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{{Short description|Class of arthropods}}
{{Other uses}}
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{{Use dmy dates|date=January 2015}}
{{Automatic taxobox
|fossil_range = {{fossilrange|Carboniferous|Present|earliest=400}}
|image = Insects - Neoptera - Paleoptera - Apterygota.jpg<!--See talk page before editing-->
|image_caption = Insects have a three-part body: head with large [[compound eye]]s and [[Antenna (biology)|antennae]], a [[Thorax (arthropod anatomy)|thorax]] with three pairs of legs, and a segmented abdomen. Many groups also have two pairs of wings.
|display_parents = 2
|taxon = Insecta
|authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]
|subdivision_ranks = Subgroups
|subdivision = * [[Archaeognatha]]
* [[Dicondylia]]
** [[Zygentoma]]
** [[Pterygota]]
|synonyms = * Ectognatha
* Entomida
}}
[[File:Insect world of motion.webm|thumb|Insects live in a world of motion. This [[Acanthocephala terminalis|leaf-footed bug]] climbs wind blown grass and flies off.]]

'''Insects''' (from [[Latin]] ''{{lang|la|insectum}}'') are [[Hexapoda|hexapod]] [[invertebrate]]s of the [[class (biology)|class]] '''Insecta'''. They are the largest group within the [[arthropod]] [[phylum]]. Insects have a [[chitin]]ous [[exoskeleton]], a three-part body ([[Insect morphology#Head|head]], [[Thorax (insect anatomy)|thorax]] and [[abdomen (insect anatomy)|abdomen]]), three pairs of jointed [[Arthropod leg|legs]], [[compound eye]]s, and a pair of [[antenna (biology)|antennae]]. Insects are the most diverse group of animals, with more than a million described [[species]]; they represent more than half of all animal species. 

The insect [[nervous system]] consists of a [[insect brain|brain]] and a [[ventral nerve cord]]. Most insects reproduce [[Oviparous|by laying eggs]]. Insects [[Respiratory system of insects|breathe air]] through a system of [[Spiracle (arthropods)|paired openings]] along their sides, connected to [[Trachea#Invertebrates|small tubes]] that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open [[hemocoel]]. Insect vision is mainly through their [[compound eye]]s, with additional small [[ocelli]]. Many insects can hear, using [[tympanal organ]]s, which may be on the legs or other parts of the body. Their [[Insect olfaction|sense of smell]] is via receptors, usually on the antennae and the mouthparts. 

Nearly all insects hatch from [[egg]]s. Insect growth is constrained by the inelastic exoskeleton, so development involves a series of [[Moulting|molts]]. The immature stages often differ from the adults in structure, habit, and habitat. Groups that undergo [[holometabolism|four-stage metamorphosis]] often have a nearly immobile [[pupa]]. Insects that undergo [[hemimetabolism|three-stage metamorphosis]] lack a pupa, developing through a series of increasingly adult-like [[Nymph (biology)|nymphal]] stages. The higher level relationship of the [[Hexapoda|insects]] is unclear. Fossilized insects of enormous size have been found from the [[Paleozoic]] Era, including [[Meganisoptera|giant dragonfly-like insects]] with wingspans of {{convert|55|to|70|cm|in|abbr=on}}. The most diverse insect groups appear to have [[coevolution|coevolved]] with [[flowering plant]]s.

Adult insects typically move about by walking and flying; some can swim. Insects are the only invertebrates that can achieve sustained powered flight; [[insect flight]] evolved just once. Many insects are at least partly [[Aquatic insect|aquatic]], and have [[larva]]e with gills; in some species, the adults too are aquatic. Some species, such as [[Gerridae|water striders]], can walk on the surface of water. Insects are mostly solitary, but some, such as [[bee]]s, [[ant]]s and [[termite]]s, are [[Eusociality|social]] and live in large, well-organized [[Colony (biology)|colonies]]. Others, such as [[earwig]]s, provide maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male [[moth]]s can sense the [[pheromone]]s of female moths over great distances. Other species communicate with sounds: [[Cricket (insect)|crickets]] [[Stridulation|stridulate]], or rub their wings together, to attract a mate and repel other males. [[Firefly|Lampyrid]] [[beetle]]s communicate with light.

Humans regard many insects as [[Pest (organism)|pests]], especially those that damage crops, and attempt to control them using [[insecticide]]s and other techniques. Others are [[parasitism|parasitic]], and may act as [[Vector (epidemiology)|vectors]] of [[List of diseases caused by insects|diseases]]. Insect [[Pollination|pollinators]] are essential to the reproduction of many flowering plants and so to their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefit. Two species in particular are economically important and were domesticated many centuries ago: [[silkworm]]s for [[silk]] and [[honey bee]]s for [[honey]]. Insects are consumed as food in 80% of the world's nations, by people in roughly 3,000 ethnic groups. Human activities are having serious effects on [[insect biodiversity]].

== Etymology ==

The word ''insect'' comes from the [[Latin language|Latin]] word {{lang|la|insectum}}<!--past participle--> from {{lang|la|in}} + {{lang|la|sĕco}}, "cut up",<ref>{{cite dictionary |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dinseco1 |title=insĕco |last1=Lewis |first1=Charlton T. |last2= Short |first2=Charles |dictionary=A Latin Dictionary |publisher=Perseus Digital Library |date=1879 }}</ref> as insects appear to be cut into three parts. The Latin word was introduced by [[Pliny the Elder]] who [[calque]]d the [[Ancient Greek]] word {{lang|grc|ἔντομον}} ''éntomon'' "insect" (as in [[entomology]]) from {{lang|grc|[[wikt:ἔντομος|ἔντομος]]}} ''éntomos'' "cut in pieces";<ref name="Liddell">{{cite web |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=e)/ntomos |title=ἔντομος |last1=Liddell |first1=Henry George |last2=Scott |first2=Robert |work=A Greek-English Lexicon |publisher=Perseus Digital Library |date=1940 }}</ref> this was [[Aristotle]]'s term for this [[Aristotle's biology#Classification|class of life in his biology]], also in reference to their notched bodies. The English word ''insect'' first appears in 1601 in [[Philemon Holland]]'s translation of Pliny.<ref name="etymonline">{{cite web |url=http://www.etymonline.com/index.php?term=insect&allowed_in_frame=0 |title=Online Etymological Dictionary |last=Harper |first=Douglas |author2=McCormack, Dan |date=November 2001 |publisher=LogoBee.com |page=1 |access-date=1 November 2011 |archive-url=https://web.archive.org/web/20120111164850/http://www.etymonline.com/index.php?term=insect&allowed_in_frame=0 |archive-date=11 January 2012 |url-status=live }}</ref><ref>{{Cite web|url=https://www.ezglot.com/words.php?w=insect&l=eng |title=insect translations |website=ezglot.com}}</ref>

== Insects and other bugs ==

=== Distinguishing features ===

In common speech, insects and other terrestrial [[arthropod]]s are often called '''bugs'''.{{efn|The [[Museum of New Zealand]] notes that "in everyday conversation", ''bug'' "refers to land arthropods with at least six legs, such as insects, spiders, and centipedes".<ref>[https://www.tepapa.govt.nz/discover-collections/read-watch-play/science/your-bug-questions-answered/what-bug-insects-arachnids "What is a bug? Insects, arachnids, and myriapods"] at Museum of New Zealand Te Papa Tongarewa website. Accessed 10 March 2022.</ref> In a chapter on "Bugs That Are Not Insects", entomologist Gilbert Walbauer specifies centipedes, millipedes, arachnids (spiders, [[Opiliones|daddy longlegs]], scorpions, [[mite]]s, [[chigger]]s and ticks) as well as the few terrestrial crustaceans ([[sowbug]]s and [[pillbug]]s).<ref name="Waldbauer 1998">{{cite book |last=Waldbauer |first=Gilbert |title=The Handy Bug Answer Book |publisher=Visible Ink |year=1998 |url=https://archive.org/details/handybuganswerbo00wald/page/5/mode/2up |pages=1, 5–26 |isbn=9781578590490}}</ref>}} Entomologists to some extent reserve the name "bugs" for a narrow category of "[[true bugs]]", insects of the order [[Hemiptera]], such as [[cicada]]s and [[shield bug]]s.<ref name="Waldbauer 1998"/> Other terrestrial arthropods, such as [[centipede]]s, [[millipede]]s, [[woodlouse|woodlice]], [[spider]]s, [[mite]]s and [[scorpion]]s, are sometimes confused with insects, since they have a jointed exoskeleton.<ref name="Chinery intro 1993">{{cite book |last=Chinery |first=Michael |date=1993 |chapter=Introduction |title=Insects of Britain & Northern Europe |edition=3rd |location=London |publisher=HarperCollins |isbn=978-0-00-219918-6 |pages=11–13}}</ref> Adult insects are the only arthropods that ever have wings, with up to two pairs on the thorax. Whether winged or not, adult insects can be distinguished by their three-part body plan, with head, thorax, and abdomen; they have three pairs of legs on the thorax.{{sfn|Gullan|Cranston|2005|pp=22–48}} 

<gallery class=center mode=nolines widths=175 heights=175 caption="Insects and other bugs that could be confused with them">
File:Gemeine Heidelibelle (Sympetrum vulgatum) 4 (cropped).jpg|'''Insect'''<!--indicating THIS group-->: Six legs, three-part body<br/>(head, thorax, abdomen),<br/>up to two pairs of wings
File:Wolfsspinne Trochosa Rose-20190905-RM-081613 (cropped).jpg|[[Spider]]: eight legs,<br/>two-part body
File:Armadillidium_vulgare_001.jpg|[[Woodlouse]]: seven pairs of legs, seven body<!--thoracic, but that's too tech. for this caption--> segments (plus head and tail)
File:Scolopendra viridicornis nigra (cropped).jpg|[[Centipede]]: many legs,<br/>one pair per segment
File:Milli's on the back (cropped).jpg|[[Millipede]]: many legs,<br/>two pairs per segment
</gallery>

=== Diversity ===

{{Main|Insect biodiversity}}

[[File:Eukaryote species pie tree.png|thumb|upright=1.75|About half of all [[eukaryote]]s are insects (left side of diagram).]]

Estimates of the total number of insect species vary considerably, suggesting that there are perhaps some 5.5 million insect species in existence, of which about one million have been described and named.<ref name="Stork2018">{{cite journal |last=Stork |first=Nigel E. |title=How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth? |journal=Annual Review of Entomology |date=7 January 2018 |volume=63 |issue=1 |pages=31–45 |doi=10.1146/annurev-ento-020117-043348 |pmid=28938083 |s2cid=23755007 |doi-access=free }}</ref> These constitute around half of all [[eukaryote]] species, including [[animal]]s, [[plant]]s, and [[Fungus|fungi]].<ref>{{cite journal |last=Erwin |first=Terry L. |year=1982 |title=Tropical forests: their richness in Coleoptera and other arthropod species |journal=The Coleopterists Bulletin |volume=36 |pages=74–75 |url=https://repository.si.edu/bitstream/handle/10088/4383/Classic_papers_in_Foundations.pdf?sequence=1&isAllowed=y |access-date=16 September 2018 |archive-url=https://web.archive.org/web/20150923014947/https://repository.si.edu/bitstream/handle/10088/4383/Classic_papers_in_Foundations.pdf?sequence=1&isAllowed=y |archive-date=23 September 2015 |url-status=live}}</ref> The most diverse insect [[Order (biology)|order]]s are the Hemiptera (true bugs<!--intentional repeat, this is an introductory section-->), Lepidoptera (butterflies and moths), Diptera (true flies), Hymenoptera (wasps, ants, and bees), and Coleoptera (beetles), each with more than 100,000 described species.<ref name="Stork2018"/>{{-}}

<gallery class=center mode=packed widths=150 heights=150 caption="Insects are extremely diverse. Five groups each have over 100,000 described species.">
File:Palomena prasina MHNT Léguevin Blanc.jpg|True bugs<br/>([[Hemiptera]])
File:Peacock butterfly (Aglais io) 2.jpg|Butterflies and moths<br/>([[Lepidoptera]])
File:Asilidae by kadavoor.jpg|Flies<br/>([[Diptera]])
File:Specimen of Podalonia tydei (Le Guillou, 1841) (cropped).jpg|Wasps<br/>([[Hymenoptera]])
File:7-Spotted-Ladybug-Coccinella-septempunctata-sq1.jpg|Beetles<br/>([[beetle|Coleoptera]])
</gallery>

=== Distribution and habitats ===

<gallery class=center mode=packed widths=165 heights=165 caption="Insects occur in habitats as varied as snow, freshwater, the tropics, desert, and even the sea.">
File:Boreus hyemalis 5930585 (cropped).jpg|The snow scorpionfly ''[[Boreus hyemalis]]'' on snow
File:Dytiscus marginalis larva.jpg|The great diving beetle ''[[Dytiscus marginalis]]'' larva in a pond
File:Green Orchid Bee (Euglossa dilemma) (7406599274).jpg|The green orchid bee ''[[Euglossa dilemma]]'' of Central America
File:SGR laying (cropped).jpg|The desert locust ''[[Schistocerca gregaria]]'' laying eggs in sand
File:Halobates sp. (Heteroptera Gerridae), 20 August 2011, Castle Beach, Kailua (Oahu), Hawaii03 (cropped).jpg|Sea skater ''[[Halobates]]'' on a Hawaii beach
</gallery>

Insects are distributed over every continent and almost every terrestrial habitat. There are many more species in the [[tropics]], especially in [[Tropical rainforest|rainforests]], than in temperate zones.{{sfn|Gullan|Cranston|2014|p=8}} The world's regions have received widely differing amounts of attention from entomologists. The British Isles have been thoroughly surveyed, so that Gullan and Cranston 2014 state that the total of around 22,500 species is probably within 5% of the actual number there; they comment that Canada's list of 30,000 described species is surely over half of the actual total. They add that the 3,000 species of the American Arctic must be broadly accurate. In contrast, a large majority of the insect species of the tropics and the [[Southern Hemisphere|southern hemisphere]] are probably undescribed.{{sfn|Gullan|Cranston|2014|p=8}} Some 30–40,000 species [[Aquatic insect|inhabit freshwater]]; very few insects, perhaps a hundred species, are marine.<ref name="Crook 2018">{{cite web |last1=Crook |first1=Glynis |title=Marine insects: small but significant |url=https://www.dw.com/en/why-are-there-so-few-insects-at-sea/a-44388348 |publisher=Deutsche Welle |access-date=2 December 2023}}</ref> Insects such as [[Snow scorpionfly|snow scorpionflies]] flourish in cold habitats including the [[Boreal ecosystem|Arctic]] and at high altitude.<ref>{{cite journal |last=Hågvar |first=Sigmund |title=A review of Fennoscandian arthropods living on and in snow |journal=European Journal of Entomology |volume=107 |issue=3 |year=2010 |pages=281–298 |doi=10.14411/eje.2010.037 |url=http://www.eje.cz/pdfs/eje/2010/03/01.pdf |archive-url=https://web.archive.org/web/20170922003622/http://www.eje.cz/pdfs/eje/2010/03/01.pdf |archive-date=22 September 2017 |url-status=live}}</ref> Insects such as [[desert locust]]s, ants, beetles, and termites are adapted to some of the hottest and driest environments on earth, such as the [[Sonoran Desert]].<ref>{{cite web |url=http://www.desertmuseum.org/books/nhsd_inverts.php |title=Invertebrates: A Vertebrate Looks at Arthropods |publisher=Arizona-Sonora Desert Museum |access-date=2013-05-21}}</ref>

== Phylogeny and evolution ==

=== External phylogeny ===

Insects form a [[clade]], a natural group with a common ancestor, among the [[arthropod]]s.<ref name=misof>{{cite journal |last1=Misof |first1=Bernhard |display-authors=etal |title=Phylogenomics resolves the timing and pattern of insect evolution |journal=Science |date=7 November 2014 |volume=346 |issue=6210 |pages=763–767 |doi=10.1126/science.1257570 |pmid=25378627 |url=https://www.science.org/ |bibcode=2014Sci...346..763M |s2cid=36008925 |access-date=17 October 2009 |archive-url=https://web.archive.org/web/20091018132952/http://www.sciencemag.org/ |archive-date=18 October 2009 |url-status=live }}</ref> A [[phylogenetic]] analysis by Kjer et al. (2016) places the insects among the [[Hexapoda]], six-legged animals with segmented bodies; their closest relatives are the [[Diplura]] (bristletails).<ref name="Kjer Simon Yavorskaya 2016">{{Cite journal |last1=Kjer |first1=Karl M. |last2=Simon |first2=Chris |author2-link=Chris Simon (biologist)|last3=Yavorskaya |first3=Margarita |last4=Beutel |first4=Rolf G. |date=2016 |title=Progress, pitfalls and parallel universes: a history of insect phylogenetics |journal=Journal of the Royal Society Interface |volume=13 |issue=121 |page=121 |doi=10.1098/rsif.2016.0363 |pmid=27558853 |pmc=5014063 }}</ref>

{{clade|style=line-height:100%
|label1=[[Hexapoda]]
|1={{clade
   |1={{clade
      |1=[[Collembola]] (springtails) [[File:Orchesella cincta (43480678534) (cropped).jpg|80px]]
      |2=[[Protura]] (coneheads) [[File:Protura.png|80px]]
      }}
   |2={{clade
      |1=[[Diplura]] (two-pronged bristletails) [[File:Campodea spec. (Campodeidae), Elst (Gld), the Netherlands (cropped).jpg|120px]]
      |2='''Insecta''' (=Ectognatha) [[File:Scorpionfly (white background).jpg|100px]]
      }}
   }}
}}

