Insect

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Insects or Insecta (from Latin insectum,) are by far the largest group ofhexapod invertebrates within the arthropod phylum. Definitions and circumscriptions vary; in one approach insects comprise a class within the Phylum Arthropoda. As the term is used here, it is synonymous with Ectognatha.

Insects 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 the most diverse group of animals on the planet, including more than a million described species and representing more than half of all known living organisms. The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the differing animal life forms on Earth. Insects may be found in nearly allenvironments, although only a small number of species reside in the oceans, a habitat dominated by another arthropod group,crustaceans.

The life cycles of insects vary but most 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, and can include a passivepupal stage in those groups that undergo 4-stage metamorphosis. Insects that undergo 3-stage metamorphosisl ack a pupal stage and adults develop through a series of nymphal stages. The higher level relationship of the Hexapoda is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm. The most diverse insect groups appear to have coevolved with flowering plants.