View source for Evolution of insects

{{Short description|Development of insects from an ancestral crustacean and their subsequent radiation}}
[[File:Insect antennae.jpg|thumb|upright=0.6|Evolution has produced astonishing variety of appendages in insects, such as these [[Antenna (biology)|antennae]].]]
The most recent understanding of the '''evolution of insects''' is based on studies of the following branches of science: molecular biology, insect morphology, paleontology, insect taxonomy, evolution, embryology, bioinformatics and scientific computing. The study of insect fossils is known as '''paleoentomology'''. It is estimated that the class of [[insect]]s originated on Earth about 480 million years ago, in the [[Ordovician]], at about the same time [[terrestrial plant]]s appeared.<ref name=":0">{{Cite web|url=https://www.sciencedaily.com/releases/2014/11/141106143709.htm|title=Landmark study on the evolution of insects|date=November 6, 2014|website=Sciencedaily.com}}</ref> Insects are thought to have evolved from a group of [[crustacean]]s.<ref>{{Cite web|url=https://www.academic.oup.com/icb/article/55/5/765/604304|title=Linking Insects with Crustacea: Physiology of the Pancrustacea: An Introduction to the Symposium|date=August 5, 2015|website=Oxford Academic|access-date=May 25, 2015}}</ref> The first insects were landbound, but about 400 million years ago in the [[Devonian]] period one lineage of insects evolved flight, the first animals to do so.<ref name=":0"/> The oldest insect fossil has been proposed to be ''[[Rhyniognatha hirsti]]'', estimated to be 400 million years old, but the insect identity of the fossil has been contested.<ref name="Haug C., Haug J. T. (2017).">{{cite journal|last1=Haug|first1=Carolin|year=2017|title=The presumed oldest flying insect: More likely a myriapod?|journal=PeerJ|volume=5|pages=e3402|doi=10.7717/peerj.3402|pmc=5452959|pmid=28584727|ref=72 |doi-access=free }}</ref> Global climate conditions changed several times during the history of Earth, and along with it the [[diversity of insects]]. The [[Pterygotes]] (winged insects) underwent a major [[Adaptive radiation|radiation]] in the [[Carboniferous]] (358 to 299 million years ago) while the [[Endopterygota]] (insects that go through different life stages with [[metamorphosis]]) underwent another major radiation in the [[Permian]] (299 to 252 million years ago).

Most extant [[Order (biology)|orders]] of insects developed during the [[Permian]] period. Many of the early groups became extinct during the [[Permian–Triassic extinction event|mass extinction at the Permo-Triassic boundary]], the largest extinction event in the history of the Earth, around 252 million years ago.<ref name="History of Insects">{{cite book |author-link1=Alex Rasnitsyn |last1=Rasnitsyn |first1=A.P. |last2=Quicke |first2=D.L.J. |title=History of Insects |year=2002 |publisher=[[Kluwer Academic Publishers]] |isbn=978-1-4020-0026-3}}{{page needed|date=September 2013}}</ref> The survivors of this event evolved in the [[Triassic]] (252 to 201 million years ago) to what are essentially the modern insect orders that persist to this day. Most modern insect [[Family (biology)|families]] appeared in the [[Jurassic]] (201 to 145 million years ago).

In an important example of [[co-evolution]], a number of highly successful insect groups — especially the [[Hymenoptera]] (wasps, bees and ants) and [[Lepidoptera]] (butterflies) as well as many types of [[Diptera]] (flies) and [[Coleoptera]] (beetles) — evolved in conjunction with [[flowering plant]]s during the [[Cretaceous]] (145 to 66 million years ago).<ref name="Biology-coevolution">{{cite web | author=J. Stein Carter | title=Coevolution and Pollination | url=http://biology.clc.uc.edu/courses/bio303/coevolution.htm | publisher=University of Cincinnati | date=2005-03-29 | access-date=2009-05-09 | url-status=dead | archive-url=https://web.archive.org/web/20090430183230/http://biology.clc.uc.edu/Courses/bio303/coevolution.htm | archive-date=2009-04-30}}</ref><ref>{{cite journal |last1=Renne |first1=Paul R. |last2=Deino |first2=Alan L. |last3=Hilgen |first3=Frederik J. |last4=Kuiper |first4=Klaudia F. |last5=Mark |first5=Darren F. |last6=Mitchell|first6=William S. |last7=Morgan |first7=Leah E. |last8=Mundil |first8=Roland |last9=Smit |first9=Jan |title=Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary |journal=Science |date=7 February 2013 |volume=339 |issue=6120 |pages=684–687 |doi=10.1126/science.1230492 |pmid=23393261 |url=http://www.cugb.edu.cn/uploadCms/file/20600/20131028144132060.pdf |bibcode=2013Sci...339..684R|s2cid=6112274}}</ref>

Many modern insect [[genera]] developed during the [[Cenozoic]] that began about 66 million years ago; insects from this period onwards frequently became preserved in [[amber]], often in perfect condition. Such specimens are easily compared with modern species, and most of them are members of extant genera.