=== Internal phylogeny ===

The internal phylogeny is based on the works of Wipfler et al. 2019 for the [[Polyneoptera]],<ref name="Wipfler Letsch Frandsen 2019">{{cite journal |last1=Wipfler |first1=Benjamin |last2=Letsch |first2=Harald |last3=Frandsen |first3=Paul B. |last4=Kapli |first4=Paschalia |last5=Mayer |first5=Christoph |last6=Bartel |first6=Daniela |last7=Buckley |first7=Thomas R. |last8=Donath |first8=Alexander |last9=Edgerly-Rooks |first9=Janice S. |last10=Fujita |first10=Mari |last11=Liu |first11=Shanlin |display-authors=5 |date=February 2019 |title=Evolutionary history of Polyneoptera and its implications for our understanding of early winged insects |journal=Proceedings of the National Academy of Sciences |volume=116 |issue=8 |pages=3024–3029 |doi=10.1073/pnas.1817794116 |pmid=30642969 |pmc=6386694 |bibcode=2019PNAS..116.3024W |doi-access=free }}</ref> Johnson et al. 2018 for the [[Paraneoptera]],<ref name="Johnson 2018">{{cite journal |last1=Johnson |first1=Kevin P. |last2=Dietrich |first2=Christopher H. |last3=Friedrich |first3=Frank |last4=Beutel |first4=Rolf G. |last5=Wipfler |first5=Benjamin |last6=Peters |first6=Ralph S. |last7=Allen |first7=Julie M. |last8=Petersen |first8=Malte |last9=Donath |first9=Alexander |last10=Walden |first10=Kimberly K. O. |last11=Kozlov |first11=Alexey M. |last12=Podsiadlowski |first12=Lars |last13=Mayer |first13=Christoph |last14=Meusemann |first14=Karen |last15=Vasilikopoulos |first15=Alexandros |last16=Waterhouse |first16=Robert M. |last17=Cameron |first17=Stephen L. |last18=Weirauch |first18=Christiane |last19=Swanson |first19=Daniel R. |last20=Percy |first20=Diana M. |last21=Hardy |first21=Nate B. |last22=Terry |first22=Irene |last23=Liu |first23=Shanlin |last24=Zhou |first24=Xin |last25=Misof |first25=Bernhard |last26=Robertson |first26=Hugh M. |last27=Yoshizawa |first27=Kazunori |display-authors=5 |title=Phylogenomics and the evolution of hemipteroid insects |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=50 |date=26 November 2018 |issn=0027-8424 |doi=10.1073/pnas.1815820115 |pages=12775–12780 |pmid=30478043 |pmc=6294958 |bibcode=2018PNAS..11512775J |doi-access=free }}</ref> and Kjer et al. 2016 for the [[Holometabola]].<ref name=Kjer>{{Cite journal |last1=Kjer |first1=Karl M. |last2=Simon |first2=Chris |author2-link=Chris Simon (biologist) |last3=Yavorskaya |first3=Margarita |last4=Beutel |first4=Rolf G. |date=2016 |title=Progress, pitfalls and parallel universes: a history of insect phylogenetics |journal=Journal of the Royal Society Interface |volume=13 |issue=121 |page=121 |doi=10.1098/rsif.2016.0363 |pmid=27558853 |pmc=5014063 }}</ref> The numbers of described [[Neontology#Extant taxa versus extinct taxa |extant]] species (boldface for groups with over 100,000 species) are from Stork 2018.<ref name="Stork2018"/>

{{clade|style=font-size:85%;line-height:85%
|label1='''Insecta'''
|1={{clade
   |label1=[[Monocondylia]]
   |1=[[Archaeognatha]] (hump-backed/jumping bristletails, 513 spp) [[File:Arch-borstehale (white background).png|70px]]
   |label2=[[Dicondylia]]
   |2={{clade
      |1=[[Zygentoma]] (silverfish, firebrats, fishmoths, 560 spp) [[File:Silverfish 2007-2 (white background).jpg|100px]]
      |label2=[[Pterygota]]
      |sublabel2=''[[Insect wing|wings]]''
      |2={{clade
         |label1=[[Palaeoptera]]
         |1={{clade
            |1=[[Odonata]] (dragonflies and damselflies, 5,899 spp) [[File:Illustrations of Exotic Entomology II 47 ('LIBELLULA AXILENA').jpg|70px]]
            |2=[[Ephemeroptera]] (mayflies, 3,240 spp) [[File:Isonychia sicca.jpeg|80px]]
            }}
         |label2=[[Neoptera]] 
         |sublabel2=''wings flex over abdomen''
         |2={{clade
            |label1=[[Polyneoptera]]
            |1={{clade
              |1={{clade
                 |1=[[Zoraptera]] (angel insects, 37 spp) [[File:Zorotypus guineensis (white background).jpg|60px]]
                 |2=[[Dermaptera]] (earwigs, 1,978 spp) [[File:Earwig on white background.jpg|60px]]
                 }}
              |2={{clade
                 |1=[[Plecoptera]] (stoneflies, 3,743 spp) [[File:Neoperla_clymene_hor.png|70px]]
                 |2={{clade
                    |1=[[Orthoptera]] (grasshoppers, crickets, katydids, 23,855 spp) [[File:Gryllidae_usda.png|50px]]
                    |2={{clade
                       |1={{clade
                          |label1=[[Notoptera]]
                          |1={{clade
                             |1=[[Grylloblattodea]] (ice crawlers, 34 spp) [[File:Grylloblattidae (white background).jpg|70px]]
                             |2=[[Mantophasmatodea]] (gladiators, 15 spp) [[File:Mantophasma zephyra (white background).jpg|70px]]
                             }}
                          |2={{clade
                             |1=[[Phasmatodea]] (stick insects, 3,014 spp) [[File:Stick insect line diagram.png|70px]]
                             |2=[[Embioptera]] (webspinners, 463 spp) [[File:Embia_major_hor.png|80px]]
                             }}
                          }}
                       |label2=[[Dictyoptera]]
                       |2={{clade
                          |1=[[Mantodea]] (mantises, 2,400 spp) [[File:Stagmomantis carolina usda hor.png|80px]]
                          |2=[[Blattodea]] (cockroaches and termites, 7,314 spp) [[File:Temnopteryx species Zebra Cockroach (white background).jpg|60px]]
                         }}
                      }}
                   }}
                }}
             }}
          |label2=[[Eumetabola]]
          |2={{clade
            |label1=[[Paraneoptera]] 
            |1={{clade
              |1=[[Psocodea]] (book lice, barklice and sucking lice, 11,000 spp)  [[File:Psocoptera (white background).jpg|45px]] [[File:Lipeurus forficulatus f.png|40px]]
              |2={{clade
                |1=[[Hemiptera]] (true bugs, '''103,590''' spp) [[File:Dorisiana bicolor MHNT, Montsinéry, Guyane dos vol 2.jpg|75px]]
                |2=[[Thysanoptera]] (thrips, 5,864 spp) [[File:Thrips (PSF) (white background).png|40px]]
                 }}
               }}
            |label2=[[Holometabola]]
            |sublabel2=''larvae, pupae''
            |2={{clade
              |label1=
              |1=[[Hymenoptera]] (sawflies, wasps, bees, ants, '''116,861''' spp) [[File:European wasp white bg.jpg|70px]]
              |label2=
              |2={{clade
                |label1=[[Neuropteroidea]]
                |1={{clade
                  |label1=[[Coleopterida]]
                  |1={{clade
                    |1=[[Strepsiptera]] (twisted-wing flies, 609 spp) [[File:Elenchus koebelei.jpg|70px]]
                    |2=[[Coleoptera]] (beetles, '''386,500''' spp) [[File:Pseudacrossus przewalskyi (Reitter, 1887).jpg|50px]]
                     }}
                  |label2=[[Neuropterida]]
                  |2={{clade
                    |1=[[Raphidioptera]] (snakeflies, 254 spp) [[File:Raphidia icon.png|70px]]
                    |2={{clade
                      |1=[[Neuroptera]] (lacewings, 5,868 spp) [[File:Osmylus_(white_background).jpg|70px]]
                      |2=[[Megaloptera]] (alderflies and dobsonflies, 354 spp) [[File:Corydalus cornutus illustration (rotated).png|70px]]
                       }}
                     }}
                   }}
                |label2=[[Panorpida]]
                |2={{clade
                  |label1=[[Amphiesmenoptera]]
                  |1={{clade
                    |1=[[Lepidoptera]] (butterflies and moths, '''157,338''' spp) [[File:Arctia villica SLU.JPG|60px]]
                    |2=[[Trichoptera]] (caddisflies, 14,391 spp) [[File:RHYACOPHILA DORSALIS Male Pont Forge de Sailly Watigny 02 MHNT.jpg|70px]] 
                     }}
                  |label2=[[Antliophora]]
                  |2={{clade
                    |1=[[Diptera]] (true flies, '''155,477''' spp) [[File:Common house fly, Musca domestica.jpg|70px]] 
                    |2={{clade
                       |1=[[Mecoptera]] (scorpionflies, 757 spp) [[File:Scorpionfly (white background).jpg|70px]]
                       |2=[[Siphonaptera]] (fleas, 2,075 spp) [[File:Pulex irritans female ZSM (white background).jpg|40px]]
                       }}
                     }}
                   }}
                 }}
               }}
             }}
           }}
        }}
      }}
   }}
}}

=== Taxonomy ===

==== Early ====

{{further|Aristotle's biology#Classification|Insecta in the 10th edition of Systema Naturae}}

{{cladogram|title=Diagram of Linnaeus's key to his seven orders of insect, 1758<ref name="Winsor 1976"/>
|cladogram={{clade|style=font-size:85%;line-height:95%
|sublabel1='''[[Insecta in the 10th edition of Systema Naturae|Insecta]]'''
|1={{clade
   |sublabel1=wingless
   |label1=[[Aptera in the 10th edition of Systema Naturae|Aptera]] 
   |1=[[File:Arch-borstehale (white background).png|70px]]
   |sublabel2=winged
   |2={{clade
      |sublabel1=2-winged
      |label1=[[Diptera in the 10th edition of Systema Naturae|Diptera]]
      |1=[[File:Common house fly, Musca domestica.jpg|70px]] 
      |sublabel2=4-winged
      |2={{clade
         |sublabel1=dissimilar pairs
         |1={{clade
            |sublabel1=forewings fully hardened
            |label1=[[Coleoptera in the 10th edition of Systema Naturae|Coleoptera]] 
            |1=[[File:Pseudacrossus przewalskyi (Reitter, 1887).jpg|50px]]
            |sublabel2=forewings partly hardened
            |label2=[[Hemiptera in the 10th edition of Systema Naturae|Hemiptera]]
            |2=[[File:Dorisiana bicolor MHNT, Montsinéry, Guyane dos vol 2.jpg|75px]]
            }}
         |sublabel2=similar pairs
         |2={{clade
            |sublabel1=wings scaly
            |label1=[[Lepidoptera in the 10th edition of Systema Naturae|Lepidoptera]]
            |1=[[File:Arctia villica SLU.JPG|60px]]
            |sublabel2=wings membranous
            |2={{clade
               |sublabel1=no sting
               |label1=[[Neuroptera in the 10th edition of Systema Naturae|Neuroptera]] 
               |1=[[File:Osmylus_(white_background).jpg|70px]]
               |sublabel2=sting
               |label2=[[Hymenoptera in the 10th edition of Systema Naturae|Hymenoptera]] 
               |2=[[File:European wasp white bg.jpg|70px]]
               }}
            }}
        }}
      }}
   }}
}}
}}

[[Aristotle]] was the first to describe the insects as a distinct group. He placed them as the second-lowest level of animals on his ''[[scala naturae]]'', above the [[Spontaneous generation|spontaneously generating]] sponges and worms, but below the hard-shelled marine snails. His classification remained in use for many centuries.<ref>{{cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2014 |isbn=978-1-4088-3622-4 |pages=111–119}}</ref>

In 1758, in his ''[[Systema Naturae]]'',<ref name=Linn1758>{{cite book |last=Linnaeus |first=Carl |author-link=Carl Linnaeus |title=Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. |edition=[[10th edition of Systema Naturae|10th]] |publisher=Holmiae (Laurentii Salvii) |year=1758 |url=https://www.biodiversitylibrary.org/bibliography/542 |access-date=22 September 2008 |language=la |archive-url=https://web.archive.org/web/20081010032456/https://www.biodiversitylibrary.org/bibliography/542 |archive-date=10 October 2008 |url-status=live}}</ref> [[Carl Linnaeus]] divided the animal kingdom into six classes including [[Insecta in the 10th edition of Systema Naturae|Insecta]]. He created seven orders of insect according to the structure of their wings. These were the wingless Aptera, the two-winged Diptera, and five four-winged orders: the Coleoptera with fully-hardened forewings; the Hemiptera with partly-hardened forewings; the Lepidoptera with scaly wings; the Neuroptera with membranous wings but no [[stinger|sting]]; and the Hymenoptera, with membranous wings and a sting.<ref name="Winsor 1976">{{cite journal |last=Winsor |first=Mary P. |year=1976 |title=The development of Linnaean insect classification |journal=[[Taxon (journal)|Taxon]] |volume=25 |issue=1 |pages=57–67 |jstor=1220406 |doi=10.2307/1220406 |bibcode=1976Taxon..25...57W }}</ref> 

[[Jean-Baptiste de Lamarck]], in his 1809 ''[[Philosophie Zoologique]]'', treated the insects as one of nine invertebrate [[phylum|phyla]].<ref name=Gould2011>{{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |title=The Lying Stones of Marrakech |url={{GBurl|id=wApMpVmi-5gC|p=130}} |year=2011 |publisher=Harvard University Press |isbn=978-0-674-06167-5 |pages=130–134}}</ref> In his 1817 ''[[Le Règne Animal]]'', [[Georges Cuvier]] grouped all animals into four ''embranchements'' ("branches" with different body plans), one of which was the articulated animals, containing arthropods and annelids.<ref>{{cite book |last=De Wit |first=Hendrik C. D. |title=Histoire du Développement de la Biologie, Volume III |publisher=Presses Polytechniques et Universitaires Romandes |date=1994 |pages=94–96 |isbn=978-2-88074-264-5}}</ref> This arrangement was followed by the embryologist [[Karl Ernst von Baer]] in 1828, the zoologist [[Louis Agassiz]] in 1857, and the comparative anatomist [[Richard Owen]] in 1860.<ref name=Valentine2004/> In 1874, [[Ernst Haeckel]] divided the animal kingdom into two subkingdoms, one of which was Metazoa for the multicellular animals. It had five phyla, including the articulates.<ref>{{cite book |last1=Haeckel |first1=Ernst |author-link=Ernst Haeckel |title=Anthropogenie oder Entwickelungsgeschichte des menschen |url=https://archive.org/details/anthropogenieod05haecgoog |year=1874 |page=202 |language=de |publisher=W. Engelmann }}</ref><ref name=Valentine2004>{{cite book |last=Valentine |first=James W. |title=On the Origin of Phyla |url={{GBurl|id=DMBkmHm5fe4C|p=8}} |year=2004 |publisher=University of Chicago Press |isbn=978-0-226-84548-7 |pages=7–8}}</ref>

==== Modern ====

{{See also|List of insect orders}}

Traditional morphology-based [[systematics]] have usually given the [[Hexapoda]] the rank of [[superclass (biology)|superclass]],{{sfn|Gullan|Cranston|2005|p=180}} and identified four groups within it: insects (Ectognatha), [[Springtail|Collembola]], [[Protura]], and [[Diplura]], the latter three being grouped together as the [[Entognatha]] on the basis of internalized mouth parts.<ref name="Kendall 2009"/>

The use of phylogenetic data has brought about numerous changes in relationships above the level of [[Order (biology)|orders]].<ref name="Kendall 2009">{{cite web |title=Classification of Bugs |last=Kendall |first=David A. |url=http://www.kendall-bioresearch.co.uk/class.htm |year=2009 |access-date=9 May 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090520201746/http://www.kendall-bioresearch.co.uk/class.htm |archive-date=20 May 2009 |df=dmy-all }}</ref> Insects can be divided into two groups historically treated as subclasses: wingless insects or [[Apterygota]], and winged insects or [[Pterygota]]. The Apterygota traditionally consisted of the primitively wingless orders [[Archaeognatha]] (jumping bristletails) and [[Zygentoma]] (silverfish). However, Apterygota is not [[monophyletic]], as Archaeognatha are sister to all other insects, based on the arrangement of their [[Mandible (insect mouthpart)|mandibles]], while the Pterygota, the winged insects, emerged from within the [[Dicondylia]], alongside the Zygentoma.<ref name="BlankeEtal">{{cite journal |last1=Blanke |first1=Alexander |last2=Machida |first2=Ryuichiro |last3=Szucsich |first3=Nikolaus Urban |last4=Wilde |first4=Fabian |last5=Misoe |first5=Bernhard |title=Mandibles with two joints evolved much earlier in the history of insects: dicondyly is a synapomorphy of bristletails, silverfish and winged insects |journal=Systematic Entomology |publisher=Wiley |volume=40 |issue=2 |date=15 October 2014 |doi=10.1111/syen.12107 |pages=357–364 |s2cid=85309726 }}</ref>

The Pterygota ([[Palaeoptera]] and [[Neoptera]]) are winged and have [[sclerite|hardened plates]] on the outside of their body segments; the Neoptera have muscles that allow their wings to fold flat over the abdomen. Neoptera can be divided into groups with incomplete metamorphosis ([[Polyneoptera]] and [[Paraneoptera]]) and those with complete metamorphosis ([[Holometabola]]). The molecular finding that the traditional [[louse]] orders [[Mallophaga]] and [[Anoplura]] are within [[Psocoptera]] has led to the new taxon [[Psocodea]].<ref>{{cite journal |last1=Johnson |first1=K. P. |last2=Yoshizawa |first2=K. |last3=Smith |first3=V. S. |year=2004 |title=Multiple origins of parasitism in lice |journal=Proceedings of the Royal Society of London |volume=271 |issue=1550 |pages=1771–1776 |doi=10.1098/rspb.2004.2798 |pmid=15315891 |pmc=1691793}}</ref> [[Phasmatodea]] and [[Embiidina]] have been suggested to form the Eukinolabia.<ref>{{cite journal |author1=Terry, M. D. |author2=Whiting, M. F. |year=2005 |title=Mantophasmatodea and phylogeny of the lower neopterous insects |doi=10.1111/j.1096-0031.2005.00062.x |url=https://www.researchgate.net/publication/227604832 |journal=Cladistics |volume=21 |issue=3 |pages=240–257 |s2cid=86259809 |doi-access=free }}</ref> Mantodea, Blattodea, and Isoptera form a monophyletic group, [[Dictyoptera]].<ref>{{cite journal |doi=10.1016/S0960-9822(00)00561-3 |year=2000 |first1=Nathan |last1=Lo |first2=Gaku |last2=Tokuda |first3=Hirofumi |last3=Watanabe |first4=Harley |last4=Rose |first5=Michael |last5=Slaytor |first6=Kiyoto |last6=Maekawa |first7=Claudio |last7=Bandi |first8=Hiroaki |last8=Noda |display-authors=5 |title=Evidence from multiple gene sequences indicates that termites evolved from wood-feeding cockroaches |journal=Current Biology |volume=10 |issue= 13|pages=801–804 |pmid=10898984|s2cid=14059547 |doi-access=free |bibcode=2000CBio...10..801L }}</ref> Fleas are now thought to be closely related to boreid mecopterans.<ref>{{cite journal |last=Whiting |first=M. F. |title=Mecoptera is paraphyletic: multiple genes and phylogeny of Mecoptera and Siphonaptera |year=2002 |journal=Zoologica Scripta |volume=31 |issue=1 |pages=93–104 |doi=10.1046/j.0300-3256.2001.00095.x |s2cid=56100681 }}</ref>

