Insect

Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the earth, with over a million described species — about three quarters of all animal species. Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans where crustaceans tend to predominate instead.

Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favoring a lower figure of about six to ten million.

The study of insects (from Latin insectus, meaning "cut into sections") is called entomology, from the Greek εντομος, also meaning "cut into sections." This is from the three body sections that characterize most insects: head, thorax and abdomen.

Longhorn beetle on a car hood, showing mouthparts

Silver-spotted skipper nectaring on button bush

Assassin bug Zelus sp. on boxwood, Georgetown, SC

Insects also generally have three pairs of legs and one or two pairs of wings arising from the thorax, and one pair of sensory antennae arising from the head, although there are some exceptions. Their bodies are segmented with an external skeleton composed mostly of chitin. Insects generally have compound eyes.

Other terrestrial arthropods, such as centipedes, millipedes, scorpions and spiders, are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointed exoskeleton. However upon closer examination their features differ significantly; most noticeably they do not have the six legs characteristic of adult insects.

Roles in the environment and human society

Many insects are considered pests by humans. Insects commonly regarded as pests include those that are parasitic such as (mosquitoes, lice, and bedbugs. Pest insects also may transmit diseases. (Mosquitoes are vectors of malaria, dengue, encephalitis, yellow fever and west Nile virus among others. Flies may carry diarrhea, typhoid fever, dysentery, tuberculosis, and shigellosis as well as many other diseases. Kissing bugs) are vectors of Chagas disease. Other pest insects may damage structures - (termites, carpenter ants, powderpost beetles).

agricultural crops or stored foods may be damaged or destroyed by insects such as (locusts, plum Curculio, aphids, leaf miners, Japanes beetles, tent caterpillars, gypsy moths, and weevils). It is impossible to entirely eliminate pest insects from the human food chain; insects already are present in many foods, especially grains. Food laws in many countries do not prohibit insect parts in food, but rather limit the quantity.

Many entomologists are involved in various forms of pest control, often using insecticides, but more and more relying on methods of biocontrol.

Although pest insects tend to attract the most attention, more insects are considered (-a value judgement from a human point of view) beneficial to the environment and to humans. Less than one per cent of all insects are pests, and only a few hundred species are serious pests. Likewise some insects function in mixed roles, for example species of moths that may be pests in the larval stage of life, but pollinators in the adult stage.

Pollination of flowering plants is a major role of bees, syrphid flies, butterflies, wasps, beetles, and ants. Pollination is a trade between plants that need sexual reproduction, and pollinators that receive nutritional rewards of nectar and pollen.

A serious environmental problem today is the decline of populations of pollinator insects, and a number of species of insects are now cultured primarily for pollination management in order to have sufficient pollinators in the field, orchard or greenhouse at bloom time. Some species of plants are threatened primarily because of the loss of their pollinators.

Insects also produce useful substances such as honey, wax, lacquer and silk. Honey bees have been cultured by humans for thousands of years for honey, although contracting for crop pollination is becoming more significant for beekeepers. The silkworm has greatly affected human history, as silk-driven trade established relationships between China and the rest of the world. Fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene, as they would only consume dead flesh. This treatment is finding modern usage in some hospitals. Adult insects such as crickets, and insect larvae of various kinds are also commonly used as fishing bait.

In some parts of the world, insects are used for human food ("entomophagy"), while being a taboo in other places.