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Insects are a class of invertebrates within the arthropod phylum that have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all known living organisms. Insects may be found in nearly all environments, although only a small number of species reside in the oceans.
The life cycles of insects vary but most insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages can differ from the adults in structure, habit and habitat. Insects that undergoincomplete metamorphosis lack a pupal stage and adults develop through a series of nymphal stages. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants.
Adult insects typically move about by walking, flying, or sometimes swimming. Insects are the only invertebrates to have evolved flight. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. (see more...)