=== Evolutionary history ===

{{main|Evolution of insects}}

The oldest fossil that may be a primitive wingless insect is ''[[Leverhulmia]]'' from the Early [[Devonian]] [[Windyfield chert]].<ref>{{Cite journal |last=Ross |first=Andrew |date=August 2022 |title=Evolution: The origin of insect wings revisited |journal=Current Biology |volume=32 |issue=15 |pages=R851–R853 |doi=10.1016/j.cub.2022.06.087|pmid=35944489 |s2cid=251464185 |doi-access=free |bibcode=2022CBio...32.R851R }}</ref> The oldest known flying insects are from the mid-[[Carboniferous]], around 328–324 million years ago. The group subsequently underwent a rapid [[Evolutionary radiation|explosive diversification]]. Claims that they originated substantially earlier, during the [[Silurian]] or [[Devonian]] (some 400 million years ago) based on [[molecular clock]] estimates, are unlikely to be correct, given the fossil record.<ref>{{cite journal |last1=Schachat |first1=Sandra R. |last2=Goldstein |first2=Paul Z. |last3=Desalle |first3=Rob |last4=Bobo |first4=Dean M. |last5=Boyce |first5=C. Kevin |last6=Payne |first6=Jonathan L. |last7=Labandeira |first7=Conrad C. |date=2023-02-02 |title=Illusion of flight? Absence, evidence and the age of winged insects |url=https://academic.oup.com/biolinnean/article/138/2/143/6936511 |journal=Biological Journal of the Linnean Society |volume=138 |issue=2 |pages=143–168 |doi=10.1093/biolinnean/blac137 |doi-access=free }}</ref>

Four [[Evolutionary radiation|large-scale radiations]] of insects have occurred: [[beetle]]s (from about 300 million years ago), [[fly|flies]] (from about 250 million years ago), [[moth]]s and [[wasp]]s (both from about 150 million years ago).<ref name="Wiegmann Trautwein Winkler Barr 2011">{{cite journal |last1=Wiegmann |first1=Brian M. |last2=Trautwein |first2=Michelle D. |last3=Winkler |first3=Isaac S. |last4=Barr |first4=Norman B. |last5=Kim |first5=Jung-Wook |last6=Lambkin |first6=Christine |last7=Bertone |first7=Matthew A. |last8=Cassel |first8=Brian K. |last9=Bayless |first9=Keith M. |last10=Heimberg |first10=Alysha M. |last11=Wheeler |first11=Benjamin M. |last12=Peterson |first12=Kevin J. |last13=Pape |first13=Thomas |last14=Sinclair |first14=Bradley J. |last15=Skevington |first15=Jeffrey H. |last16=Blagoderov |first16=Vladimir |last17=Caravas |first17=Jason |last18=Kutty |first18=Sujatha Narayanan |last19=Schmidt-Ott |first19=Urs |last20=Kampmeier |first20=Gail E. |last21=Thompson |first21=F. Christian |last22=Grimaldi |first22=David A. |last23=Beckenbach |first23=Andrew T. |last24=Courtney |first24=Gregory W. |last25=Friedrich |first25=Markus |last26=Meier |first26=Rudolf |last27=Yeates |first27=David K. |display-authors=5 |date=14 March 2011 |title=Episodic radiations in the fly tree of life |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=14 |pages=5690–5695 |bibcode=2011PNAS..108.5690W |doi=10.1073/pnas.1012675108 |pmc=3078341 |pmid=21402926 |doi-access=free }}</ref><!-- These four groups account for the majority of described species.-->

The remarkably successful [[Hymenoptera]] (wasps, bees, and ants) appeared some 200 million years ago in the [[Triassic]] period, but achieved their wide diversity more recently in the [[Cenozoic]] era, which began 66 million years ago. Some highly successful insect groups evolved in conjunction with [[flowering plant]]s, a powerful illustration of [[coevolution]]. Insects were among the earliest terrestrial [[herbivore]]s and acted as major selection agents on plants.<ref name="Biology-coevolution">{{cite web |last=Carter |first=J. Stein |date=29 March 2005 |title=Coevolution and Pollination |url=http://biology.clc.uc.edu/courses/bio303/coevolution.htm |url-status=dead |archive-url=https://web.archive.org/web/20090430183230/http://biology.clc.uc.edu/Courses/bio303/coevolution.htm |archive-date=30 April 2009 |access-date=9 May 2009 |publisher=University of Cincinnati}}</ref> Plants evolved chemical [[Plant defense against herbivory|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]].<ref name="autogenerated2">{{cite web|title=Coevolution and Pollination |url=http://biology.clc.uc.edu/courses/bio303/coevolution.htm |publisher=[[University of Cincinnati]] |access-date=9 May 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090430183230/http://biology.clc.uc.edu/Courses/bio303/coevolution.htm |archive-date=30 April 2009}}</ref> 

<gallery mode=packed heights=120px>
File:Meganeura.png|The giant dragonfly-like insect ''[[Meganeura monyi]]'' grew to wingspans of {{cvt|75|cm|ftin|0}} in the late [[Carboniferous]], around 300 million years ago.<ref>{{cite book |last1=Taylor |last2=Lewis |year=2007 |title=Fossil Invertebrates |first1=Paul D. |first2=David N. |edition=repeated  |publisher=[[Harvard University Press]] |isbn=978-0674025745 |page=160 |url=https://books.google.com/books?id=7kPwZ2LeSAoC }}</ref>
File:Moravocoleus permianus.jpg|[[Beetle]] ''[[Tshekardocoleidae|Moravocoleus permianus]]'', fossil and reconstruction, from the Early [[Permian]] 
File:Fossil Wasp ( Iberomaimetsha ).jpg|[[Hymenoptera]] such as this ''[[Maimetshidae|Iberomaimetsha]]'' from the Early [[Cretaceous]], around 100 million years ago.
</gallery>

== Morphology and physiology ==

{{Main|Insect morphology}}

=== External ===

[[File:Insect anatomy diagram.svg|right|upright=1.35|thumb|Insect morphology<br/>
'''A'''- Head '''B'''- Thorax '''C'''- Abdomen
{{image key
|list type=ordered
|[[Antenna (biology)|antenna]]
|[[ocellus]] (lower)
|ocellus (upper)
|[[compound eye]]
|brain (cerebral [[Ganglion|ganglia]])
|[[prothorax]]
|dorsal blood vessel
|[[invertebrate trachea|tracheal]] tubes (trunk with [[Spiracle (arthropods)|spiracle]])
|[[mesothorax]]
|[[metathorax]]
|[[insect wing|forewing]]
|[[insect wing|hindwing]]
|midgut (stomach)
|dorsal tube (heart)
|ovary
|[[hindgut]] (intestine, rectum, anus)
|anus
|oviduct
|[[ventral nerve cord|nerve cord (abdominal ganglia)]]
|[[Malpighian tubule system|Malpighian tubules]]
|tarsal pads
|claws
|[[Arthropod leg|tarsus]]
|[[Arthropod leg|tibia]]
|[[Arthropod leg|femur]]
|[[Arthropod leg|trochanter]]
|foregut (crop, gizzard)
|[[ventral nerve cord|thoracic ganglion]]
|[[Arthropod leg|coxa]]
|salivary gland
|[[subesophageal ganglion]]
|[[insect mouthparts|mouthparts]]
}}]]

==== Three-part body ====

Insects have a [[segmentation (biology)|segmented]] body supported by an [[Arthropod exoskeleton|exoskeleton]], the hard outer covering made mostly of [[chitin]]. The body is organized into three [[tagma (biology)|interconnected units]]: the [[Insect morphology#Head|head]], [[Thorax (insect anatomy)|thorax]] and [[abdomen (insect anatomy)|abdomen]]. The head supports a pair of sensory [[Antenna (biology)|antennae]], a pair of [[compound eye]]s, zero to three simple eyes (or [[ocelli]]) and three sets of variously modified appendages that form the [[insect mouthparts|mouthparts]]. The thorax carries the three pairs of legs and up to two pairs of [[Insect wing|wings]]. The abdomen contains most of the digestive, respiratory, excretory and reproductive structures.{{sfn|Gullan|Cranston|2005|pp=22–48}}

==== Segmentation ====

{{further|Insect morphology}}

The head is enclosed in a hard, heavily [[Sclerotin|sclerotized]], unsegmented [[head capsule]], which contains most of the sensing organs, including the antennae, compound eyes, ocelli, and  mouthparts.{{sfn|Resh|Carde|2009|p=13}} The thorax is composed of three sections named (from front to back) the [[prothorax]], [[mesothorax]] and [[metathorax]]. The prothorax carries the first pair of legs. The mesothorax carries the second pair of legs and the front wings. The metathorax carries the third pair of legs and the hind wings.{{sfn|Gullan|Cranston|2005|pp=22–48}}{{sfn|Resh|Carde|2009|p=13}} The abdomen is the largest part of the insect, typically with 11–12 segments, and is less strongly sclerotized than the head or thorax. Each segment of the abdomen has sclerotized upper and lower plates (the tergum and sternum), connected to adjacent sclerotized parts by membranes. Each segment carries a pair of [[Spiracle (arthropods)|spiracle]]s.{{sfn|Resh|Carde|2009|p=13}}

==== Exoskeleton ====

{{main|Arthropod cuticle}}

The outer skeleton, the [[arthropod cuticle|cuticle]], is made up of two layers: the epicuticle, a thin and waxy water-resistant outer layer without [[chitin]], and a lower layer, the thick chitinous procuticle. The procuticle has two layers: an outer exocuticle and an inner endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each other in a sandwich pattern, while the exocuticle is rigid and sclerotized.{{sfn|Gullan|Cranston|2005|p=22–24}}<ref>{{Cite journal |last=Belles |first=Xavier |title=The innovation of the final moult and the origin of insect metamorphosis |date=14 October 2019 |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=374 |issue=1783 |pages=20180415 |doi=10.1098/rstb.2018.0415 |pmid=31438822 |pmc=6711288}}</ref> As an adaptation to life on land, insects have an [[enzyme]] <!--multicopper oxidase-2--> that uses atmospheric oxygen to harden their cuticle, unlike crustaceans which use heavy calcium compounds for the same purpose. This makes the insect exoskeleton a lightweight material.<ref name=AsanHashEver23>{{Cite journal |last1=Asano |first1=Tsunaki |last2=Hashimoto |first2=Kosei |last3=Everroad |first3=R. Craig |date=2023 |title=Eco-evolutionary implications for a possible contribution of cuticle hardening system in insect evolution and terrestrialisation |journal=Physiological Entomology |volume=48 |issue=2–3 |pages=55–60 |doi=10.1111/phen.12406 |s2cid=258209514 |doi-access=free }}</ref>

=== Internal systems ===

{{main|Insect physiology}}

==== Nervous ====

The [[nervous system]] of an insect consists of a [[insect brain|brain]] and a [[ventral nerve cord]]. The head capsule is made up of six fused segments, each with either a pair of [[ganglion|ganglia]], or a cluster of nerve cells outside of the brain. The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect's [[esophagus]], called the [[subesophageal ganglion]].{{sfn|Gullan|Cranston|2005|p=57}} The [[thoracic]] segments have one ganglion on each side, connected into a pair per segment. This arrangement is also seen in the first eight segments of the abdomen. Many insects have fewer ganglia than this.<ref name="INSECTS: ROLE OF THE SPIRACLES">{{cite journal |doi=10.2307/1539265|last=Schneiderman|first=Howard A. |year=1960 |title=Discontinuous respiration in insects: role of the spiracles |journal=[[The Biological Bulletin]] |issue=3 |pages=494–528|url=http://www.biolbull.org/cgi/reprint/119/3/494?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=insect+thoracic+spiracle&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT |volume=119 |jstor=1539265 |access-date=22 May 2009 |archive-url=https://web.archive.org/web/20090625011339/http://www.biolbull.org/cgi/reprint/119/3/494?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=insect+thoracic+spiracle&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT|archive-date=25 June 2009 |url-status=live}}</ref> Insects are capable of learning.<ref>{{Cite journal |last=Dukas |first=Reuven |date=1 January 2008 |title=Evolutionary Biology of Insect Learning |url=https://www.annualreviews.org/doi/10.1146/annurev.ento.53.103106.093343|journal=Annual Review of Entomology |volume=53 |issue=1 |pages=145–160 |doi=10.1146/annurev.ento.53.103106.093343 |pmid=17803459 }}</ref>

==== Digestive ====

An insect uses its digestive system to extract nutrients and other substances from the food it consumes.<ref name="genent">{{cite web |title=General Entomology – Digestive and Excritory system |url=http://www.cals.ncsu.edu/course/ent425/library/tutorials/internal_anatomy/digestive.html |publisher=NC state University |access-date=3 May 2009 |archive-url=https://web.archive.org/web/20090523024349/http://www.cals.ncsu.edu/course/ent425/library/tutorials/internal_anatomy/digestive.html |archive-date=23 May 2009 |url-status=live }}</ref> There is extensive variation among different [[Taxonomic order|orders]], [[Metamorphosis|life stages]], and even [[Caste (insect)|castes]] in the digestive system of insects.<ref>{{Cite journal |last1=Bueno |first1=Odair Correa |last2=Tanaka |first2=Francisco André Ossamu |last3=de Lima Nogueira |first3=Neusa |last4=Fox |first4=Eduardo Gonçalves Paterson |last5=Rossi |first5=Mônica Lanzoni |last6=Solis |first6=Daniel Russ |date=1 January 2013 |title=On the morphology of the digestive system of two Monomorium ant species |journal=Journal of Insect Science |volume=13 |issue=1 |page=70 |doi=10.1673/031.013.7001 |pmc=3835044 |pmid=24224520}}</ref> The [[alimentary canal|gut]] runs lengthwise through the body. It has three sections, with paired [[salivary gland]]s and salivary reservoirs.{{sfn|Gullan|Cranston|2005|pp=70–77}} By moving its mouthparts the insect mixes its food with saliva.<ref name="autogenerated1">{{cite web |title=General Entomology – Digestive and Excretory system |url=http://www.cals.ncsu.edu/course/ent425/library/tutorials/internal_anatomy/digestive.html |publisher=North Carolina State University |access-date=3 May 2009 |archive-url=https://web.archive.org/web/20090523024349/http://www.cals.ncsu.edu/course/ent425/library/tutorials/internal_anatomy/digestive.html |archive-date=23 May 2009 |url-status=live }}</ref><ref name="Duncan1939">{{cite book |last=Duncan |first=Carl D. |title=A Contribution to The Biology of North American Vespine Wasps |publisher=[[Stanford University Press]] |location=Stanford |year=1939 |edition=1st |pages=24–29}}</ref> Some insects, like [[Diptera|flies]], expel [[digestive enzyme]]s onto their food to break it down, but most insects digest their food in the gut.{{sfn|Nation|2001|p=31}} The [[foregut]] is lined with cuticule as protection from tough food. It includes the [[buccal cavity|mouth]], pharynx, and [[Crop (anatomy)|crop]] which stores food.{{sfn|Gullan|Cranston|2005|p=70}} Digestion starts in the mouth with enzymes in the saliva. Strong muscles in the pharynx pump fluid into the mouth, lubricating the food, and enabling certain insects to feed on blood or from the [[xylem]] and [[phloem]] transport vessels of plants.{{sfn|Nation|2001|p=30–31}} Once food leaves the crop, it passes to the [[stomach|midgut]], where the majority of digestion takes place. Microscopic projections, [[microvillus|microvilli]], increase the surface area of the wall to absorb nutrients.{{sfn|Nation|2001|p=32}} In the [[hindgut]], undigested food particles are joined by [[uric acid]] to form fecal pellets; most of the water is absorbed, leaving a dry pellet to be eliminated. Insects may have one to hundreds of [[Malpighian tubule system|Malpighian tubules]]. These remove nitrogenous wastes from the hemolymph of the insect and regulate osmotic balance. Wastes and solutes are emptied directly into the alimentary canal, at the junction between the midgut and hindgut.{{sfn|Gullan|Cranston|2005|pp=71–72, 78–80}}

==== Reproductive ====

{{main|Insect reproductive system}}

The [[Insect reproductive system|reproductive system of female insects]] consist of a pair of [[ovaries]], accessory glands, one or more [[spermatheca]]e to store sperm, and ducts connecting these parts. The ovaries are made up of a variable number of egg tubes, [[ovarioles]]. Female insects make eggs, receive and store sperm, manipulate sperm from different males, and lay eggs. Accessory glands produce substances to maintain sperm and to protect the eggs. They can produce glue and protective substances for coating eggs, or tough coverings for a batch of eggs called [[ootheca]]e.{{sfn|Resh|Carde||2009|p=880}}

For males, the reproductive system consists of one or two [[testis|testes]], suspended in the body cavity by [[Invertebrate trachea|tracheae]]. The testes contain sperm tubes or follicles in a membranous sac. These connect to a duct that leads to the outside. The terminal portion of the duct may be sclerotized to form the [[intromittent organ]], the [[aedeagus]].{{sfn|Resh|Carde|2009|p=885}}

==== Respiratory ====

{{main|Respiratory system of insects}}

[[File:Structural organization of the heart of the mosquito Anopheles gambiae - image.ppat.v08.i11.g001.png|thumb|The tube-like heart (green) of the mosquito ''[[Anopheles gambiae]]'' extends horizontally across the body, interlinked with the diamond-shaped [[insect wing#Muscles|wing muscles]] (also green) and surrounded by [[pericardium|pericardial cells]] (red). Blue depicts [[cell nuclei]].]]

