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Insects or Insecta (from Latin insectum) are hexapod invertebrates and the largest group within the arthropod phylum. Definitions and circumscriptions vary; usually, insects comprise a class within the Arthropoda. As used here, the term Insecta 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. Insects are the most diverse group of animals ; they include more than a million described species and represent more than half of all known living organisms. The total number of extant species is estimated at between six and ten million ; potentially over 90% of the animal life forms on Earth are insects. Insects may be found in nearly all environments, although only a small number of species reside in the oceans , which are dominated by another arthropod group, crustaceans. Nearly all insects hatch from eggs . Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages often differ from the adults in structure , habit and habitat, and can include a passive pupal stage in those groups that undergo four -stage metamorphosis (see holometabolism).