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insect
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{{other uses}}{{pp-pc1|small=yes}}{{pp-move-indef}}{{good article}}{{Use dmy dates|date=January 2015}}{{Automatic taxobox| Protura (coneheads)| Diplura (two-pronged bristletails)| Zygentoma (silverfish)
396Devonian (but see text) – Holocene>Present| image = Insect collage.png| image_caption = 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)| image2 = A Magicicada chorus containing M. septendecim, M. cassini, and M. septendecula - pone.0000892.s004.oga| image2_caption = A chorus of several Magicicada species| display_parents = 2| taxon = InsectaCarl Linnaeus>Linnaeus, 1758| subdivision_ranks = Subgroups#Taxonomy>See text.| synonyms =
  • Ectognatha
  • Entomida
}}Insects or Insecta (from Latin ') 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.BOOK, Chapman, A.D., 2006, Numbers of living species in Australia and the World, Canberra: Australian Biological Resources Study, 978-0-642-56850-2,weblink WEB,weblinkweblink" title="web.archive.org/web/20150220154543weblink">weblink 20 February 2015, Threats to Global Diversity, Wilson, E.O., 17 May 2009, The total number of extant species is estimated at between six and ten million;JOURNAL, Novotny, Vojtech, Basset, Yves, Miller, Scott E., Weiblen, George D., Bremer, Birgitta, Cizek, Lukas, Drozd, Pavel, 2002, Low host specificity of herbivorous insects in a tropical forest, Nature (journal), Nature, 11976681, 416, 6883, 841–844, 10.1038/416841a, 2002Natur.416..841N, BOOK, Erwin, Terry L., 1997, Biodiversity at its utmost: Tropical Forest Beetles, 27–40,weblink's%20updated%20pub%20PDFs%2010Jan2014/121_1995_Biodiversity-at-its-utmost.pdf, In: BOOK, Reaka-Kudla, M.L., Wilson, D.E., Wilson, E.O., Biodiversity II, Joseph Henry Press, Washington, D.C., potentially over 90% of the animal life forms on Earth are insects.JOURNAL, Erwin, Terry L., 1982, Tropical forests: their richness in Coleoptera and other arthropod species, The Coleopterists Bulletin, 36, 74–75,weblink 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. Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages."insect physiology" McGraw-Hill Encyclopedia of Science and Technology, Ch. 9, p. 233, 2007 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.ENCYCLOPEDIA,weblink Vincent Brian Wigglesworth, Insect, Encyclopædia Britannica online, 19 April 2012, 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.Damian Carrington. "Insects could be the key to meeting food needs of growing global population", The Guardian 1 August 2010. Retrieved 27 February 2011.BOOK, Creepy crawly cuisine: the gourmet guide to edible insects, Ramos-Elorduy, Julieta, Menzel, Peter, 1998, Inner Traditions / Bear & Company, 978-0-89281-747-4, 44,weblink 23 April 2014,

Etymology

The word "insect" comes from the Latin word , meaning "with a notched or divided body", or literally "cut into", from the neuter singular perfect passive participle of , "to cut into, to cut up", from in- "into" and secare "to cut";WEB,weblink Online Etymological Dictionary, Harper, Douglas, Dan McCormack, November 2001, LogoBee.com, 1, 1 November 2011,
because insects appear "cut into" three sections. A calque of Greek ['], "cut into sections", Pliny the Elder introduced the Latin designation as a loan-translation of the Greek word (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 (, from ' "to cut" and mil, "animal"), Serbo-Croatian (zareznik, from rezati, "to cut"), Russian ( nasekomoje, from seč'/-sekat', "to cut"), etc.WEB,weblink Insect translations,

Definitions

The precise definition of the taxon Insecta and the equivalent English name "insect" varies; three alternative definitions are shown in the table.{| class="wikitable"|+ Definition of Insecta! Group !!colspan=3| Alternative definitions
Collembola (springtails) >Insecta sensu lato=Hexapoda rowspan=3Entognatha(paraphyletic) >Apterygota(wingless hexapods)(paraphyletic)
Archaeognatha (jumping bristletails) >Insecta sensu stricto=Ectognatha
Pterygota (winged insects) >|Insecta sensu strictissimo
In the broadest circumscription, Insecta sensu lato consists of all hexapods.{{sfn|Chinery|1993|p=10}} 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.{{sfn|Chinery|1993|pp=34–35}} 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