[[Respiratory system of insects|Insect respiration]] is accomplished without [[lung]]s. Instead, insects have a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need it via their [[invertebrate trachea|tracheae]] and tracheoles. In most insects, air is taken in through paired [[Spiracle (arthropods)|spiracle]]s, openings on the sides of the abdomen and thorax. The respiratory system limits the size of insects. As insects get larger, [[gas exchange#Invertebrates|gas exchange]] via spiracles becomes less efficient, and thus the heaviest insect currently weighs less than 100&nbsp;g. However, with increased atmospheric oxygen levels, as were present in the late [[Paleozoic]], larger insects were possible, such as dragonflies with wingspans of more than {{Convert |2 |ft |cm |sigfig=1 |spell=in}}.<ref>{{cite web|title=What Keeps Bugs from Being Bigger?|date=8 August 2007|publisher=Argonne National Laboratory|url=https://www1.aps.anl.gov/APS-Science-Highlight/2007/What-Keeps-Bugs-from-Being-Bigger|archive-url=https://web.archive.org/web/20170514060330/https://www1.aps.anl.gov/APS-Science-Highlight/2007/What-Keeps-Bugs-from-Being-Bigger |archive-date=14 May 2017|access-date=15 July 2013}}</ref> Gas exchange patterns in insects range from continuous and [[diffusion|diffusive]] ventilation, to [[discontinuous gas exchange|discontinuous]].{{sfn|Gullan|Cranston|2005|pp=65–68}}<ref name="IPE">{{cite book |last1=Chown |first1=S. L. |last2=Nicholson |first2=S. W. |title=Insect Physiological Ecology |publisher=[[Oxford University Press]] |year=2004 |location=New York |isbn=978-0-19-851549-4}}</ref><ref name="aquins"/><ref>{{cite book |last1=Merritt |first1=R. W. |last2=Cummins |first2=K. W. |last3=Berg |first3=M. B. |title=An Introduction To The Aquatic Insects Of North America |year=2007 |publisher=Kendall Hunt Publishing |isbn=978-0-7575-4128-5}}</ref>

==== Circulatory ====

{{further|Insect physiology#Circulatory system}}

Because oxygen is delivered directly to tissues via tracheoles, the circulatory system is not used to carry oxygen, and is therefore greatly reduced. The insect circulatory system is open; it has no [[vein]]s or [[artery|arteries]], and instead consists of little more than a single, perforated dorsal tube that pulses [[peristalsis|peristaltically]]. This dorsal blood vessel is divided into two sections: the heart and aorta. The dorsal blood vessel circulates the [[hemolymph]], arthropods' fluid analog of [[blood]], from the rear of the body cavity forward.{{sfn|Gullan|Cranston|2005|pp=61–65}}<ref>{{cite web |url=http://www.cals.ncsu.edu/course/ent425/tutorial/circulatory.html |title=Circulatory System |last=Meyer|first=John R. |date=17 February 2006 |publisher=Department of Entomology, NC State University |location=NC State University |page=1 |archive-url=https://web.archive.org/web/20090927000720/http://www.cals.ncsu.edu/course/ent425/tutorial/circulatory.html |archive-date=27 September 2009 |url-status=dead |access-date=11 October 2009 |df=dmy-all}}</ref> Hemolymph is composed of plasma in which [[hemocytes]] are suspended. Nutrients, hormones, wastes, and other substances are transported throughout the insect body in the hemolymph. Hemocytes include many types of cells that are important for immune responses, wound healing, and other functions. Hemolymph pressure may be increased by muscle contractions or by swallowing air into the digestive system to aid in molting.<ref>{{cite book |last1=Triplehorn |first1=Charles |title=Borror and DeLong's introduction to the study of insects |publisher=Thompson Brooks/Cole |others=Johnson, Norman F., Borror, Donald J. |year=2005 |isbn=978-0030968358 |edition=7th |location=Belmont, California |pages=27–28 |oclc=55793895 }}</ref>

==== Sensory ====

{{further|Insect physiology#Sensory organs}}

[[File:Calliphora sp Portrait.jpg|thumb|upright|Most insects have a pair of large [[compound eye]]s and other sensory organs such as antennae able to detect movements and chemical stimuli on their heads.]]

Many insects possess numerous specialized [[Sensilla|sensory organs]] able to detect stimuli including limb position ([[proprioception]]) by [[campaniform sensilla]], light, [[Hygroreception|water]], chemicals (senses of [[taste]] and [[Sense of smell|smell]]), sound, and heat.{{sfn|Gullan|Cranston|2014|pp=95–124}} Some insects such as [[bees]] can perceive [[ultraviolet]] wavelengths, or detect [[polarized light]], while the [[antenna (biology)|antennae]] of male moths can detect the [[pheromone]]s of female moths over distances of over a kilometer.<ref>{{cite web |url=http://crazydaz.com/insects.pdf |title=Insects |publisher=Alien Life Forms |page=4 |access-date=17 May 2009 |archive-url=https://web.archive.org/web/20110708201217/http://crazydaz.com/insects.pdf |archive-date=8 July 2011 |url-status=live}}</ref> There is a trade-off between visual acuity and chemical or tactile acuity, such that most insects with well-developed eyes have reduced or simple antennae, and vice versa. Insects perceive sound by different mechanisms, such as thin vibrating membranes ([[Tympanal organ|tympana]]).{{sfn|Gullan|Cranston|2014|pp=97–103}} Insects were the earliest organisms to produce and sense sounds. Hearing has evolved independently at least 19 times in different insect groups.<ref name="Warren Nowotny 2021">{{Cite journal |first1=Ben |last1=Warren |first2=Manuela |last2=Nowotny |title=Bridging the Gap Between Mammal and Insect Ears – A Comparative and Evolutionary View of Sound-Reception |date=11 April 2021 |journal=Frontiers in Ecology and Evolution |volume=9 |doi=10.3389/fevo.2021.667218 |doi-access=free |bibcode=2021FrEEv...967218W }}</ref>

Most insects, except some [[Rhaphidophoridae|cave crickets]], are able to perceive light and dark. Many have acute vision capable of detecting small and rapid movements. The eyes may include simple eyes or [[Ocellus|ocelli]] as well as larger [[compound eye]]s. Many species can detect light in the [[infrared]], [[ultraviolet]] and [[visible light]] wavelengths, with color vision. Phylogenetic analysis suggests that UV-green-blue [[trichromacy]] existed from at least the [[Devonian]] period, some 400 million years ago.<ref name="insvision">{{cite journal |last1=Briscoe |first1=A. D. |last2=Chittka |first2=L. |title=The evolution of color vision in insects |year=2001 |journal=Annual Review of Entomology |volume=46 |pages=471–510 |doi=10.1146/annurev.ento.46.1.471 |pmid=11112177 |s2cid=20894534 }}</ref>

The individual lenses in compound eyes are immobile, but fruit flies have photoreceptor cells underneath each lens which move rapidly in and out of focus, in a series of movements called photoreceptor microsaccades. This gives them, and possibly many other insects, a much clearer image of the world than previously assumed.<ref>{{Cite journal |title=Binocular mirror–symmetric microsaccadic sampling enables Drosophila hyperacute 3D vision |first1=Joni |last1=Kemppainen |first2=Ben |last2=Scales |first3=Keivan |last3=Razban Haghighi |first4=Jouni |last4=Takalo |first5=Neveen |last5=Mansour |first6=James |last6=McManus |first7=Gabor|last7=Leko |first8=Paulus |last8=Saari |first9=James |last9=Hurcomb |first10=Andra |last10=Antohi |first11=Jussi-Petteri |last11=Suuronen |first12=Florence |last12=Blanchard |first13=Roger C.|last13=Hardie |first14=Zhuoyi |last14=Song |first15=Mark |last15=Hampton |first16=Marina |last16=Eckermann |first17=Fabian |last17=Westermeier |first18=Jasper |last18=Frohn |first19=Hugo |last19=Hoekstra |first20=Chi-Hon |last20=Lee |first21=Marko |last21=Huttula |first22=Rajmund |last22=Mokso |first23=Mikko |last23=Juusola |display-authors=5 |date=22 March 2022 |journal=Proceedings of the National Academy of Sciences |volume=119 |issue=12 |pages=e2109717119 |doi=10.1073/pnas.2109717119 |doi-access=free |pmid=35298337 |pmc=8944591 |bibcode=2022PNAS..11909717K }}</ref>

An insect's [[Insect olfaction |sense of smell]] is via [[Chemoreceptor|chemical receptors]], usually on the antennae and the mouthparts. These detect both airborne [[Volatile organic compound|volatile compounds]] and odorants on surfaces, including pheromones from other insects and compounds released by food plants. Insects use olfaction to locate mating partners, food, and places to lay eggs, and to avoid predators. It is thus an extremely important sense, enabling insects to discriminate between thousands of volatile compounds.<ref name="Carraher Dalziel Jordan 2015">{{Cite journal |last1=Carraher |first1=Colm |last2=Dalziel |first2=Julie |last3=Jordan |first3=Melissa D. |last4=Christie |first4=David L. |last5=Newcomb |first5=Richard D. |last6=Kralicek |first6=Andrew V. |title=Towards an understanding of the structural basis for insect olfaction by odorant receptors |journal=Insect Biochemistry and Molecular Biology |volume=66 |pages=31–41 |doi=10.1016/j.ibmb.2015.09.010 |year=2015 |pmid=26416146|bibcode=2015IBMB...66...31C |url=https://figshare.com/articles/journal_contribution/22745009 }}</ref>

Some insects are capable of [[magnetoreception]]; ants and bees navigate using it both locally (near their nests) and when migrating.<ref name="Wajnberg 2010">{{cite journal |last1=Wajnberg |first1=E. |author2=Acosta-Avalos, D. |author3=Alves, O.C. |author4=de Oliveira, J.F. |author5=Srygley, R.B. |author6=Esquivel, D.M. |year=2010 |title=Magnetoreception in eusocial insects: An update |journal=[[Journal of the Royal Society Interface]] |volume=7 |issue=Suppl 2 |pages=S207–S225 |doi=10.1098/rsif.2009.0526.focus |pmid=20106876 |pmc=2843992}}</ref> The [[Schwarziana quadripunctata|Brazilian stingless bee]] detects magnetic fields using the hair-like [[sensillum|sensilla]] on its antennae.<ref name="Esquivel 2005">{{cite journal |last1=Esquivel |first1=Darci M.S. |last2=Wajnberg |first2=E. |last3=do Nascimento |first3=F.S. |last4=Pinho |first4=M.B. |last5=Lins de Barros |first5=H.G.P. |last6=Eizemberg |first6=R. |year=2005 |title=Do Magnetic Storms Change Behavior of the Stingless Bee Guiriçu (''Schwarziana quadripunctata'')? |journal=[[Naturwissenschaften]] |volume=94 |issue=2 |pages=139–142 |doi=10.1007/s00114-006-0169-z |pmid=17028885|s2cid=10746883 }}</ref><ref name="Lucano 2005">{{cite journal |last1=Lucano |first1=M.J. |last2=Cernicchiaro |first2=G. |last3=Wajnberg |first3=E. |last4=Esquivel |first4=D.M.S. |year=2005 |title=Stingless Bee Antennae: A Magnetic Sensory Organ? |journal=[[BioMetals (journal)|BioMetals]] |volume=19 |issue=3 |pages=295–300 |doi=10.1007/s10534-005-0520-4 |pmid=16799867 |s2cid=10162385 }}</ref>

== Reproduction and development ==

=== Life-cycles ===

[[File:Mating Pair of Spotted Fritillaries on Greater Pignut.JPG|thumb|left|[[Butterfly|Butterflies]] mating ]]

The majority of insects hatch from [[egg (biology)|eggs]]. The fertilization and development takes place inside the egg, enclosed by a shell ([[chorion]]) that consists of maternal tissue. In contrast to eggs of other arthropods, most insect eggs are drought resistant. This is because inside the chorion two additional membranes develop from embryonic tissue, the [[amnion]] and the [[serosa]]. This serosa secretes a [[cuticle]] rich in [[chitin]] that protects the embryo against desiccation.<ref>{{cite journal |last1=Jacobs |first1=C. G. |last2=Rezende |first2=G. L. |last3=Lamers |first3=G. E. |last4=van der Zee |first4=M. |year=2013 |title=The extraembryonic serosa protects the insect egg against desiccation |journal=Proceedings of the Royal Society of London B |volume=280 |issue=1764 |page=20131082 |pmid=23782888 |doi=10.1098/rspb.2013.1082 |pmc=3712428 }}</ref> Some species of insects, like aphids and tsetse flies, are [[Ovoviviparity|ovoviviparous]]: their eggs develop entirely inside the female, and then hatch immediately upon being laid.<ref name="mcgraw-hill">"insect physiology" ''McGraw-Hill Encyclopedia of Science and Technology'', Ch. 9, p. 233, 2007</ref> Some other species, such as in the cockroach genus ''[[Diploptera]]'', are [[viviparous]], [[Gestation|gestating]] inside the mother and [[Viviparity|born alive]].{{sfn|Gullan|Cranston|2005|pp=129, 131, 134–135}} Some insects, like [[parasitoid wasp]]s, are [[polyembryony|polyembryonic]], meaning that a single fertilized egg divides into many separate embryos.{{sfn|Gullan|Cranston|2005|pp=136–137}} Insects may be [[Voltinism|univoltine]], bivoltine or multivoltine, having one, two or many broods in a year.<ref name="OdonateGlossary">{{cite web |title=Glossary of Lepidopteran and Odonate anatomy |url=http://www.vararespecies.org/glossary.shtml |website=Rare species atlas |publisher=Virginia Department of Conservation and Recreation |year=2013 |access-date=14 June 2013 |archive-url=https://web.archive.org/web/20131004213832/http://www.vararespecies.org/glossary.shtml |archive-date=4 October 2013|url-status=live }}</ref>

[[File:Aphid-giving-birth.jpg|thumb|[[Aphid]] giving birth to live female young by [[parthenogenesis]] from unfertilized eggs]]
[[File:Acanthocephala terminalis egg depositing.webm|thumb|thumbtime=80|A female [[Acanthocephala terminalis|leaf-footed bug]] deposits an egg before flying off.]]

Other developmental and reproductive variations include [[haplodiploidy]], [[Polymorphism (biology)|polymorphism]], [[paedomorphosis]] or [[peramorphosis]], [[sexual dimorphism]], parthenogenesis, and more rarely [[hermaphroditism]].{{sfn|Gullan|Cranston|2005|p=143}}<ref>{{Cite book |last=Judson |first=Olivia |url=https://books.google.com/books?id=HzzdYcX9P54C&pg=PA198 |title=Dr. Tatiana's Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex |date=2002-08-14 |publisher=Macmillan |isbn=978-0-8050-6331-8 |page=198}}</ref> In [[haplodiploidy]], which is a type of [[sex-determination system]], the offspring's sex is determined by the number of sets of [[chromosome]]s an individual receives. This system is typical in bees and wasps.<ref>{{cite journal |author1=Hughes, William O. H. |author2=Oldroyd, Benjamin P. |author3=Beekman, Madeleine |author4=Ratnieks, Francis L. W. |title=Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality|journal=[[Science (journal)|Science]] |volume=320 |issue=5880 |pages=1213–1216 |date=2008 |pmid=18511689 |doi=10.1126/science.1156108 |bibcode=2008Sci...320.1213H |s2cid=20388889 }}</ref> 

Some insects are [[parthenogenesis|parthenogenetic]], meaning that the female can reproduce and give birth without having the eggs [[fertilization|fertilized]] by a [[male]]. Many aphids undergo a cyclical form of parthenogenesis in which they alternate between one or many generations of asexual and sexual reproduction.<ref>{{cite journal  |last1=Nevo |first1=E. |last2=Coll |first2=M. |title=Effect of nitrogen fertilization on ''Aphis gossypii'' (Homoptera: Aphididae): variation in size, color, and reproduction |journal=Journal of Economic Entomology |volume=94 |issue=1 |pages=27–32 |year=2001 |pmid= 11233124 |doi=10.1603/0022-0493-94.1.27 |s2cid=25758038 |doi-access=free }}</ref><ref>{{cite journal |last1=Jahn |first1=G. C. |author2=Almazan, L .P. |author3=Pacia, J. |title=Effect of nitrogen fertilizer on the intrinsic rate of increase of the rusty plum aphid, ''Hysteroneura setariae'' (Thomas) (Homoptera: Aphididae) on rice (''Oryza sativa'' L.) |url=http://docserver.esa.catchword.org/deliver/cw/pdf/esa/freepdfs/0046225x/v34n4s26.pdf |archive-url=http://webarchive.nationalarchives.gov.uk/20100909141631/http%3A//docserver.esa.catchword.org/deliver/cw/pdf/esa/freepdfs/0046225x/v34n4s26.pdf |url-status=dead |archive-date=2010-09-09 |journal=Environmental Entomology |volume=34 |issue=4 |pages=938–943 |year=2005 |doi=10.1603/0046-225X-34.4.938 |s2cid=1941852 }}</ref> In summer, aphids are generally female and parthenogenetic; in the autumn, males may be produced for sexual reproduction. Other insects produced by parthenogenesis are bees, wasps and ants; in their [[Haplodiploidy|haplodiploid]] system, [[diploid]] females spawn many females and a few [[haploid]] males.<ref name="mcgraw-hill"/>

=== Metamorphosis ===

[[Metamorphosis]] in insects is the process of development that converts young to adults. There are two forms of metamorphosis: incomplete and complete.

==== Incomplete ====

{{Main|Hemimetabolism}}

[[File:Locust instars NMNS (aligned).jpg|thumb|upright=2|[[hemimetabolism|Incomplete metamorphosis]] in a [[locust]] with multiple [[instar]]s. Egg is not shown. The largest specimen is adult.]]

