercing, or lapping and the legs for walking, running, jumping, burrowing, or swimming. Insects may feed on plants or decaying matter or prey upon other small animals (especially other insects) or parasitize larger ones; they may be omnivorous or highly specialized in their diets. They display a remarkable variety of adaptive shapes and colors that may serve either as camouflage or as warning (see mimicry mimicry, in biology, the advantageous resemblance of one species to another, often unrelated, species or to a feature of its own environment. (When the latter results from pigmentation it is classed as protective coloration .
..... Click the link for more information. ). Some have stinging spines or hairs and blistering or noxious secretions, used for defense.

Reproduction

A few species, notably the fireflies firefly or lightning bug, small, luminescent, carnivorous beetle of the family Lampyridae. Fireflies are well represented in temperate regions, although the majority of species are tropical and subtropical.
..... Click the link for more information. , produce light, used as a signal in courtship, by a chemical reaction. The sexes are separate in insects, and reproduction is usually sexual, although in many insect groups eggs sometimes develop without fertilization by sperm (see parthenogenesis parthenogenesis (pär'thənōjĕn`əsĭs) [Gr.
..... Click the link for more information. ). In some insects, such as bees, unfertilized eggs become males and fertilized eggs females. In others, such as aphids, all-female generations are produced by parthenogenesis. Eggs are usually laid in a sheltered place; in a few insects they are retained and hatched internally. After hatching, the insect must molt periodically as it grows, since the rigid exoskeleton does not allow much expansion. A new, soft exoskeleton forms beneath the old one, and after each molt the insect undergoes a rapid expansion before its new covering hardens. The stages between molts are called instars; the final instar is the adult.

Metamorphosis

In nearly all insects growth involves a metamorphosis, that is, a transformation in form and in way of life. Complete, or indirect, metamorphosis is characteristic of over 80% of all insect species and has four stages: egg, larva, pupa, and adult. The wingless, wormlike larva (in many species called a grub or a caterpillar) is completely unlike the adult, and its chief activities are eating and growing. Only the simple eyes are present, and the mouth is the chewing type, even in species whose adults have other kinds of mouthparts. After several molts the larva enters a quiescent stage called the pupa; the pupa does not eat and usually does not move, but within the exoskeleton a major transformation occurs that involves the reorganization of organ systems as well as the development of such adult external structures as wings and compound eyes. In some insects the pupa is enclosed in a protective case, called the cocoon, built by the larva just before pupation. When the transformation is complete the final molt occurs: the adult emerges, its wings fill with blood and expand, and the new exoskeleton hardens. The chief function of the adult is propagation; in some species it does not eat.

Incomplete, or gradual, metamorphosis is seen in members of less advanced orders (such as locusts and their relatives and the true bugs). The larva, often called a nymph (or, if aquatic, a naiad) is usually similar in form to the adult, but lacks wings. The wings begin as external bumps on the larva, and the adult emerges from the last molt without having undergone a pupal stage.

In a few very primitive, wingless insects (such as the silverfish silverfish, common name for primitive, wingless insects of the family Lepismatidae. The silverfish, which has two long antennae and three long tail bristles, is named for its covering of tiny, silvery scales.
..... Click the link for more information. ) there is no metamorphosis. The insect emerges from the egg as a miniature adult and the only futher changes are in size and in maturation of the reproductive organs.

Insect Pests

Plant-eating insects cause enormous damage to crops; any part of a plant is subject to attack by either the adult or the larva of some insect. Among the well-known plant pests are the locust, armyworm armyworm, larva of a moth, Pseudaletia unipuncta, found in North America E of the Rocky Mts. When numerous, armyworms move in hordes, traveling by night and devouring grasses, young grains, and some leguminous crops. The full-grown larva is about 2 in.
..... Click the link for more information. , aphid, corn borer corn borer or European corn borer, common name for the larva of a moth of the family Pyralidae, introduced from S Europe into the Boston area in 1917.
..... Click the link for more information. , coddling moth, tent caterpillar tent caterpillar, common name for the larvae of the members of a family of moths (Lasiocampidae), easily recognized by the large silk tents, or webs, that the larvae construct during the spring in the crotches of trees, particularly apple and cherry trees.
..... Click the link for more information. , Japanese beetle Japanese beetle, common name for a destructive beetle, Popillia japonica, of the scarab beetle family. Accidentally imported to the United States from Japan, it was first discovered in New Jersey in 1916 and is now widespread in the northeastern states, where
..... Click the link for more information. , gypsy moth gypsy moth, common name for a moth, Lymantria dispar, of the tussock moth family, native to Europe and Asia. Its caterpillars, or larvae, defoliate deciduous and evergreen trees and shrubs. Introduced from Europe into Massachusetts c.
..... Click the link for more information. , bagworm bagworm, common name for the larva of small moths of the family Psychidae. The larva spins a silken cocoon as it travels, hence the term bagworm. When fully grown, the bagworm fastens its covering to a twig and pupates within it.
..... Click the link for more information. , and scale insect. Insect carriers of human diseases include the mosquito, housefly housefly, common name of the fly Musca domestica, found in most parts of the world. The housefly, a scavenger, does not bite living animals but is dangerous because it carries bacteria and protozoans that cause many serious diseases, e.g.
..... Click the link for more information. , tsetse fly tsetse fly (tsĕt`sē), name for any of several bloodsucking African flies of the genus Glossina,
..... Click the link for more information. , and flea flea, common name for any of the small, wingless insects of the order Siphonaptera. The adults of both sexes eat only blood and are all external parasites of mammals and birds. Fleas have hard bodies flattened from side to side and piercing and sucking mouthparts.
..... Click the link for more information. .

