INSECTS
Insects or
Insects 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 Insects 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.
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 pupil stage in those groups that undergo four-stage metamorphosis.
Insects that undergo three-stage metamorphosis lack a pupil stage and adults
develop through a series of nymphal stages. The higher level relationship of
the insects 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 to 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. 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,
composed of the front & rear on one side with the middle on the other side.
Insects are the only invertebrates to have evolved flight, and all flying
insects derive from one common ancestor. 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. Insects are
mostly solitary, but some, such as certain bees, ants and 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: crickets
stridulate, or rub their wings together, to attract a mate and repel other
males. Lampyrid beetles communicate with light.
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, fruits, or wood. Some species are parasitic, and may
vector diseases. 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 would be devastated. Many insects are considered
ecologically beneficial as predators and a few provide direct economic benefit.
Silkworms produce silk and honey bees produce honey and both have been
domesticated by humans. Insects are consumed as food in 80% of the world's
nations, by people in roughly 3000 ethnic groups. Human activities also have
effects on insect biodiversity.
All insects
belong to the phylum Arthropoda. But unlike other arthropods—like lobsters,
spiders, or millipedes—insects have three pairs of jointed legs, segmented
bodies, an exoskeleton, one pair of antennae, and (usually) one or two pairs of
wings.
Insects
live in nearly every habitat, and it’s estimated that there are currently 10
quintillion insects on the globe. So far scientists who study bugs, called
entomologists, have named one million insect species but studies estimate that
four million are still uncategorized.
The oldest
insect fossil—a mandible (or jaw) found in Scotland—is between 408 and 438
million years old. The oldest winged fossil dates back 330 million years ago,
suggesting that insects were among the first animals to leave the oceans for
land during the Devonian period some 400 million years ago.
Insects are
vital to every ecosystem. They pollinate plants, decompose plant and animal
matter, and are themselves a source of food. Birds alone are estimated to eat
400 to 500 million tons of insects per year.