[[hemimetabolism|Hemimetabolous]] insects, those with incomplete metamorphosis, change gradually after hatching from the [[Egg (biology)|egg]] by undergoing a series of [[ecdysis|molts]] through stages called [[instar]]s, until the final, [[adult]], stage is reached. An insect molts when it outgrows its exoskeleton, which does not stretch and would otherwise restrict the insect's growth. The molting process begins as the insect's [[epidermis (skin)|epidermis]] secretes a new [[epicuticle]] inside the old one. After this new epicuticle is secreted, the epidermis releases a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. When this stage is complete, the insect makes its body swell by taking in a large quantity of water or air; this makes the old cuticle split along predefined weaknesses where it was thinnest.{{sfn|Gullan|Cranston|2005|p=142}}<ref name="RuppertFoxBarnes2004P523To524">{{cite book |author1=Ruppert, E. E. |author2=Fox, R. S. |author3=Barnes, R. D. |title=Invertebrate Zoology |publisher=Brooks / Cole |edition=7th |isbn=978-0-03-025982-1 |year=2004 |pages=[https://archive.org/details/isbn_9780030259821/page/523 523–524] |url=https://archive.org/details/isbn_9780030259821/page/523 }}</ref>

==== Complete ====

{{Main|Holometabolism}}

[[File:Monarch life-cycle.jpg|thumb|upright=2|Life-cycle of butterfly, undergoing [[Holometabolism|complete metamorphosis]] from egg through [[caterpillar]] larvae to [[pupa]] and adult]]

[[Holometabolism]], or complete metamorphosis, is where the insect changes in four stages, an egg or [[embryo]], a [[Insect larvae|larva]], a [[pupa]] and the adult or [[imago]]. In these species, an egg hatches to produce a larva, which is generally worm-like in form. This can be eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform (elongated, flattened and active), elateriform (wireworm-like) or vermiform (maggot-like). The larva grows and eventually becomes a pupa, a stage marked by reduced movement. There are three [[Pupa#Types|types of pupae]]: obtect, exarate or coarctate. Obtect pupae are compact, with the legs and other appendages enclosed. Exarate pupae have their legs and other appendages free and extended. Coarctate pupae develop inside the larval skin.{{sfn|Gullan|Cranston|2005|p=151}} Insects undergo considerable change in form during the pupal stage, and emerge as adults. Butterflies are well-known for undergoing complete metamorphosis; most insects use this life cycle. Some insects have evolved this system to [[hypermetamorphosis]]. Complete metamorphosis is a trait of the most diverse insect group, the [[Endopterygota]].{{sfn|Gullan|Cranston|2005|p=143}}

== Communication ==

Insects that produce sound can generally hear it. Most [[Animal hearing|insects can hear]] only a [[Hearing range|narrow range]] of [[Frequency|frequencies]] related to the frequency of the sounds they can produce. Mosquitoes can hear up to 2 [[Hertz|kilohertz]].<ref>{{cite journal |last1=Cator |first1=L.J. |date= 2009|title=Harmonic convergence in the love songs of the dengue vector mosquito |journal=Science |volume=323 |issue=5917 |pages=1077–1079 |doi=10.1126/science.1166541 |pmc=2847473 |pmid=19131593 |last2=Arthur |first2=B.J. |last3=Harrington |first3=L.C. |last4=Hoy |first4=R.R. |bibcode=2009Sci...323.1077C}}</ref> Certain predatory and parasitic insects can detect the characteristic sounds made by their prey or hosts, respectively. Likewise, some nocturnal moths can perceive the [[Ultrasound|ultrasonic]] emissions of [[bat]]s, which helps them [[Ultrasound avoidance|avoid predation]].{{sfn|Gullan|Cranston|2005|pp=87–94}} 

=== Light production ===

A few insects, such as [[Mycetophilidae]] (Diptera) and the beetle families [[Lampyridae]], [[Phengodidae]], [[Elateridae]] and [[Staphylinidae]] are [[bioluminescent]]. The most familiar group are the [[fireflies]], beetles of the family Lampyridae. Some species are able to control this light generation to produce flashes. The function varies with some species using them to attract mates, while others use them to lure prey. Cave dwelling larvae of ''[[Arachnocampa]]'' (Mycetophilidae, fungus gnats) glow to lure small flying insects into sticky strands of silk.<ref>{{cite journal |last=Pugsley |first=Chris W. |title=Literature review of the New Zealand glowworm ''Arachnocampa luminosa'' (Diptera: Keroplatidae) and related cave-dwelling Diptera |journal=New Zealand Entomologist |year=1983 |volume=7 |issue=4 |pages=419–424 |url=http://www.ento.org.nz/nzentomologist/free_issues/NZEnto07_4_1983/Volume%207-4-419-424.pdf |archive-url=https://web.archive.org/web/20071020094802/http://www.ento.org.nz/nzentomologist/free_issues/NZEnto07_4_1983/Volume%207-4-419-424.pdf |url-status=dead |archive-date=2007-10-20 |doi=10.1080/00779962.1983.9722435|bibcode=1983NZEnt...7..419P }}</ref> Some fireflies of the genus ''[[Photuris]]'' [[mimicry|mimic]] the flashing of female ''[[Photinus (beetle)|Photinus]]'' species to attract males of that species, which are then captured and devoured.<ref>{{cite journal |first=James E. |last=Lloyd |title=Occurrence of Aggressive Mimicry in Fireflies |journal=The Florida Entomologist |volume=67 |issue=3 |year=1984 |pages=368–376 |doi=10.2307/3494715 |jstor=3494715 |s2cid=86502129}}</ref> The colors of emitted light vary from dull blue (''Orfelia fultoni'', Mycetophilidae) to the familiar greens and the rare reds (''Phrixothrix tiemanni'', Phengodidae).<ref>{{cite book |first1=James E. |last1=Lloyd |last2=Gentry |first2=Erin C. |title=The Encyclopedia of Insects |pages=[https://archive.org/details/encyclopediaofin00bada/page/115 115–120] |year=2003 |publisher=Academic Press |isbn=978-0-12-586990-4 |url=https://archive.org/details/encyclopediaofin00bada/page/115 }}</ref>

=== Sound production ===

Insects make sounds mostly by mechanical action of appendages. In [[grasshopper]]s and crickets, this is achieved by [[stridulation]]. [[Cicada]]s make the loudest sounds among the insects by producing and amplifying sounds with special modifications to their body to form [[tymbal]]s and associated musculature. The African [[cicada]] ''[[Brevisana brevis]]'' has been measured at 106.7&nbsp;[[decibel]]s at a distance of {{convert|50|cm|in|abbr=on}}.<ref name="walker">{{Cite web |title=The University of Florida Book of Insect Records|publisher=Department of Entomology & Nematology, UF/IFAS |url=https://entnemdept.ufl.edu/walker/ufbir/index.shtml |access-date=2022-01-13 |website=entnemdept.ufl.edu }}</ref> Some insects, such as the ''[[Helicoverpa zea]]'' moths, [[hawk moth]]s and [[Hedylidae|Hedylid]] butterflies, can hear [[ultrasound]] and [[Ultrasound avoidance|take evasive action]] when they sense that they have been detected by bats.<ref>{{cite journal |last=Kay |first=Robert E. |title=Acoustic signalling and its possible relationship to assembling and navigation in the moth, ''Heliothis zea''|journal= Journal of Insect Physiology |volume=15 |issue=6 |year=1969|pages= 989–1001 |doi=10.1016/0022-1910(69)90139-5|bibcode=1969JInsP..15..989K }}</ref><ref>{{cite journal |last=Spangler |first=Hayward G. |title=Moth hearing, defense, and communication|journal= Annual Review of Entomology |volume=33 |issue=1 |year=1988 |pages=59–81 |doi=10.1146/annurev.ento.33.1.59}}</ref> Some moths produce ultrasonic clicks that warn predatory bats of their unpalatability (acoustic [[aposematism]]),<ref>{{cite journal |last1=Hristov |first1=N. I. |last2=Conner |first2=William E. |year=2005 |title=Sound strategy: acoustic aposematism in the bat–tiger moth arms race |journal=Naturwissenschaften |volume=92 |pages=164–169 |doi=10.1007/s00114-005-0611-7 |pmid=15772807 |issue=4 |bibcode=2005NW.....92..164H |s2cid=18306198 }}</ref> while some palatable moths have evolved to mimic these calls (acoustic [[Batesian mimicry]]).<ref>{{cite journal |last1=Barber |first1=J. R. |last2=Conner |first2=W. E. |year=2007 |title=Acoustic mimicry in a predator–prey interaction |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=22 |pages=9331–9334 |doi=10.1073/pnas.0703627104|pmid=17517637 |pmc=1890494 |bibcode=2007PNAS..104.9331B |doi-access=free }}</ref> The claim that some moths can [[Echolocation jamming#Jamming by prey|jam bat sonar]] has been revisited. Ultrasonic recording and high-speed infrared videography of bat-moth interactions suggest the palatable tiger moth really does defend against attacking big brown bats using ultrasonic clicks that jam bat sonar.<ref>{{cite journal |last=Corcoran |first=Aaron J. |author2=Barber, Jesse R. |last3=Conner |first3=William E. |year=2009 |title=Tiger Moth Jams Bat Sonar |journal=Science |volume=325 |issue=5938 |pages=325–327 |doi=10.1126/science.1174096 |bibcode=2009Sci...325..325C |pmid=19608920 |s2cid=206520028 }}</ref>

{{Listen |filename=Grasshoppers.ogg |title=Grasshopper stridulation |description=Several unidentified grasshoppers stridulating |format=[[Ogg]] |pos=right}}

Very low sounds are produced in various species of [[Coleoptera]], [[Hymenoptera]], [[Lepidoptera]], [[Mantodea]] and [[Neuroptera]]. These low sounds are produced by the insect's movement, amplified by stridulatory structures on the insect's muscles and joints; these sounds can be used to warn or communicate with other insects. Most sound-making insects also have [[tympanal organ]]s that can perceive airborne sounds. Some [[hemiptera]]ns, such as the [[Corixidae|water boatmen]], communicate via underwater sounds.<ref>{{cite journal |last=Theiss |first=Joachim |year=1982 |title=Generation and radiation of sound by stridulating water insects as exemplified by the corixids |journal=Behavioral Ecology and Sociobiology |volume=10 |pages=225–235 |doi=10.1007/BF00299689 |issue=3 |bibcode=1982BEcoS..10..225T |s2cid=10338592}}</ref>

[[File:Field cricket Gryllus pennsylvanicus.ogg|thumb|Cricket in garage with familiar call]]

Communication using surface-borne vibrational signals is more widespread among insects because of size constraints in producing air-borne sounds.<ref>{{cite journal |last1=Virant-Doberlet |first1=M. |last2=Čokl |first2=Andrej |year=2004 |title=Vibrational communication in insects |journal=Neotropical Entomology |volume=33 |issue=2 |pages=121–134|doi=10.1590/S1519-566X2004000200001 |doi-access=free }}</ref> Insects cannot effectively produce low-frequency sounds, and high-frequency sounds tend to disperse more in a dense environment (such as [[foliage]]), so insects living in such environments communicate primarily using substrate-borne vibrations.<ref>{{cite journal |last=Bennet-Clark |first=H. C. |year=1998 |title=Size and scale effects as constraints in insect sound communication |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=353 |pages=407–419 |doi=10.1098/rstb.1998.0219 |issue=1367 |pmc=1692226 }}</ref> 

Some species use vibrations for communicating, such as to attract mates as in the songs of the [[shield bug]] ''[[Nezara viridula]]''.<ref>{{cite journal |journal=Journal of Insect Behavior |title=The Influence of Substrate on Male Responsiveness to the Female Calling Song in ''Nezara viridula'' |volume=14 |issue=3 |pages=313–332 |year=2001 |first1=Nadège |last1=Miklas |author2=Stritih, Nataša |author3=Čokl, Andrej |author4=Virant-Doberlet, Meta |author5=Renou, Michel |doi=10.1023/A:1011115111592|bibcode=2001JIBeh..14..313M |s2cid=11369425 }}</ref> Vibrations can also be used to communicate between species; [[lycaenid]] caterpillars, which [[Myrmecophile|form a mutualistic association with ants]] communicate with ants in this way.<ref>{{cite journal |last=DeVries |first=P. J. |year=1990 |title=Enhancement of symbiosis between butterfly caterpillars and ants by vibrational communication |journal=Science |volume=248 |pages=1104–1106 |doi=10.1126/science.248.4959.1104 |pmid=17733373 |issue=4959|bibcode=1990Sci...248.1104D |s2cid=35812411 }}</ref> The [[Madagascar hissing cockroach]] has the ability to press air through its spiracles to make a hissing noise as a sign of aggression;<ref>{{cite journal |last=Nelson |first=Margaret C. |author2=Fraser, Jean |year=1980 |title=Sound production in the cockroach, Gromphadorhina portentosa: evidence for communication by hissing |journal=Behavioral Ecology and Sociobiology |volume=6 |issue=4 |pages=305–314 |doi=10.1007/BF00292773 |bibcode=1980BEcoS...6..305N |s2cid=9637568 }}</ref> the [[death's-head hawkmoth]] makes a squeaking noise by forcing air out of their pharynx when agitated, which may also reduce aggressive worker honey bee behavior when the two are close.<ref>{{cite journal |last1=Moritz |first1=R. F. A. |last2=Kirchner |first2=W. H. |last3=Crewe |first3=R. M. |date=1991 |title=Chemical camouflage of the death's head hawkmoth (Acherontia atropos L.) in honeybee colonies |journal=Naturwissenschaften |volume=78 |issue=4 |doi=10.1007/BF01136209 |bibcode=1991NW.....78..179M |pages=179–182 |s2cid=45597312 }}</ref>

=== Chemical communication ===

{{main |Chemical communication in insects |Insect olfaction}}

[[File:Pheromonal glands in ants.png|thumb|Social insects such as ants have multiple types of [[Pheromone|pheromonal]] glands, producing different [[semiochemical]]s for [[Chemical communication in insects|communication with other insects]].{{sfn |Gullan |Cranston |2005 |pp=96–105}}]]

Many insects have evolved [[chemical communication in insects |chemical means for communication]]. These [[semiochemical]]s are often derived from plant metabolites including those meant to attract, repel and provide other kinds of information. [[Pheromone]]s are used for attracting mates of the opposite sex, for aggregating [[conspecificity|conspecific]] individuals of both sexes, for deterring other individuals from approaching, to mark a trail, and to trigger aggression in nearby individuals. [[Allomone]]s benefit their producer by the effect they have upon the receiver. [[Kairomone]]s benefit their receiver instead of their producer. Synomones benefit the producer and the receiver. While some chemicals are targeted at individuals of the same species, others are used for communication across species. The use of scents is especially well-developed in social insects.{{sfn |Gullan |Cranston |2005 |pp=96–105}} {{visible anchor |Cuticular hydrocarbons |Cuticular hydrocarbon}} are nonstructural materials produced and secreted to the cuticle surface to fight [[desiccation]] and [[pathogen]]s. They are important, too, as pheromones, especially in social insects.<ref name="Yan-Liebig-2021">{{cite journal |last1=Yan |first1=Hua |last2=Liebig |first2=Jürgen |title=Genetic basis of chemical communication in eusocial insects |journal=[[Genes & Development]] |publisher=[[Cold Spring Harbor Laboratory Press]] & [[The Genetics Society]] |volume=35 |issue=7–8 |date=1 April 2021 |doi=10.1101/gad.346965.120 |pages=470–482 |pmid=33861721 |pmc=8015721 }}</ref>

== Social behavior ==

{{main|Eusociality}}

{{multiple image
|total_width=300
|image1=Termite Cathedral DSC03570.jpg
|caption1=A [[Nasutitermes triodiae|cathedral mound]] created by [[Eusociality|eusocial]] [[mound-building termites]].
|image2=Bee waggle dance.png
|caption2=[[Honey bee]]'s figure-eight [[waggle dance]]. An orientation 45° to the right of ‘up' on the comb indicates food 45° to the right of the sun. The dancer's rapid waggling blurs her abdomen.
}}

[[Social insect]]s, such as [[termite]]s, [[ant]]s and many [[bee]]s and [[wasp]]s, are [[Eusociality|eusocial]].<ref>{{cite web |title=Social insects |last=Brewer |first=Gary |url=http://www.ndsu.nodak.edu/entomology/topics/societies.htm |publisher=North Dakota State University |access-date=6 May 2009 |archive-url=https://web.archive.org/web/20080321171246/http://www.ndsu.nodak.edu/entomology/topics/societies.htm |archive-date=21 March 2008}}</ref> They live together in such large well-organized colonies of genetically similar individuals that they are sometimes considered [[superorganism]]s. In particular, reproduction is largely limited to a [[Gyne|queen caste]]; other females are [[worker caste|workers]], prevented from reproducing by [[worker policing]]. [[Honey bee]]s have evolved a system of abstract symbolic communication where a behavior is used to represent and convey specific information about the environment. In this communication system, called [[Bee learning and communication|dance language]], the angle at which a bee dances represents a direction relative to the sun, and the length of the dance represents the distance to be flown.{{sfn|Gullan|Cranston|2005|pp=309–311}} [[Bumblebee]]s too have some social communication behaviors. ''[[Bombus terrestris]]'', for example, more rapidly learns about visiting unfamiliar, yet rewarding flowers, when they can see a conspecific foraging on the same species.<ref>{{cite journal |author1=Leadbeater, E. |author2=Chittka, L. |year=2007 |title=The dynamics of social learning in an insect model, the bumblebee (''Bombus terrestris'') |journal=Behavioral Ecology and Sociobiology |volume=61 |issue=11 |pages=1789–1796 |doi=10.1007/s00265-007-0412-4 |bibcode=2007BEcoS..61.1789L |s2cid=569654 }}</ref>

Only insects that live in nests or colonies possess fine-scale spatial orientation. Some can [[Animal navigation|navigate]] unerringly to a single hole a few millimeters in diameter among thousands of similar holes, after a trip of several kilometers. In [[philopatry]], insects that [[hibernation|hibernate]] are able to recall a specific location up to a year after last viewing the area of interest.<ref>{{cite journal |last=Salt |first=R. W. |date=1961 |title=Principles of Insect Cold-Hardiness |journal=Annual Review of Entomology |doi=10.1146/annurev.en.06.010161.000415 |volume=6 |pages=55–74 }}</ref> A few insects seasonally [[Insect migration|migrate]] large distances between different geographic regions, as in the continent-wide [[monarch butterfly migration]].{{sfn|Gullan|Cranston|2005|p=14}}

