Insects or Insecta (from
Latin insectum, a
Greek ἔντομον [éntomon], "cut into sections") are by far the largest group of
invertebrates within the
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
exoskeleton, a three-part body (
abdomen), three pairs of jointed
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
 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 all
environments, although only a small number of species reside in the oceans, a habitat dominated by another arthropod group,
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 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
 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 (22–28 in). The most diverse insect groups appear to have
Adult insects typically move about by walking, flying or sometimes swimming 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. Insects are mostly solitary, but some, such as certain
termites, are social and live in large, well-organized colonies. Some insects, such as
earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male
moths can sense the
pheromones of female moths over great distances. Other species communicate with sounds:
stridulate, or rub their wings together, to attract a mate and repel other males.
Lampyridae in the
beetle order communicate with light.
. As it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles. Insects are the only invertebrates to have evolved flight. Many insects spend at least part of their lives under water, with
Humans regard certain insects as
pests, and attempt to control them using
insecticides and a host of other techniques. Some insects damage crops by feeding on sap, leaves or fruits. A few
parasitic species are
pathogenic. Some insects perform complex ecological roles;
blow-flies, for example, help consume
carrion but also spread diseases. Insect
pollinators are essential to the life cycle of many flowering plant species on which most organisms, including humans, are at least partly dependent; without them, the terrestrial portion of the biosphere (including humans) would be devastated.
 Many other insects are considered ecologically beneficial as predators and a few provide direct economic benefit.
Silkworms and bees have been used extensively by humans for the production of
honey, respectively. In some cultures, people eat the larvae or adults of certain insects.