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INSECTS or INSECTA (from Latin _insectum_, a calque of Greek ἔντομον , "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 Arthopoda. 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 all environments , 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 passive pupal stage in those groups that undergo 4-stage metamorphosis (see holometabolism ). Insects that undergo 3-stage metamorphosis lack 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 (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants .

Adult insects typically move about by walking, flying or sometimes swimming (see § Locomotion below). 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 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. Lampyridae in the beetle order 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 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 silk and honey , respectively. In some cultures, people eat the larvae or adults of certain insects.

CONTENTS

* 1 Etymology * 2 Definitions

* 3 Phylogeny and evolution

* 3.1 Evolutionary relationships * 3.2 Taxonomy

* 4 Diversity

* 5 Morphology and physiology

* 5.1 External

* 5.2 Segmentation

* 5.2.1 Exoskeleton

* 5.3 Internal

* 5.3.1 Nervous system

* 5.3.2 Digestive system

* 5.3.2.1 Foregut * 5.3.2.2 Midgut * 5.3.2.3 Hindgut

* 5.3.3 Reproductive system * 5.3.4 Respiratory system * 5.3.5 Circulatory system

* 6 Reproduction and development

* 6.1 Metamorphosis

* 6.1.1 Incomplete metamorphosis * 6.1.2 Complete metamorphosis

* 7 Senses and communication

* 7.1 Light production and vision * 7.2 Sound production and hearing * 7.3 Chemical communication

* 8 Social behavior

* 8.1 Care of young

* 9 Locomotion

* 9.1 Flight

* 9.2 Walking

* 9.2.1 Use in robotics

* 9.3 Swimming

* 10 Ecology

* 10.1 Defense and predation * 10.2 Pollination * 10.3 Parasitism

* 11 Relationship to humans

* 11.1 As pests * 11.2 In beneficial roles * 11.3 In research * 11.4 As food * 11.5 In culture

* 12 See also * 13 References * 14 Bibliography * 15 Further reading * 16 External links

ETYMOLOGY

The word "insect" comes from the Latin word _insectum_, meaning "with a notched or divided body", or literally "cut into", from the neuter singular perfect passive participle of _insectare_, "to cut into, to cut up", from _in_- "into" and _secare_ "to cut"; because insects appear "cut into" three sections. Pliny the Elder introduced the Latin designation as a loan-translation of the Greek word ἔντομος (_éntomos_) or "insect" (as in entomology ), which was Aristotle 's term for this class of life, also in reference to their "notched" bodies. "Insect" first appears documented in English in 1601 in Holland 's translation of Pliny. Translations of Aristotle's term also form the usual word for "insect" in Welsh (trychfil, from _trychu_ "to cut" and _mil_, "animal"), Serbo-Croatian (_zareznik_, from _rezati_, "to cut"), Russian (насекомое _nasekomoje_, from _seč'/-sekat'_, "to cut"), etc.

DEFINITIONS

The precise definition of the taxon Insecta and the equivalent English name "insect" varies; three alternative definitions are shown in the table.

Definition of Insecta GROUP ALTERNATIVE DEFINITIONS

Collembola (springtails) Insecta _sensu lato_ = Hexapoda

Protura (coneheads)

Diplura (two-pronged bristletails)

Archaeognatha (jumping bristletails) Insecta _sensu stricto_ =Ectognatha

Zygentoma (silverfish)

Pterygota (winged insects) Insecta _sensu strictissimo_

In the broadest circumscription , Insecta _sensu lato _ consists of all hexapods . Traditionally, insects defined in this way were divided into "Apterygota" (the first five groups in the table) – the wingless insects – and Pterygota – the winged insects. However, modern phylogenetic studies have shown that "Apterygota" is not monophyletic, and so does not form a good taxon. A narrower circumscription restricts insects to those hexapods with external mouthparts, and comprises only the last three groups in the table. In this sense, Insecta _sensu stricto _ is equivalent to Ectognatha. In the narrowest circumscription, insects are restricted to hexapods that are either winged or descended from winged ancestors. Insecta _sensu strictissimo _ is then equivalent to Pterygota. For the purposes of this article, the middle definition is used; insects consist of two wingless taxa, Archaeognatha (jumping bristletails) and Zygentoma (silverfish), plus the winged or secondarily wingless Pterygota.

PHYLOGENY AND EVOLUTION

_ This section needs to be UPDATED. Please update this article to reflect recent events or newly available information. (July 2017)_

Evolution has produced enormous variety in insects. Pictured are some of the possible shapes of antennae . Main article: Evolution of insects

The evolutionary relationship of insects to other animal groups remains unclear.

Although traditionally grouped with millipedes and centipedes —possibly on the basis of convergent adaptations to terrestrialisation —evidence has emerged favoring closer evolutionary ties with crustaceans . In the Pancrustacea theory, insects, together with Entognatha , Remipedia , and Cephalocarida , make up a natural clade labeled Miracrustacea .

A report in November 2014 unambiguously places the insects in one clade, with the crustaceans and myriapods , as the nearest sister clades. This study resolved insect phylogeny of all extant insect orders, and provides "a robust phylogenetic backbone tree and reliable time estimates of insect evolution."

Other terrestrial arthropods, such as centipedes, millipedes, scorpions , and spiders , are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointed exoskeleton. However, upon closer examination, their features differ significantly; most noticeably, they do not have the six-legged characteristic of adult insects.

Hexapoda (Insecta, Collembola , Diplura , Protura )

Crustacea (crabs , shrimp , isopods , etc.)

Myriapoda

Pauropoda

Diplopoda (millipedes)

Chilopoda (centipedes)

Symphyla

Chelicerata

Arachnida (spiders , scorpions , mites , ticks , etc.)

Eurypterida (sea scorpions: extinct)

Xiphosura (horseshoe crabs)

Pycnogonida (sea spiders)

Trilobites (extinct)

A phylogenetic tree of the arthropods and related groups

The higher-level phylogeny of the arthropods continues to be a matter of debate and research. In 2008, researchers at Tufts University uncovered what they believe is the world's oldest known full-body impression of a primitive flying insect, a 300 million-year-old specimen from the Carboniferous period . The oldest definitive insect fossil is the Devonian _ Rhyniognatha hirsti _, from the 396-million-year-old Rhynie chert . It may have superficially resembled a modern-day silverfish insect. This species already possessed dicondylic mandibles (two articulations in the mandible), a feature associated with winged insects, suggesting that wings may already have evolved at this time. Thus, the first insects probably appeared earlier, in the Silurian period.

Four super radiations of insects have occurred: beetles (evolved about 300 million years ago), flies (evolved about 250 million years ago), and moths and wasps (evolved about 150 million years ago). These four groups account for the majority of described species. The flies and moths along with the fleas evolved from the Mecoptera .

The origins of insect flight remain obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects had an additional pair of winglets attaching to the first segment of the thorax, for a total of three pairs. As of 2009, no evidence suggests the insects were a particularly successful group of animals before they evolved to have wings.

Late Carboniferous and Early Permian insect orders include both extant groups, their stem groups, and a number of Paleozoic groups, now extinct. During this era, some giant dragonfly-like forms reached wingspans of 55 to 70 cm (22 to 28 in), making them far larger than any living insect. This gigantism may have been due to higher atmospheric oxygen levels that allowed increased respiratory efficiency relative to today. The lack of flying vertebrates could have been another factor. Most extinct orders of insects developed during the Permian period that began around 270 mill