=== Care of young ===

[[Eusocial]] insects build nests, guard eggs, and provide food for offspring full-time. Most insects, however, lead short lives as adults, and rarely interact with one another except to mate or compete for mates. A small number provide [[parental care]], where they at least guard their eggs, and sometimes guard their offspring until adulthood, possibly even feeding them. Many wasps and bees construct a nest or burrow, [[Mass provisioning|store provisions]] in it, and lay an egg upon those provisions, providing no further care.<ref>{{cite web |title=Social Insects |url=http://www.ndsu.nodak.edu/entomology/topics/societies.htm |publisher=North Dakota State University |access-date=12 October 2009 |archive-url=https://web.archive.org/web/20080321171246/http://www.ndsu.nodak.edu/entomology/topics/societies.htm |archive-date=21 March 2008}}</ref>

== Locomotion ==

=== Flight ===

{{main|Insect flight}}

[[File:Hoverfly September 2007-7 (cropped).jpg|thumb|Insects such as [[hoverflies]] are capable of rapid and agile [[insect flight|flight]]. ]]

Insects are the only group of [[invertebrate]]s to have developed flight. The ancient groups of insects in the Palaeoptera, the dragonflies, damselflies and mayflies, operate their wings directly by paired muscles attached to points on each wing base that raise and lower them. This can only be done at a relatively slow rate. All other insects, the Neoptera, have [[indirect flight]], in which the flight muscles cause rapid oscillation of the thorax: there can be more wingbeats than nerve impulses commanding the muscles. One pair of flight muscles is aligned vertically, contracting to pull the top of the thorax down, and the wings up. The other pair runs longitudinally, contracting to force the top of the thorax up and the wings down.<ref name="Chapman">{{cite book |last=Chapman |first=A. D. |year=2006 |title=Numbers of living species in Australia and the World |publisher=Canberra: [[Australian Biological Resources Study]] |isbn=978-0-642-56850-2 |url=http://www.deh.gov.au/biodiversity/abrs/publications/other/species-numbers/index.html |archive-url=https://archive.today/20121130101928/http://www.deh.gov.au/biodiversity/abrs/publications/other/species-numbers/index.html |url-status=dead |archive-date=30 November 2012 }}</ref><ref>{{cite journal |last=Smith |first=D. S. |title=Flight muscles of insects |journal=[[Scientific American]] |volume=212 |pages=76–88 |year=1965 |issue=6 |pmid=14327957 |bibcode=1965SciAm.212f..76S |doi=10.1038/scientificamerican0665-76 }}</ref> Most insects gain [[aerodynamic lift]] by creating a spiralling [[vortex]] at the [[leading edge]] of the wings.<ref name="aerodynamics">{{cite journal |last=Sane |first=Sanjay P.  |title=The aerodynamics of insect flight |journal=[[Journal of Experimental Biology]] |year=2003 |volume=206 |pages=4191–4208 |doi=10.1242/jeb.00663 |pmid=14581590 |issue=23 |s2cid=17453426 |doi-access=free |bibcode=2003JExpB.206.4191S |url=http://www.cs.washington.edu/homes/diorio/MURI2003/Publications/sane_review.pdf}}</ref> Small insects like thrips with tiny feathery wings gain lift using the [[clap and fling]] mechanism; the wings are clapped together and pulled apart, flinging vortices into the air at the leading edges and at the wingtips.<ref>{{cite journal |last=Weis-Fogh |first=Torkel |author-link=Torkel Weis-Fogh |date=1973 |title=Quick estimates of flight fitness in hovering animals, including novel mechanisms of lift production |journal=[[Journal of Experimental Biology]] |volume=59 |issue=1 |pages=169–230 |doi=10.1242/jeb.59.1.169 |bibcode=1973JExpB..59..169W }}</ref><ref>{{Cite journal |last=Bennett |first=L. |date=1977 |title=Clap and fling aerodynamics- an experimental evaluation |journal=[[Journal of Experimental Biology]] |volume=69 |issue=1 |pages=261–272 |doi=10.1242/jeb.69.1.261 |bibcode=1977JExpB..69..261B }}</ref>

The evolution of [[insect wing]]s has been [[Insect wing#Hypotheses|a subject of debate]]; it has been suggested they came from modified gills, flaps on the spiracles, or an appendage, the epicoxa, at the base of the legs.<ref>{{cite journal |last1=Jockusch |first1=E. L. |last2=Ober |first2=K. A. |title=Hypothesis testing in evolutionary developmental biology: a case study from insect wings |journal=Journal of Heredity |date=September 2004 |volume=95 |issue=5 |pages=382–396 |doi=10.1093/jhered/esh064 |pmid=15388766 |doi-access=free }}</ref> More recently, entomologists have favored evolution of wings from lobes of the [[notum]], of the [[Pleuron (insect anatomy)|pleuron]], or more likely both.<ref>{{cite book |last=Grimaldi |first=David A. |title=The Complete Insect: Anatomy, Physiology, Evolution, and Ecology |url=https://books.google.com/books?id=xGquEAAAQBAJ&dq=insects+wings+notum+pleuron&pg=PA135 |date=2023 |publisher=[[Princeton University Press]] |page=135 |isbn=9780691243115}}</ref> 
In the [[Carboniferous]] age, the dragonfly-like ''[[Meganeura]]'' had as much as a {{convert|50|cm|in|abbr=on}} wide wingspan. The appearance of gigantic insects is consistent with high atmospheric oxygen at that time, as the respiratory system of insects constrains their size.<ref>{{cite journal |last=Dudley |first=R. |year=1998 |title=Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance |journal=Journal of Experimental Biology |volume=201 |issue=8 |pages=1043–1050 |doi=10.1242/jeb.201.8.1043 |pmid=9510518 |bibcode=1998JExpB.201.1043D |url=http://jeb.biologists.org/content/201/8/1043.full.pdf |access-date=8 December 2012 |archive-url=https://web.archive.org/web/20130124032600/http://jeb.biologists.org/content/201/8/1043.full.pdf |archive-date=24 January 2013 |url-status=live}}</ref> The largest flying insects today are much smaller, with the largest wingspan belonging to the white witch moth (''[[Thysania agrippina]]''), at approximately {{convert|28|cm|in|abbr=on}}.<ref name="BIR">{{Cite web|title=Chapter 32: Largest Lepidopteran Wing Span &#124; The University of Florida Book of Insect Records &#124; Department of Entomology & Nematology &#124; UF/IFAS|url=https://entnemdept.ufl.edu/walker/ufbir/chapters/chapter_32.shtml|access-date=2022-01-13|website=entnemdept.ufl.edu}}</ref>

Unlike [[bird]]s, small insects are swept along by the [[prevailing winds]]<ref>{{cite web |last=Yates |first=Diana |title=Birds migrate together at night in dispersed flocks, new study indicates |url=https://news.illinois.edu/view/6367/206246 |access-date=2022-01-13 |website=news.illinois.edu }}</ref> although many larger insects [[insect migration|migrate]]. [[Aphid]]s are transported long distances by low-level [[jet stream]]s.<ref>{{cite journal |last=Drake |first=V. A. |author2=Farrow, R. A. |year=1988 |title=The Influence of Atmospheric Structure and Motions on Insect Migration |journal=[[Annual Review of Entomology]] |volume=33 |pages=183–210 |doi=10.1146/annurev.en.33.010188.001151 }}</ref>

=== Walking ===

{{further|Walking#Insects}}

[[File:Alternating Tripod Gait.webm|upright=1.15|right|thumb|Spatial and temporal stepping pattern of walking desert ants performing an alternating tripod gait. Recording rate: 500 fps, Playback rate: 10 fps.]]

Many adult insects use six legs for walking, with an alternating [[tripod gait]]. This allows for rapid walking with a stable stance; it has been studied extensively in [[cockroach]]es and [[ant]]s. For the first step, the middle right leg and the front and rear left legs are in contact with the ground and move the insect forward, while the front and rear right leg and the middle left leg are lifted and moved forward to a new position. When they touch the ground to form a new stable triangle, the other legs can be lifted and brought forward in turn.<ref name="Locomotion">{{cite book |last=Biewener |first=Andrew A. |title=Animal Locomotion |publisher=[[Oxford University Press]] |year=2003 |isbn=978-0-19-850022-3 |pages= }}{{pn|date=July 2022}}</ref> The purest form of the tripedal gait is seen in insects moving at high speeds. However, this type of locomotion is not rigid and insects can adapt a variety of gaits. For example, when moving slowly, turning, avoiding obstacles, climbing or slippery surfaces, four (tetrapodal) or more feet ([[Walking#Insects|wave-gait]]) may be touching the ground.<ref>{{cite journal |last1=Grabowska |first1=Martyna |author2=Godlewska, Elzbieta |author3=Schmidt, Joachim |author4=Daun-Gruhn, Silvia |title= Quadrupedal gaits in hexapod animals – inter-leg coordination in free-walking adult stick insects |journal=Journal of Experimental Biology |volume=215 |issue=24 |pages=4255–4266 |year=2012|doi=10.1242/jeb.073643 |pmid=22972892 |doi-access=free }}</ref> Cockroaches are among the fastest insect runners and, at full speed, adopt a bipedal run. More sedate locomotion is seen in the well-[[camouflage]]d stick insects ([[Phasmatodea]]). A small number of species such as [[Water strider]]s can move on the surface of water; their claws are recessed in a special groove, preventing the claws from piercing the water's surface film.<ref name="aquins"/> The ocean-skaters in the genus ''[[Halobates]]'' even live on the surface of open oceans, a habitat that has few insect species.<ref>{{cite journal |last1=Ikawa |first1=Terumi |author2=Okabe, Hidehiko |author3=Hoshizaki, Sugihiko |author4=Kamikado, Takahiro |author5=Cheng, Lanna |title=Distribution of the oceanic insects ''Halobates'' (Hemiptera: Gerridae) off the south coast of Japan |journal=Entomological Science |volume=7 |issue=4 |pages=351–357 |year=2004 |doi=10.1111/j.1479-8298.2004.00083.x |s2cid=85017400 |url=https://escholarship.org/uc/item/9r50k828 }}</ref> 

=== Swimming ===

{{Main|Aquatic insects}}

[[File:Notonecta glauca01.jpg|thumb|The [[backswimmer]] ''[[Notonecta glauca]]'' underwater, showing its paddle-like hindleg adaptation]]

A large number of insects live either part or the whole of their lives underwater. In many of the more primitive orders of insect, the immature stages are aquatic. In some groups, such as [[water beetle]]s, the adults too are aquatic.<ref name="aquins">{{cite book |editor1=Richard W. Merritt |editor2=Kenneth W. Cummins |editor3=Martin B. Berg |year=2007 |title=An Introduction to the Aquatic Insects of North America |publisher=Kendall Hunt Publishers |isbn=978-0-7575-5049-2 |edition=4th}}</ref>

Many of these species are adapted for under-water locomotion. Water beetles and water bugs have legs adapted into paddle-like structures. Dragonfly [[nymph (biology)|naiads]] use jet propulsion, forcibly expelling water out of their rectal chamber.<ref>{{cite journal |last=Mill |first=P. J. |author2=Pickard, R. S. |year=1975 |title=Jet-propulsion in anisopteran dragonfly larvae |journal=Journal of Comparative Physiology A |volume=97 |issue=4 |pages=329–338 |doi=10.1007/BF00631969|s2cid=45066664 }}</ref> Other insects such as the [[Staphylinidae|rove beetle]] ''[[Stenus]]'' emit [[Pygidium#Insects|pygidial]] gland [[surfactant]] secretions that reduce surface tension; this enables them to move on the surface of water by [[Marangoni effect|Marangoni propulsion]].<ref>{{cite journal |last=Linsenmair |first=K. |author2=Jander, R. |year=1976 |title=Das "entspannungsschwimmen" von ''Velia'' and ''Stenus'' |journal=Naturwissenschaften |volume=50 |page=231 |doi=10.1007/BF00639292 |bibcode=1963NW.....50..231L |issue=6 |s2cid=40832917 |url=https://nbn-resolving.org/urn:nbn:de:bvb:20-opus-44663 }}</ref><ref>{{cite journal |last=Bush |first=J. W. M. |author2=Hu, David L. Hu |year=2006 |title=Walking on Water: Biolocomotion at the Interface |journal=Annual Review of Fluid Mechanics |volume=38 |issue=1 |pages=339–369 |url=http://www.cims.nyu.edu/~dhu/Pubweb/Bush_Hu_06.pdf |doi=10.1146/annurev.fluid.38.050304.092157 |bibcode=2006AnRFM..38..339B |url-status=dead |archive-url=https://web.archive.org/web/20070710022624/http://www.cims.nyu.edu/~dhu/Pubweb/Bush_Hu_06.pdf |archive-date=10 July 2007}}</ref>

== Ecology ==

{{main|Insect ecology}}

Insects play many critical roles in [[ecosystem]]s, including soil turning and aeration, dung burial, pest control, pollination and wildlife nutrition.{{sfn|Schowalter|2006|pp=3, 572}} For instance, termites modify the environment around their nests, encouraging grass growth;{{sfn|Gullan|Cranston|2014|p=257}} many [[beetle]]s are [[scavenger]]s; dung beetles [[recycling|recycle]] biological materials into forms useful to other [[organism]]s.{{sfn|Gullan|Cranston|2014|pp=261–264}}<ref>{{cite journal |last1=Losey |first1=John E. |last2=Vaughan |first2=Mace |title=The Economic Value of Ecological Services Provided by Insects |journal=BioScience |date=2006 |volume=56 |issue=4 |pages=311–323(13) |doi=10.1641/0006-3568(2006)56[311:TEVOES]2.0.CO;2 |url=http://wordinfo.info/unit/3562?letter=I&spage=3  |access-date=8 November 2011 |archive-url=https://web.archive.org/web/20120112041609/http://wordinfo.info/unit/3562?letter=I&spage=3 |archive-date=12 January 2012 |url-status=live |doi-access=free }}</ref> Insects are responsible for much of the process by which [[topsoil]] is created.{{sfn|Gullan|Cranston|2005|pp=3, 218–228}}

=== Defense ===

{{main|Defense in insects}}

[[File:Reduvius personatus, Masked Hunter Bug nymph camouflaged with sand grains.JPG|thumb|''[[Reduvius personatus]]'', the masked hunter bug [[Nymph (biology)|nymph]], [[camouflage]]s itself with sand grains to avoid [[Predation|predator]]s.]]

Insects are mostly small, soft bodied, and fragile compared to larger lifeforms. The immature stages are small, move slowly or are immobile, and so all stages are exposed to [[predation]] and [[parasitism]]. Insects accordingly employ multiple [[Defense in insects|defensive strategies]], including [[camouflage]], [[mimicry]], toxicity and active defense.<ref name="Evans126">{{cite book |last1=Evans |first1=Arthur V. |last2=Bellamy |first2=Charles |year=2000 |title=An Inordinate Fondness for Beetles |publisher=[[University of California Press]] |isbn=978-0-520-22323-3 |url=https://books.google.com/books?id=ZZ_hfpMo8oAC&pg=PA31 |page=31}}</ref> 
Many [[Defense in insects#Hiding|insects rely on camouflage]] to avoid being noticed by their predators or prey.<ref>{{cite web |title=Photos: Masters of Disguise – Amazing Insect Camouflage |url=http://news.nationalgeographic.com/news/2014/03/140321-insects-fossil-camouflage-mimicry-pictures/ |access-date=11 June 2015 |date=2014-03-24 |archive-url=https://web.archive.org/web/20150612201555/http://news.nationalgeographic.com/news/2014/03/140321-insects-fossil-camouflage-mimicry-pictures/ |archive-date=12 June 2015 |url-status=dead}}</ref> It is common among [[leaf beetle]]s and [[weevil]]s that feed on wood or vegetation.<ref name="Evans126"/> [[Stick insect]]s mimic the forms of sticks and leaves.<ref name="bedford">{{cite journal |last=Bedford |first=Geoffrey O. |title=Biology and Ecology of the Phasmatodea |journal=Annual Review of Entomology |year=1978 |volume=23 |pages=125–149 |doi=10.1146/annurev.en.23.010178.001013 }}</ref>
Many insects use [[mimicry]] to deceive predators into avoiding them. In [[Batesian mimicry]], edible species, such as of [[hoverflies]] (the mimics), gain a survival advantage by resembling inedible species (the models).<ref name="Evans126"/><ref name="Ritland & Brower">{{cite journal |last1=Ritland |first1=D. B. |last2=Brower |first2=Lincoln P. |author-link2=Lincoln Brower |year=1991 |title=The viceroy butterfly is not a Batesian mimic |journal=[[Nature (journal)|Nature]] |volume=350 |issue= 6318|pages=497–498 |doi =10.1038/350497a0 |quote=Viceroys are as unpalatable as monarchs, and significantly more unpalatable than queens from representative Florida populations. |bibcode=1991Natur.350..497R|s2cid=28667520 }}</ref> In [[Müllerian mimicry]], inedible species, such as of wasps and bees, resemble each other so as to reduce the sampling rate by predators who need to learn that those insects are inedible. ''[[Heliconius]]'' butterflies, many of which are toxic, form Müllerian complexes, advertising their inedibility.<ref>{{cite journal |last=Meyer |first=A. |year=2006 |title=Repeating Patterns of Mimicry |journal=[[PLOS Biology]] |volume=4 |issue=10 |page=e341 |doi=10.1371/journal.pbio.0040341 |pmid=17048984 |pmc=1617347 |doi-access=free }}</ref>
[[Chemical defense#Terrestrial arthropods|Chemical defense]] is common among Coleoptera and Lepidoptera, usually being advertised by bright warning colors ([[aposematism]]), as in the [[monarch butterfly]]. As larvae, they obtain their [[Caterpillar#Chemical defences|toxicity]] by sequestering chemicals from the plants they eat into their own tissues. Some manufacture their own toxins. Predators that eat poisonous butterflies and moths may vomit violently, learning not to eat insects with similar markings; this is the basis of Müllerian mimicry.<ref name="ANC">{{cite book |last=Kricher |first=John |title=A Neotropical Companion |pages=157–158 |chapter=6 |chapter-url=https://books.google.com/books?id=Z3pgdvrSmG8C&pg=PA158 |publisher=[[Princeton University Press]] |isbn=978-0-691-00974-2 |date=1999}}</ref> 
Some [[ground beetle]]s of the family Carabidae actively defend themselves, spraying chemicals from their abdomen with great accuracy, to repel predators.<ref name="Evans126"/>