{{Update|section|date=July 2017}}{| class="wikitable" style="float:right; width:20em;"|{{clade| style=font-size:75%;line-height:75%|1={{clade|1={{cladeHexapoda (Insecta, Springtail>Collembola, Diplura, Protura)Crustacean>Crustacea (crabs, shrimp, isopods, etc.)}}|label2=Myriapoda|2={{clade|1=PauropodaMillipede>Diplopoda (millipedes)Centipede>Chilopoda (centipedes)|4=Symphyla}}|label3=Chelicerata|3={{clade|1=Arachnida (spiders, scorpions, mites, ticks, etc.)|2=Eurypterida (sea scorpions: extinct)|3=Xiphosura (horseshoe crabs)Sea spider>Pycnogonida (sea spiders)}}|4=†Trilobites (extinct)}}}}|A phylogenetic tree of the arthropods and related groupsWEB, Arthropoda, Tree of Life,weblink Tree of Life Web Project, 1995, 9 May 2009, File:Insect antennae comparison.jpg|thumb|Evolution has produced enormous variety in insects. Pictured are some possible shapes of antennae.]]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 terrestrialisationJOURNAL, Russell Garwood, Gregory Edgecombe, 2011, Early terrestrial animals, evolution and uncertainty, Evolution, Education, and Outreach, 4, 3, 489–501, 10.1007/s12052-011-0357-y,weblink —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.WEB, Palaeos invertebrates:Arthropoda,weblink Palaeos Invertebrates, 3 May 2002, 6 May 2009,weblink" title="web.archive.org/web/20090215010139weblink">weblink 15 February 2009, Insects form a single clade, closely related to crustaceans and myriapods.JOURNAL, Misof, et.al., Bernhard, Phylogenomics resolves the timing and pattern of insect evolution, Science, 7 November 2014, 346, 6210, 763–767, 10.1126/science.1257570, 25378627,weblink 2014Sci...346..763M, 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.WEB, Evolution of insect flight,weblink Malcolm W. Browne, 25 October 1994, 6 May 2009, 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.WEB, Researchers Discover Oldest Fossil Impression of a Flying Insect, 14 October 2008, Newswise,weblink 21 September 2014, 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.JOURNAL, Engel, Michael S., David A. Grimaldi, 2004, New light shed on the oldest insect, Nature, 427, 627–630, 10.1038/nature02291, 14961119, 6975, 2004Natur.427..627E, JOURNAL, Rice, C.M., Ashcroft, W.A., Batten, D.J., Boyce, A.J., Caulfield, J.B.D., Fallick, A.E., Hole, M.J., Jones, E., Pearson, M.J., Rogers, G., Saxton, J.M., Stuart, F.M., Trewin, N.H., Turner, G., 1995, A Devonian auriferous hot spring system, Rhynie, Scotland, Journal of the Geological Society, London, 152, 229–250, 10.1144/gsjgs.152.2.0229, 2, 1995JGSoc.152..229R, 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).JOURNAL, Wiegmann BM, Trautwein MD, Winkler IS, Barr NB, Kim JW, Lambkin C, Bertone MA, Cassel BK, Bayless KM, Heimberg AM, Wheeler BM, Peterson KJ, Pape T, Sinclair BJ, Skevington JH, Blagoderov V, Caravas J, Kutty SN, Schmidt-Ott U, Kampmeier GE, Thompson FC, Grimaldi DA, Beckenbach AT, Courtney GW, Friedrich M, Meier R, Yeates DK, 2011, Episodic radiations in the fly tree of life, Proceedings of the National Academy of Sciences, 108, 14, 5690–5695, 10.1073/pnas.1012675108, 21402926, 3078341, 2011PNAS..108.5690W, 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.BOOK, David Grimaldi (entomologist), Grimaldi, D., Michael S. Engel, Engel, M. S., Evolution of the Insects, 2005, Cambridge University Press, 978-0-521-82149-0, Late Carboniferous and Early Permian insect orders include both extant groups, their stem groups,JOURNAL, X-ray micro-tomography of Carboniferous stem-Dictyoptera: New insights into early insects, Russell J., Garwood, Mark D., Sutton, 2010, Biology Letters, 6, 5, 699–702,weblink 9 June 2015, 10.1098/rsbl.2010.0199, 20392720, 2936155, and a number of Paleozoic groups, now extinct. During this era, some giant dragonfly-like forms reached wingspans of {{convert|55|to|70|cm|in|abbr=on}}, 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 million years ago. Many of the early groups became extinct during the Permian-Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago.BOOK, Alex Rasnitsyn, Rasnitsyn, A.P., Quicke, D.L.J., History of Insects, 2002, Kluwer Academic Publishers, 978-1-4020-0026-3, The remarkably successful Hymenoptera appeared as long as 146 million years ago in the Cretaceous period, but achieved their wide diversity more recently in the Cenozoic era, which began 66 million years ago. A number of highly successful insect groups evolved in conjunction with flowering plants, a powerful illustration of coevolution.WEB, J. Stein Carter, Coevolution and Pollination,weblink University of Cincinnati, 29 March 2005, 9 May 2009, yes,weblink" title="web.archive.org/web/20090430183230weblink">weblink 30 April 2009, Many modern insect genera developed during the Cenozoic. Insects from this period on are often found preserved in amber, often in perfect condition. The body plan, or morphology, of such specimens is thus easily compared with modern species. The study of fossilized insects is called paleoentomology.

Evolutionary relationships

Insects are prey for a variety of organisms, including terrestrial vertebrates. The earliest vertebrates on land existed 400 million years ago and were large amphibious piscivores. Through gradual evolutionary change, insectivory was the next diet type to evolve.JOURNAL, 10.1130/G31182.1, Sahney, S., Benton, M.J., Falcon-Lang, H.J., 2010, Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica, Geology, 38, 12, 1079–1082, 2010Geo....38.1079S, Insects were among the earliest terrestrial herbivores and acted as major selection agents on plants. Plants evolved chemical defenses against this herbivory and the insects, in turn, evolved mechanisms to deal with plant toxins. Many insects make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity using warning colors.WEB, Coevolution and Pollination,weblink University of Cincinnati, 9 May 2009, yes,weblink" title="web.archive.org/web/20090430183230weblink">weblink 30 April 2009, This successful evolutionary pattern has also been used by mimics. Over time, this has led to complex groups of coevolved species. Conversely, some interactions between plants and insects, like pollination, are beneficial to both organisms. Coevolution has led to the development of very specific mutualisms in such systems.

Taxonomy

{{See also|Category:Insect orders|Category:Insect families}}{| class="wikitable" style="float:right; width:20em;"|{| style="background:Transparent; border:solid 0 #503df9;"! colspan="4" style="background:#E6D09D"| Classification| Insecta{| class="collapsible collapsed" style="float:right; background:Transparent;"! Monocondylia -Archaeognatha - 470 Dicondylia{| class="collapsible collapsed" style="float:right; background:Transparent;"! Apterygota -Zygentoma

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