insect, invertebrate animal of the class Insecta of the phylum Arthropoda. 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.
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 (Coleoptera). Next, in order of size, are the moths and butterflies (Lepidoptera); the wasps, ants, and bees (Hymenoptera); and the flies and mosquitoes (Diptera). Other major orders are the true bugs (Hemiptera); the cicadas, aphids, and scale insects (Homoptera); the grasshoppers and crickets (Orthoptera); the cockroaches (Blattodea); and the mantids (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). Some have stinging spines or hairs and blistering or noxious secretions, used for defense.
A few species, notably the fireflies, 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). 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.
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) 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.
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, aphid, corn borer, coddling moth, tent caterpillar, Japanese beetle, gypsy moth, bagworm, and scale insect. Insect carriers of human diseases include the mosquito, housefly, tsetse fly, and flea.
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). 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, beeswax, silk, lac, and cochineal. 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 has been the major experimental animal used in genetics.
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); H. Raffles, Insectopedia (2010); M. Zuk, Sex on Six Legs (2011).