=== Pollination ===

{{main|Entomophily}}

[[File:Apis mellifera flying.jpg|thumb|[[Apis mellifera|European honey bee]] carrying pollen in a [[pollen basket]] back to the hive]]

Pollination is the process by which [[pollen]] is transferred in the reproduction of plants, thereby enabling [[fertilisation]] and [[sexual reproduction]].<ref>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/289001/insect |last=Wigglesworth |first=Vincent Brian |author-link=Vincent Wigglesworth |title=Insect |encyclopedia=[[Encyclopædia Britannica]] online |access-date=19 April 2012 |archive-url=https://web.archive.org/web/20120504183402/https://www.britannica.com/EBchecked/topic/289001/insect |archive-date=4 May 2012 |url-status=live }}</ref> Most flowering plants require an animal to do the transportation. The majority of [[Entomophily|pollination is by insects]].<ref name="USFS">{{cite web|url=http://www.fs.fed.us/wildflowers/pollinators/documents/factsheet_pollinator.pdf |publisher=[[United States Forest Service]] |title=Pollinator Factsheet |access-date=19 April 2012 |url-status=dead |archive-url=https://web.archive.org/web/20080410135644/http://www.fs.fed.us/wildflowers/pollinators/documents/factsheet_pollinator.pdf |archive-date=10 April 2008}}</ref> Because insects usually receive benefit for the pollination in the form of energy rich nectar it is a [[Mutualism (biology)|mutualism]]. The various flower traits, such as bright colors and [[Insect pheromones|pheromones]] that [[Coevolution|coevolved]] with their pollinators, have been called [[pollination syndrome]]s, though around one third of flowers cannot be assigned to a single syndrome.<ref name="Ollerton Alarcón Waser Price 2009 pp. 1471–1480">{{cite journal |last1=Ollerton |first1=Jeff |last2=Alarcón |first2=Ruben |last3=Waser |first3=Nickolas M. |last4=Price |first4=Mary V. |last5=Watts |first5=Stella |last6=Cranmer |first6=Louise |last7=Hingston |first7=Andrew |last8=Peter |first8=Craig I. |last9=Rotenberry |first9=John |display-authors=4 |title=A global test of the pollination syndrome hypothesis |journal=Annals of Botany |publisher=Oxford University Press |volume=103 |issue=9 |date=14 February 2009 |doi=10.1093/aob/mcp031 |pages=1471–1480|pmid=19218577 |pmc=2701765 |doi-access=free }}</ref>

=== Parasitism ===

{{further|Parasitism|Parasitoid wasp}}

Many insects are [[Parasitism|parasitic]]. The largest group, with over 100,000 species<ref name="Polaszek Vilhemsen 2023">{{cite journal |last1=Polaszek |first1=Andrew |last2=Vilhemsen |first2=Lars |title=Biodiversity of hymenopteran parasitoids |journal=Current Opinion in Insect Science |date=2023 |volume=56 |page=101026 |doi=10.1016/j.cois.2023.101026 |pmid=36966863 |s2cid=257756440 |doi-access=free |bibcode=2023COIS...5601026P }}</ref> and perhaps over a million,<ref name="Forbes Bagley Beer Hippee 2018">{{cite journal |last1=Forbes |first1=Andrew A. |last2=Bagley |first2=Robin K. |last3=Beer |first3=Marc A. |last4=Hippee |first4=Alaine C. |last5=Widmayer |first5=Heather A. |display-authors=3 |title=Quantifying the unquantifiable: why Hymenoptera, not Coleoptera, is the most speciose animal order |journal=BMC Ecology |volume=18 |issue=1 |date=12 July 2018 |page=21 |doi=10.1186/s12898-018-0176-x|pmid=30001194 |pmc=6042248 |bibcode=2018BMCE...18...21F |doi-access=free }}</ref> consists of a single [[clade]] of [[parasitoid wasp]]s among the Hymenoptera.<ref name="Zhang Kopylov 2020">{{Cite journal |last1=Zhang |first1=Qi |last2=Kopylov |first2=Dmitry S. |last3=Rasnitsyn |first3=Alexandr P. |last4=Zheng |first4=Yan |last5=Zhang |first5=Haichun |date=November 2020 |editor-last=Smith |editor-first=Andrew |title=Burmorussidae, a new family of parasitic wasps (Insecta, Hymenoptera) from mid-Cretaceous Burmese amber |url=https://onlinelibrary.wiley.com/doi/10.1002/spp2.1312 |journal=Papers in Palaeontology |volume=6 |issue=4|pages=593–603|doi=10.1002/spp2.1312 |bibcode=2020PPal....6..593Z |s2cid=219039881}}</ref> These are parasites of other insects, eventually killing their hosts.<ref name="Polaszek Vilhemsen 2023"/> Some are hyper-parasites, as their hosts are other parasitoid wasps.<ref name="Polaszek Vilhemsen 2023"/><ref>{{Cite journal |last1=Tanaka |first1=S. |last2=Ohsaki |first2=N. |doi=10.1007/s11284-006-0153-2 |title=Behavioral manipulation of host caterpillars by the primary parasitoid wasp Cotesia glomerata (L.) to construct defensive webs against hyperparasitism |journal=Ecological Research |volume=21 |issue=4 |pages=570 |year=2006 |bibcode=2006EcoR...21..570T |s2cid=23457678 }}</ref> Several groups of insects can be considered as either [[micropredator]]s or [[ectoparasites|external parasites]];<ref name=Rollinson>{{cite journal |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |editor1=Rollinson, D. |editor2=Hay, S. I. |title=The Many Roads to Parasitism: A Tale of Convergence |journal=Advances in Parasitology |url=https://books.google.com/books?id=9y4AlXka7t0C&pg=PA28 |year=2011 |volume=74 |publisher=Academic Press |isbn=978-0-12-385897-9 |pages=27–28 |doi=10.1016/B978-0-12-385897-9.00001-X |pmid=21295676 }}</ref><ref name=PoulinRandhawa2015>{{cite journal |last1=Poulin |first1=Robert |author1-link=Robert Poulin (zoologist) |last2=Randhawa |first2=Haseeb S. |title=Evolution of parasitism along convergent lines: from ecology to genomics |journal=Parasitology |date=February 2015 |volume=142 |issue=Suppl 1 |pages=S6–S15 |doi=10.1017/S0031182013001674 |pmc=4413784 |pmid=24229807}} {{open access}}</ref>  for example, many [[hemiptera]]n bugs have piercing and sucking mouthparts, adapted for feeding on plant sap,<ref name=Gullan>{{cite book |author1=Gullan, P.J. |author2=Cranston, P.S. |title=The Insects: An Outline of Entomology, 5th Edition |url=https://books.google.com/books?id=lF5hBAAAQBAJ&pg=PT790 |year=2014 |publisher=Wiley |isbn=978-1-118-84616-2 |pages=80–81, 790–}}</ref><ref>{{cite journal |last1=Labandeira |first1=Conrad C. |title=Early History of Arthropod and Vascular Plant Associations |journal=Annual Review of Earth and Planetary Sciences |date=May 1998 |volume=26 |pages=329–377 |doi=10.1146/annurev.earth.26.1.329 |bibcode=1998AREPS..26..329L }}</ref> while species in groups such as [[flea]]s, [[lice]], and [[mosquito]]es are [[Hematophagy|hematophagous]], feeding on the [[blood]] of animals.<ref name=PoulinRandhawa2015/>

<gallery class=center mode=nolines widths=180 heights=180>
File:CSIRO ScienceImage 2357 Spotted alfalfa aphid being attacked by parasitic wasp.jpg|A [[parasitoid wasp]] ovipositing into an [[aphid]]<ref name="WilsonSwincer1982">{{cite journal |last1=Wilson |first1=C. G. |last2=Swincer |first2=D. E. |last3=Walden |first3=K. J. |title=The introduction of ''Trioxys complanatus'' Quilis (Hymenoptera: Aphidiidae), an internal parasite of the Spotted Alfalfa Aphid, into South Australia |journal=Australian Journal of Entomology |volume=21 |issue=1 |year=1982 |pages=13–27 |doi=10.1111/j.1440-6055.1982.tb01758.x|s2cid=84996305 }}</ref>
File:Wilting a twig (4608641762).jpg|Plant parasite or [[micropredator]]: a [[Coreidae|coreid]] bug sucking plant sap
File:Male human head louse.jpg|[[Pediculus humanus capitis|Human head-lice]] are directly transmitted [[Obligate parasite|obligate]] ectoparasites.
</gallery>

== Relationship to humans ==

{{main|Human interactions with insects}}

=== As pests ===

[[File:Aedes aegypti biting human.jpg|thumb|upright=0.9|''[[Aedes aegypti]]'', the [[yellow fever]] mosquito, is a [[Disease vector|vector]] of several [[mosquito-borne disease|diseases]].]]

{{main|Pest insect}}

Many insects are considered [[Pest (organism)|pests]] by humans. These include parasites of people and livestock, such as [[lice]] and [[Cimex|bed bugs]]; [[mosquito]]es act as [[Disease vector|vectors]] of [[mosquito-borne disease|several diseases]]. Other pests include insects like [[termite]]s that damage wooden structures; herbivorous insects such as [[locust]]s, aphids, and [[thrips]] that destroy agricultural crops, or like [[wheat weevil]]s damage stored agricultural produce. Farmers have often attempted to control insects with chemical [[insecticide]]s, but increasingly rely on [[biological pest control]]. This uses one organism to reduce the population density of a pest organism; it is a key element of [[integrated pest management]].<ref>{{cite journal |title=Biological control and sustainable food production |last1=Bale |first1=J. S. |last2=van Lenteren |first2=J. C. |last3=Bigler |first3=F. |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |date=27 February 2008|volume=363 |issue=1492 |pages=761–776 |doi=10.1098/rstb.2007.2182 |pmid=17827110 |pmc=2610108}}</ref><ref>{{cite book |last=Davidson |first=E. |title=Big Fleas Have Little Fleas: How Discoveries of Invertebrate Diseases Are Advancing Modern Science |year=2006 |publisher=University of Arizona Press |location=Tucson, Arizona |url=https://archive.org/details/bigfleashavelitt0000davi |url-access=registration |isbn=978-0-8165-2544-7 }}</ref> Biological control is favored because insecticides can cause harm to ecosystems far beyond the intended pest targets.<ref>{{cite journal |last1=Colborn |first1=T. |last2=vom Saal |first2=F. S. |last3=Soto |first3=A. M. |title=Developmental effects of endocrine-disrupting chemicals in wildlife and humans |journal=Environmental Health Perspectives |date=October 1993 |volume=101 |issue=5 |pages=378–384 |pmc=1519860 |pmid=8080506 |doi=10.2307/3431890 |jstor=3431890}}</ref><ref>{{cite journal |title=Extinction risk to bird populations caused by DDT exposure |last1=Nakamaru |first1=M. |last2=Iwasab |first2=Y. |last3=Nakanishic |first3=J. |journal=Chemosphere |date=October 2003 |volume=53 |issue=4 |pages=377–387 |doi=10.1016/S0045-6535(03)00010-9 |pmid=12946395 |bibcode=2003Chmsp..53..377N }}</ref>

=== In beneficial roles ===

{{See also|Economic entomology#Beneficial insects}}

[[File:008 Fàbrica de seda Yodgorlik, Imom Zahiriddin Ko'chasi 138 (Marguilan), bullint els capolls (cropped).jpg|thumb|upright|[[Silkworm]]s were [[Domestication|domesticated]] for [[silk]] over 5,000 years ago.<ref>{{cite journal |last1=Yu |first1=Hong-Song |last2=Shen |first2=Yi-Hong |last3=Yuan |first3=Gang-Xiang |display-authors=et al |title=Evidence of selection at melanin synthesis pathway loci during silkworm domestication |journal=Molecular Biology and Evolution |volume=28 |issue=6 |pages=1785–99 |year=2011 |pmid =21212153 |doi=10.1093/molbev/msr002 |doi-access= }}</ref><ref>{{cite journal |last=Normile |first=Dennis |title=Sequencing 40 Silkworm Genomes Unravels History of Cultivation |journal=Science |volume=325 |issue=5944 |pages=1058–1059 |year=2009 |pmid=19713499 |doi=10.1126/science.325_1058a |bibcode = 2009Sci...325.1058N |doi-access=free }}</ref> Here, silk cocoons are being unrolled. ]]

[[Pollination]] of flowering plants by insects including [[bee]]s, [[butterfly|butterflies]], [[Diptera|flies]], and [[beetle]]s, is economically important.<ref>{{cite book |last=Holldobler |first=Wilson |title=Journey to the ants: a story of scientific exploration |year=1994 |location=Cambridge, Massachusetts |publisher=Belknap Press |isbn=978-0-674-48525-9 |pages=[https://archive.org/details/journeytoantss00holl/page/196 196–199] |url=https://archive.org/details/journeytoantss00holl/page/196}}</ref> The value of insect pollination of crops and fruit trees was estimated in 2021 to be about $34 billion in the US alone.<ref>{{Cite journal|last1=Jordan |first1=Alex |last2=Patch |first2=Harland M. |last3=Grozinger |first3=Christina M. |last4=Khanna |first4=Vikas |date=2021-01-26 |title=Economic Dependence and Vulnerability of United States Agricultural Sector on Insect-Mediated Pollination Service |url=https://pubs.acs.org/doi/pdf/10.1021/acs.est.0c04786 |journal=Environmental Science & Technology |volume=55 |issue=4 |pages=2243–2253 |doi=10.1021/acs.est.0c04786 |pmid=33496588 |bibcode=2021EnST...55.2243J |s2cid=231710967 }}</ref>

Insects produce useful substances such as [[honey]],<ref name="Crane1990">{{cite journal |last=Crane |first=E. |title = Honey from honeybees and other insects |journal=Ethology Ecology & Evolution |volume=3 |issue=sup1 |pages=100–105 |year=1990 |doi=10.1080/03949370.1991.10721919 |author-link=Eva Crane }}</ref> [[wax]],<ref name=Sanford>{{cite journal |last1=Sanford |first1=M.T. |last2=Dietz |first2=A. |year=1976 |title=The fine structure of the wax gland of the honey bee (Apis mellifera L.). |journal=Apidologie |volume=7 |issue=3 |pages=197–207 |doi=10.1051/apido:19760301|doi-access=free }}</ref><ref>{{Cite news |url=https://www.beeculture.com/wax-rendering/ |title=Wax Rendering |date=23 March 2016 |work=Bee Culture |access-date=26 October 2018 }}</ref> [[lacquer]]<ref>{{cite news |title=How Shellac Is Manufactured |url=http://nla.gov.au/nla.news-article55073762 |access-date=17 July 2015 |publisher=The Mail (Adelaide, SA : 1912 – 1954) |date=18 Dec 1937}}</ref> and [[silk]].<ref>{{cite web |last=Bezzina |first=Neville |title=Silk Production Process |url=http://www.senature.com/sensemagazine/research-technologies/silk-production-process-go-behind-the-scenes-1701.html |publisher=Sense of Nature Research |url-status=dead |archive-url=https://web.archive.org/web/20120629231032/http://www.senature.com/sensemagazine/research-technologies/silk-production-process-go-behind-the-scenes-1701.html |archive-date=29 June 2012 }}</ref> [[Honey bee]]s have been cultured by humans for thousands of years for honey.<ref>{{cite journal |last1=Dams |first1=M. |last2=Dams |first2=L. |title=Spanish Rock Art Depicting Honey Gathering During the Mesolithic |journal=Nature |date=21 July 1977 |volume=268 |issue=5617 |pages=228–230 |doi=10.1038/268228a0 |bibcode=1977Natur.268..228D |s2cid=4177275}}</ref> Beekeeping in pottery vessels began about 9,000 years ago in North Africa.<ref name=9kya>{{cite journal |last=Roffet-Salque |first=Mélanie |display-authors=et al|title=Widespread exploitation of the honeybee by early Neolithic farmers |journal=Nature |date=14 June 2016 |volume=534 |issue=7607 |pages=226–227 |doi=10.1038/nature18451 |pmid=26560301 |doi-access=free|hdl=10379/13692 |hdl-access=free }}</ref> The [[silkworm]] has greatly affected human history, as [[Silk Road|silk-driven trade]] established relationships between China and the rest of the world.<ref>{{cite book |title=Chinese Silk: A Cultural History |last=Vainker |first=Shelagh |year=2004 |publisher=[[Rutgers University Press]] |isbn=0813534461 |page=20}}</ref><ref>{{cite journal |last=Christian |first=David |year=2000 |title=Silk Roads or Steppe Roads? The Silk Roads in World History |journal=[[Journal of World History]]|volume=2 |issue=1 |page=1 |doi=10.1353/jwh.2000.0004 |s2cid=18008906}}</ref>

Insects that feed on or parasitise other insects are beneficial to humans if they thereby reduce damage to agriculture and human structures. For example, [[aphid]]s feed on crops, causing economic loss, but [[ladybug]]s feed on aphids, and can be used [[Biological pest control|to control them]]. Insects account for the vast majority of insect consumption.{{sfn|Gullan|Cranston|2005|p=328–348, 400}}<ref>{{cite web |title=Biocontrol Network – Beneficial Insects |url=http://www.biconet.com/biocontrol.html |publisher=Biocontrol Network |access-date=9 May 2009 |archive-url=https://web.archive.org/web/20090228043847/http://www.biconet.com/biocontrol.html |archive-date=28 February 2009 |url-status=usurped }}</ref><ref>{{cite book |last1=Davidson |first1=R. H. |last2=Lyon |first2=William F. |year=1979 |title=Insect Pests of Farm, Garden, and Orchard |publisher=John Wiley & Sons |page=38 |isbn=978-0-471-86314-4}}</ref>

