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Insects are invertebrates in the class Insecta, which contains 28 living orders. This class of the phylum Arthropoda is distinguished by a number of anatomical features, including an adult body that is typically divided into three parts (head, thorax, and abdomen), three pairs of segmented legs attached to the thorax, one pair of antennae, and ventilation of respiratory gases through pores called spiracles and along tubes called tracheae. Insect orders in the subclass Pterygota have two pair of wings as adults, but some relatively primitive orders in the subclass Apterygota are wingless.

Insects have a complex life cycle, with a series of intricate transformations (called metamorphosis) occurring between the stages, each of which is radically different in morphology, physiology, and behavior. The most complicated life cycles have four stages: egg, larva, pupa, and adult. Examples of insect orders with this life cycle include butterflies and moths (Lepidoptera) and the true flies (Diptera). Other orders of insects have a less complex, more direct development, involving egg, nymph, and adult. Insect orders with this life cycle include the relatively primitive springtails (Collembola) and the true bugs (Hemiptera).

Most insects are nonsocial. However, some species have developed remarkably complex social behaviors, with large groups of closely related individuals living together and caring for the eggs and young of the group, which are usually the progeny of a single female, known External and internal features of a generalized insect. Illustration by Hans & Cassidy. Courtesy of Gale Group.
as the queen. This social system is most common in the bees, wasps, and ants (Hymenoptera), and in the unrelated termites (Isoptera).

A few species of insects are useful to humans. Some insects, however, are important factors in the transmission of human diseases. For example, malaria, yellow fever, sleeping sickness, and certain types of encephalitis are caused by microorganisms, but are transmitted by particular species of biting flies, especially mosquitoes. Other insects are important defoliators of trees, and can thereby cause substantial damage to commercial timber stands and to shade trees. Insects may also defoliate agricultural plants, or may feed on unharvested or stored grains, thus causing great economic losses. Some insects, particularly termites, cause enormous damage to wood, literally eating buildings constructed of that material. Pesticides—chemicals that are toxic to insects—are sometimes used to control the populations of insects regarded as major pests.

Taxonomists have recognized and named more than one million species of insects—more than have been recognized in any other group of organisms. Of these, approximately three-quarter of a million have been described in some detail. To lend perspective of the vast number of insect species, there are a mere 6,200 bird and 5,800 reptile species described. There are over 10,000 known species of ants alone. In addition, biologists believe that tens of millions of species of insects remain undiscovered. One estimate is that as many as 30 million species of insects inhabit Earth. Most of these are thought to be beetles (Coleoptera). In fact, all of the insect orders are poorly known. Most of these undiscovered species of insects occur in tropical rainforests, especially in the canopy.

Globally, there is an enormous abundance and productivity of insects, and an extraordinary richness of species. These exploit a remarkable diversity of habitats, and are ecologically important as herbivores, predators, parasites, and scavengers. As a result of these attributes, insects are considered to be one of the most successful group of organisms on Earth, if not the most successful.

See also Pesticides.

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