Beneficial Insects

Many insects are valuable as predators on the harmful species, and some are important as scavengers and as aerators of the soil (see scarab beetle scarab beetle or scarab, name for members of a large family of heavy-bodied, oval beetles (the Scarabaeidae), with about 30,000 species distributed throughout most of the world and over 1,200 in North America.
..... Click the link for more information. ). Most important, many plants depend on insects as agents of pollination; in fact, flowering plants and insects evolved together. Insects are the source of useful products such as honey honey, sweet, viscid fluid produced by honeybees from the nectar of flowers. The nectar is taken from the flower by the worker bee and is carried in the honey sac back to the hive.
..... Click the link for more information. , beeswax, silk Wild silk is the product of the tussah worm of India and China, which feeds on oaks. It is now semicultivated, as groves of dwarf trees are provided for its feeding. It spins a coarser, flatter, yellower filament than the Bombyx mori,
..... Click the link for more information. , lac lac, resinous exudation from the bodies of females of a species of scale insect (Tachardia lacca), from which shellac is prepared. India is the chief source of shellac, although some is obtained from other areas in Southeast Asia.
..... Click the link for more information. , and cochineal cochineal (kŏchĭnēl`, kŏch`ĭnēl)
..... Click the link for more information. . They are a major source of food for many animals, and some are eaten by humans in many parts of the world. The fruit fly fruit fly, common name for any of the flies of the families Tephritidae and Drosophilidae. All fruit flies are very small insects that lay their eggs in various plant tissues.
..... Click the link for more information. has been the major experimental animal used in genetics.

Bibliography

See R. F. Chapman, The Insects (1982); M. V. Brian, Social Insects (1983); P. W. Price, Insect Ecology (1984); R. H. Arnett, American Insects (1985); The Audubon Society Field Guide to North American Insects and Spiders (1992).

insect

Body plan of a generalized insect. The body is usually divided into a head, thorax, and abdomen. …

(credit: © Merriam-Webster Inc.)

Any member of the class Insecta, the largest arthropod class, including nearly 1 million known species (about three-fourths of all animals) and an estimated 5–10 million undescribed species. Insect bodies have three segments: head, thorax (which bears three pairs of legs and usually two pairs of wings), and many-segmented abdomen. Many species undergo complete metamorphosis. There are two subclasses: Apterygota (primitive, wingless forms, including silverfish and bristletails) and Pterygota (more advanced, winged or secondarily wingless forms). The approximately 27 orders of Pterygota are generally classified by wing form: e.g., Coleoptera (beetles), Diptera (dipterans), Heteroptera (bugs). Insects are found in almost all terrestrial and freshwater and some marine habitats.

Insect

archy

literary cockroach that cannot reach shift-key on the typewriter. [Am. Lit.: archy and mehitabel; Benét, 46]

bread-and-butter-fly

its body is a crust; lives on weak tea. [Br. Lit.: Lewis Carroll Through the Looking-glass]

Charlotte’s Web

story of a spider who saves a young girl’s pet pig. [Am. Lit.: E. B. White Charlotte’s Web]

gnat

chicken-sized insect that tells Alice all about other strange insects. [Br. Lit.: Lewis Carroll Through the Looking-glass]

Metamorphosis

Gregor Samsa turned into a huge insect. [Ger. Lit.: Kafka Metamorphosis]

rocking-horse-fly

wooden insect feeds on sap and sawdust. [Br. Lit.: Lewis Carroll Through the Looking-glass]

snap-dragon-fly

has a body of plum pudding and lives on mince pie. [Br. Lit.: Lewis Carroll Through the Looking-glass]