The immature stages can 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).Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages.
The most diverse insect groups appear to have coevolved with flowering plants.
Adult insects typically move about by walking, flying, or sometimes swimming.
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.
Some species, such as water striders, are capable of walking on the surface of water.
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.
Clockwise from top left: dance fly (Empis livida), long-nosed weevil (Rhinotia hemistictus), mole cricket (Gryllotalpa brachyptera), German wasp (Vespula germanica), emperor gum moth (Opodiphthera eucalypti), assassin bug (Harpactorinae)], "cut into sections") are by far the largest group of hexapod invertebrates within the arthropod phylum.
Definitions and circumscriptions vary; in one approach insects comprise a class within the Phylum Arthropoda.
Humans regard certain insects as pests, and attempt to control them using insecticides and a host of other techniques.