Fly larvae ([[maggot]]s) were formerly [[Maggot therapy|used to treat wounds]] to prevent or stop [[gangrene]], as they would only consume dead flesh. This treatment is finding modern usage in some hospitals. Insects have gained attention as potential sources of drugs and other medicinal substances.<ref>{{cite journal |last=Dossey |first=Aaron T. |date= December 2010 |title=Insects and their chemical weaponry: New potential for drug discovery |journal=Natural Product Reports |volume=27 |pages=1737–1757 |doi=10.1039/c005319h |issue=12 |pmid=20957283 }}</ref> Adult insects, such as crickets and insect larvae of various kinds, are commonly used as fishing bait.<ref>{{cite journal |last=Sherman |first=Ronald A. |author2=Pechter, Edward A. |date=1987 |title=Maggot therapy: a review of the therapeutic applications of fly larvae in human medicine, especially for treating osteomyelitis |journal=Medical and Veterinary Entomology |volume=2 |issue=3 |pages=225–230 |doi=10.1111/j.1365-2915.1988.tb00188.x |pmid=2980178 |s2cid=44543735}}</ref>

=== Population declines ===

{{main|Decline in insect populations}}

At least 66 insect species extinctions have been recorded since 1500, many of them on oceanic islands.<ref name="Briggs2017">{{cite journal |last1=Briggs |first1=John C |title=Emergence of a sixth mass extinction? |journal=Biological Journal of the Linnean Society |date=October 2017 |volume=122 |issue=2 |pages=243–248 |doi=10.1093/biolinnean/blx063|doi-access=free }}</ref> [[Decline in insect populations|Declines in insect abundance]] have been attributed to human activity in the form of artificial lighting,<ref name="Owens2018">{{cite journal |last1=Owens |first1=Avalon C. S. |last2=Lewis |first2=Sara M. |title=The impact of artificial light at night on nocturnal insects: A review and synthesis |journal=Ecology and Evolution |date=November 2018 |volume=8 |issue=22 |pages=11337–11358 |doi=10.1002/ece3.4557|pmid=30519447 |pmc=6262936 |bibcode=2018EcoEv...811337O }}</ref> land use changes such as urbanization or farming,<ref name="Tscharntke2005">{{cite journal |last1=Tscharntke |first1=Teja |last2=Klein |first2=Alexandra M. |last3=Kruess |first3=Andreas |last4=Steffan-Dewenter |first4=Ingolf |last5=Thies |first5=Carsten |title=Landscape perspectives on agricultural intensification and biodiversity and ecosystem service management |journal=Ecology Letters |date=August 2005 |volume=8 |issue=8 |pages=857–874 |doi=10.1111/j.1461-0248.2005.00782.x|bibcode=2005EcolL...8..857T |s2cid=54532666 |doi-access=free }}</ref><ref>{{cite book |title=Insect-plant interactions in a crop protection perspective |isbn=978-0-12-803324-1 |pages=313–320|date=2017-01-19 |publisher=Academic Press }}</ref> pesticide use,<ref>{{cite journal |last1=Braak |first1=Nora |last2=Neve |first2=Rebecca |last3=Jones |first3=Andrew K. |last4=Gibbs |first4=Melanie |last5=Breuker |first5=Casper J. |title=The effects of insecticides on butterflies – A review |journal=Environmental Pollution |date=November 2018 |volume=242 |issue=Pt A |pages=507–518 |doi=10.1016/j.envpol.2018.06.100|pmid=30005263 |bibcode=2018EPoll.242..507B |s2cid=51625489 |url=https://radar.brookes.ac.uk/radar/items/fddc88d3-25f2-4274-9321-2f9b3b72047a/1 }}</ref> and invasive species.<ref>{{cite journal |last1=Wagner |first1=David L. |last2=Van Driesche |first2=Roy G. |title=Threats Posed to Rare or Endangered Insects by Invasions of Nonnative Species |journal=Annual Review of Entomology |date=January 2010 |volume=55 |issue=1 |pages=547–568 |doi=10.1146/annurev-ento-112408-085516|pmid=19743915 }}</ref><ref>{{cite web |url=http://www.globalchange.umich.edu/globalchange2/current/lectures/biodiversity/biodiversity.html |archive-url=https://web.archive.org/web/20150220154543/http://www.globalchange.umich.edu/globalchange2/current/lectures/biodiversity/biodiversity.html |archive-date=20 February 2015 |title=Threats to Global Diversity |last=Wilson |first=E. O. |access-date=17 May 2009}}</ref> A 2019 research review suggested that a large proportion of insect species is<!--singular: "a proportion is"--> threatened with extinction in the 21st century,<ref name="Wyckhuys2019">{{cite journal |last1=Sánchez-Bayo |first1=Francisco |last2=Wyckhuys |first2=Kris A.G. |title=Worldwide decline of the entomofauna: A review of its drivers |journal=Biological Conservation |date=April 2019 |volume=232 |pages=8–27 |doi=10.1016/j.biocon.2019.01.020 |doi-access=free |bibcode=2019BCons.232....8S }}</ref> though the details have been disputed.<ref>{{cite web |last1=Saunders |first1=Manu |title=Insectageddon is a great story. But what are the facts? |url=https://ecologyisnotadirtyword.com/2019/02/16/insectageddon-is-a-great-story-but-what-are-the-facts/ |website=Ecology is not a dirty word |access-date=24 February 2019 |date=16 February 2019 |archive-url=https://web.archive.org/web/20190225044750/https://ecologyisnotadirtyword.com/2019/02/16/insectageddon-is-a-great-story-but-what-are-the-facts/ |archive-date=25 February 2019 |url-status=live }}</ref> A larger 2020 meta-study, analyzing data from 166 long-term surveys, suggested that populations of terrestrial insects are indeed decreasing rapidly, by about 9% per decade.<ref name=vanKlink2020>{{citation |journal=[[Science (journal)|Science]] |title=Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances|first1=Roel |last1=van Klink |date=24 April 2020|volume=368 |issue=6489 |pages=417–420 |doi=10.1126/science.aax9931 |pmid=32327596 |bibcode=2020Sci...368..417V |s2cid=216106896 |doi-access=free }}</ref><ref>{{Cite news |last=McGrath |first=Matt |url=https://www.bbc.com/news/science-environment-52399373 |title='Insect apocalypse' more complex than thought |date=23 April 2020 |work=BBC News |access-date=2020-04-24 }}</ref>

=== In research ===

[[File:Drosophila melanogaster - side (aka).jpg|thumb|upright=0.9|The fruit fly ''[[Drosophila melanogaster]]'' is a widely used [[model organism]].]]

Insects play important roles in biological research. For example, because of its small size, short generation time and high [[fecundity]], the common fruit fly ''[[Drosophila melanogaster]]'' is a [[model organism]] for studies in the [[genetics]] of [[eukaryote]]s, including [[genetic linkage]], [[epistasis|interactions between genes]], [[chromosome|chromosomal]] genetics, [[evo-devo|development]], behavior and [[evolution]]. Because genetic systems are well conserved among eukaryotes, understanding basic cellular processes like [[DNA replication]] or [[Transcription (genetics)|transcription]] in fruit flies can help to understand those processes in other eukaryotes, including humans.<ref name="Pierce">{{cite book |title=Genetics: A Conceptual Approach |last=Pierce |first=B. A. |edition=2nd |publisher=W.H. Freeman and Company |location=New York |page=[https://archive.org/details/geneticsconceptu0000unse/page/87 87] |isbn=978-0-7167-8881-2 |year=2006 |url=https://archive.org/details/geneticsconceptu0000unse/page/87 }}</ref> The [[genome]] of ''D. melanogaster'' was [[Genome project|sequenced]] in 2000, reflecting the organism's important role in biological research. It was found that 70% of the fly genome is similar to the [[human genome]], supporting the theory of [[evolution]].<ref name="Adams_2000">{{cite journal |last1=Adams |first1=M. D. |title=The genome sequence of ''Drosophila melanogaster'' |journal=Science |volume=287 |issue=5461 |pages=2185–2195 |date=24 March 2000 |pmid=10731132 |doi=10.1126/science.287.5461.2185 |last2=Celniker |first2=S. E. |last3=Holt |first3=R. A. |last4=Evans |first4=C. A. |last5=Gocayne |first5=J. D. |last6=Amanatides |first6=P. G. |last7=Scherer |first7=S. E. |last8=Li |first8=P. W. |last9=Hoskins |first9=R. A. |last10=Galle |first10=R. F. |last11=George |first11=R. A. |last12=Lewis |first12=S.E. |last13=Richards |first13=S. |last14=Ashburner |first14=M. |last15=Henderson |first15=S. N. |last16=Sutton |first16=G. G. |last17=Wortman |first17=J. R. |last18=Yandell |first18=M. D. |last19=Zhang |first19=Q. |last20=Chen |first20=L.X. |last21=Brandon |first21=R. C. |last22=Rogers |first22=Y. H. |last23=Blazej |first23=R. G. |last24=Champe |first24=M. |last25=Pfeiffer |first25=B. D. |last26=Wan |first26=K. H. |last27=Doyle |first27=C. |last28=Baxter |first28=E. G. |last29=Helt |first29=G. |last30=Nelson |first30=C. R. |bibcode=2000Sci...287.2185. |display-authors=5 |citeseerx=10.1.1.549.8639 }}</ref>

=== As food ===

{{main|Insects as food}}

[[File:Witchetty grub.jpg|thumb|upright=0.9|[[Witchetty grub]]s are prized as high-protein foods by Aboriginal Australians.<ref name="Ceurstemont 2013">{{cite journal |last1=Ceurstemont |first1=Sandrine |title=Inevitable insectivores? Not so fast |date=6 July 2013 |url=https://www.sciencedirect.com/science/article/abs/pii/S0262407913616917 |journal=New Scientist |volume=219 |issue=2924 |pages=35 |doi=10.1016/S0262-4079(13)61691-7 |access-date=3 December 2021 }}</ref>]]

Insects are consumed as food in 80% of the world's nations, by people in roughly 3,000 ethnic groups.<ref name="theguardian">{{Cite news |date=2010-07-31 |title=Insects could be the key to meeting food needs of growing global population|url=http://www.theguardian.com/environment/2010/aug/01/insects-food-emissions |access-date=2022-01-13 |work=[[The Guardian]] }}</ref><ref name="Ramos-Elorduy 1998 44">{{cite book |title=Creepy crawly cuisine: the gourmet guide to edible insects |last=Ramos-Elorduy |first=Julieta |author2=Menzel, Peter |year=1998 |publisher=Inner Traditions / Bear & Company |isbn=978-0-89281-747-4 |page=44 |url=https://books.google.com/books?id=Q7f1LkFz11gC |access-date=23 April 2014}}</ref> In Africa, locally abundant species of [[locust]]s and [[termite]]s are a common traditional human food source.<ref name="buginfo_asfood">{{cite web |title=Insects as Food for Humans |url=https://www.si.edu/spotlight/buginfo/insects-food |access-date = 14 September 2022 }}</ref> Some, especially [[deep-fried]] [[cicada]]s, are considered to be [[delicacy|delicacies]]. Insects have a high protein content for their mass, and some authors suggest their potential as a major source of [[protein]] in human [[nutrition]].{{sfn|Gullan|Cranston|2005|pp=10–13}} In most first-world countries, however, [[entomophagy]] (the eating of insects), is [[taboo]].<ref>{{cite book |last=Michels |first=John |title=Science |editor=John Michels |publisher=American Association for the Advance of Science |location=New York |year=1880 |volume=1 |page=69 |url=https://books.google.com/books?id=aDkLAAAAYAAJ&pg=PA69 <!--|isbn=978-1-930775-36-7-->}}</ref> They are also recommended by [[military|armed forces]] as a [[survival skills|survival]] food for troops in adversity.<ref name="buginfo_asfood"/> Because of the abundance of insects and a worldwide concern of food shortages, the [[Food and Agriculture Organization]] of the [[United Nations]] considers that people throughout the world may have to eat insects as a food staple. Insects are noted for their nutrients, having a high content of protein, minerals and fats and are already regularly eaten by one-third of the world's population.<ref name="investvine">{{cite web |url=http://investvine.com/un-bugs-are-food-of-the-future-video/ |title=UN: Insects are 'food of the future' (video) |first=Arno |last=Maierbrugger |work=Inside Investor |date=14 May 2013 |access-date=17 May 2013 |archive-url=https://web.archive.org/web/20130910061538/http://investvine.com/un-bugs-are-food-of-the-future-video/ |archive-date=10 September 2013 |url-status=live}}</ref>

=== In other products ===

[[Black soldier fly]] larvae can provide [[protein]] and fats for use in [[cosmetics]].<ref>{{cite journal |last1=Verheyen |first1=Geert |last2=Ooms|first2=Tom |last3=Vogels|first3=Liesbeth |last4=Vreysen |first4=Steven |last5=Bovy |first5=Ann |last6=Van Miert |first6=Sabine |last7=Meersman |first7=Filip |date=2018-05-01 |title=Insects as an Alternative Source for the Production of Fats for Cosmetics |url=https://www.researchgate.net/publication/326671736 |journal=Journal of Cosmetic Science |volume=69 |issue=3 |pages=187–202 |pmid=30052193}}</ref> Insect cooking oil, insect butter and [[fatty alcohol]]s [[Biorefinery|can be made from]] such insects as the superworm (''[[Zophobas morio]]'').<ref>{{Cite web |date=2016-08-15 |title=From Pest to Pot: Can Insects Feed the World? |url=https://www.nationalgeographic.com/culture/article/sponsor-content-from-pest-pot-can-insects-feed-the-world |archive-url=https://web.archive.org/web/20210410150413/https://www.nationalgeographic.com/culture/article/sponsor-content-from-pest-pot-can-insects-feed-the-world |url-status=dead |archive-date=10 April 2021 |access-date=2022-01-13 |website=National Geographic Culture}}</ref> Insect species including the black soldier fly or the [[housefly]] in their [[maggot farming|maggot]] forms, and beetle larvae such as [[mealworms]], can be processed and [[Insects as feed|used as feed]] for farmed animals including chicken, fish and pigs.<ref>{{Cite magazine |title=How AgriProtein makes chicken food from maggots |magazine=Wired UK |url=https://www.wired.co.uk/article/food-of-larvae |access-date=2022-01-13 }}</ref> Many species of insects are sold and kept as [[pet]]s.<ref>{{Cite web |last=Bugs |title=Bugs – das Wirbellosenmagazin |url=https://www.ms-verlag.de/magazine/bugs |access-date=7 March 2021|website=NTV Verlag}}</ref>

=== In religion and folklore ===

{{further|Insects in mythology}}

[[File:WLA brooklynmuseum Scarab with Separate Wings 3.jpg|thumb|[[Ancient Egypt]]ian [[Scarab (artifact)|scarab]] with separate wings, c. 712-342 BC]]

[[Scarabaeidae|Scarab beetles]] held religious and cultural symbolism in [[ancient Egypt]], [[Greece]] and some [[Shamanism|shamanistic]] Old World cultures. The ancient [[China|Chinese]] regarded [[cicada]]s as symbols of rebirth or immortality. In [[Mesopotamia]]n literature, the epic poem of [[Gilgamesh]] has allusions to [[Odonata]] that signify the impossibility of immortality. Among the [[Indigenous Australians|Aborigines]] of [[Australia]] of the [[Arrernte language|Arrernte]] language groups, honey ants and [[witchetty grub]]s served as personal clan totems. In the case of the [[San people|'San' bush-men]] of the [[Kalahari]], it is the [[praying mantis]] that holds much cultural significance including creation and [[zen]]-like patience in waiting.{{sfn|Gullan|Cranston|2005|p=9}}

==See also==

* [[Entomology]]
* [[Ethnoentomology]]
* [[Flying and gliding animals]]
* [[:Category:Insect-borne diseases|Insect-borne diseases]]

== Notes ==

{{notelist}}

== References ==

{{Reflist|30em}}

== Sources ==

* {{cite book |last1=Gullan |first1=P. J. |last2=Cranston |first2=P. S. |title=The Insects: An Outline of Entomology |publisher=Blackwell Publishing |location=Oxford |year=2005 |edition=3rd |isbn=978-1-4051-1113-3 |url=https://archive.org/details/isbn_9781405111133 }}
* {{cite book |last1=Gullan |first1=P. J. |last2=Cranston |first2=P. S. |title=The Insects: An Outline of Entomology |publisher=Wiley Blackwell |location=Oxford |year=2014 |edition=5th |isbn=978-1-118-84616-2 |url=https://books.google.com/books?id=lF5hBAAAQBAJ }}
* {{cite book |last=Nation |first=James L. |title=Insect Physiology and Biochemistry |publisher=CRC Press |year=2001 |edition=1st |isbn=978-0-8493-1181-9 |url=https://books.google.com/books?id=l3v2tOvz1uQC}}
* {{cite book |last1=Resh |first1=Vincent H. |last2=Carde |first2=Ring T. |title=Encyclopedia of Insects |publisher=Academic Press |date=2009 |edition=2 |isbn=978-0-12-374144-8}}
* {{cite book |last=Schowalter |first=Timothy Duane |title=Insect Ecology: An Ecosystem Epproach |edition=2nd (illustrated) |year=2006 |publisher=Academic Press |isbn=978-0-12-088772-9 |url=https://books.google.com/books?id=3PD6R-AEvwEC |access-date=27 October 2015 |archive-url=https://web.archive.org/web/20160603221311/https://books.google.com/books?id=3PD6R-AEvwEC |archive-date=3 June 2016 |url-status=live}}

== External links ==

{{Spoken Wikipedia|WP en Insect Intro 30 Oct 2010.ogg|date=2010-10-30}}

* [https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=47158&view=species Insect species and observations on iNaturalist]<!--works, but totally unstructured-->
* [http://bugguide.net/node/view/222292 Overview of Orders of Insects]
* {{EOL}}
* [https://web.archive.org/web/20091122074740/http://www.spc.int/PPS/SAFRINET/inse-scr.pdf A Safrinet Manual for Entomology and Arachnology] [[Secretariat of the Pacific Community|SPC]]
* [http://tolweb.org/Insecta/8205 Tree of Life Project] – Insecta, [http://tolweb.org/movies/Insecta/8205 Insecta Movies]
* [http://edna.palass-hosting.org/ Fossil Insect Database]: Holotypes at the International Palaeoentological Society
* [http://entomology.ifas.ufl.edu/walker/ufbir/ UF Book of Insect Records]
* [http://www.insectimages.org/ InsectImages.org] 24,000 high resolution insect photographs

{{Arthropods}}
{{Orders of Insects}}
{{Insects in culture}}
{{Taxonbar|from=Q1390}}
{{Authority control}}

[[Category:Insects|Insects]]
[[Category:Entomology]]
[[Category:Extant Early Devonian first appearances]]