==Fossils==
===Preservation===
[[File:Fossil insect (Diptera, Nematocera in Baltic amber. Age 50 Mill. years (the Lower Eocene).jpg|right|thumb|Fossil [[gnat]] in [[amber]], a well-preserved insect fossil.]]
Due to their external skeleton, the fossil history of insects is not entirely dependent on [[lagerstätte]] type preservation as for many [[soft-bodied organism]]s. However, with their small size and light build, insects have not left a particularly robust fossil record. Other than insects preserved in amber, most finds are terrestrial or near terrestrial sources and only preserved under very special conditions such as at the edge of freshwater lakes. While some 1/3 of known non-insect species are extinct fossils, due to the paucity of their fossil record, only 1/100 of known insects are extinct fossils.<ref name="Virtual Museum"/>

Insect fossils are often three dimensional preservations of the original fossil. Loose wings are a common type of fossil as the wings do not readily decay or digest, and are often left behind by predators. Fossilization will often preserve their outer appearance, contrary to vertebrate fossils whom are mostly preserved just as bony remains (or inorganic casts thereof). Due to their size, vertebrate fossils with the external aspect similarly preserved are rare, and most known cases are [[subfossil]]s.<ref>{{cite journal |last1=Carpenter |first1=K. |s2cid=53487023 |title=How to Make a Fossil: Part 2 – Dinosaur Mummies and Other Soft Tissue |journal=The Journal of Paleontological Science |date=2007 |issue=C.07.0002 |pages=1–23}}</ref> Fossils of insects, when preserved, are often preserved as three-dimensional, permineralized, and charcoalified replicas; and as inclusions in amber and even within some minerals. Sometimes even their colour and patterning is still discernible.<ref name=":1">{{cite journal |last1=McNamara |first1=M.E. |last2=Briggs |first2=D.EG. |last3=Orr |first3=P.J. |last4=Gupta |first4=N.S. |last5=Locatelli |first5=E.R. |last6=Qiu |first6=L. |last7=Yang |first7=H. |last8=Wang |first8=Z. |last9=Noh |first9=H. |last10=Cao |first10=H. |title=The fossil record of insect color illuminated by maturation experiments |journal=Geology |date=April 2013 |volume=41 |issue=4 |pages=487–490 |doi=10.1130/G33836.1 |bibcode=2013Geo....41..487M}}</ref> Preservation in amber is, however, limited since  copious resin production by trees only evolved in the Mesozoic.<ref name=Grimaldi2009>{{Cite journal |doi=10.1126/science.1179328|pmid=19797645|title=Pushing Back Amber Production|year=2009|last1=Grimaldi|first1=D.|journal=Science|volume=326|issue=5949|bibcode=2009Sci...326...51G|pages=51–2|s2cid=206522565}}</ref><ref name=Bray2009>{{cite journal |doi=10.1126/science.1177539|title=Identification of Carboniferous (320 Million Years Old) Class Ic Amber|year=2009|last1=Bray|first1=P. S.|last2=Anderson|first2=K. B.|journal=Science|volume=326|issue=5949|pages=132–134|pmid=19797659|bibcode=2009Sci...326..132B|s2cid=128461248}}</ref>

There is also abundant fossil evidence for the behavior of extinct insects, including feeding damage on fossil vegetation and in wood, fecal pellets, and nests in fossil soils. Such preservation is rare in vertebrates, and is mostly confined to [[footprint]]s and [[coprolite]]s.<ref name="Grimaldi 2005"/>{{rp|42}}

===Freshwater and marine insect fossils===
The common denominator among most deposits of fossil insects and terrestrial plants is the lake environment. Those insects that became preserved were either living in the fossil lake ([[Autochthon (geology)|autochthonous]]) or carried into it from surrounding habitats by winds, stream currents, or their own flight ([[allochthonous]]). Drowning and dying insects not eaten by fish and other predators settle to the bottom, where they may be preserved in the lake's sediments, called lacustrine, under appropriate conditions. Even amber, or fossil resin from trees, requires a watery environment that is lacustrine or [[brackish]] in order to be preserved. Without protection in anoxic sediments, amber would gradually disintegrate; it is never found buried in fossil soils. Various factors contribute greatly to what kinds of insects become preserved and how well, if indeed at all, including lake depth, temperature, and alkalinity; type of sediments; whether the lake was surrounded by forest or vast and featureless salt pans; and if it was choked in anoxia or highly oxygenated.

There are some major exceptions to the lacustrine theme of fossil insects, the most famous being the Late Jurassic [[limestone]]s from [[Solnhofen]] and [[Eichstätt]], Germany, which are marine. These deposits are famous for pterosaurs and the bird-like ''[[Archaeopteryx]]''. The limestones were formed by a very fine mud of calcite that settled within stagnant, hypersaline bays isolated from inland seas. Most organisms in these [[limestone]]s, including rare insects, were preserved intact, sometimes with feathers and outlines of soft wing membranes, indicating that there was very little decay. The insects, however, are like casts or molds, having relief but little detail. In some cases iron oxides precipitated around wing veins, revealing better detail.<ref name="Grimaldi 2005"/>{{rp|42}}

===Compressions, impressions and mineralization===

There are many different ways insects can be fossilized and preserved including compressions and impressions, concretions, mineral replication, charcoalified (fusainized) remains, and their trace remains. Compressions and impressions are the most extensive types of insect fossils, occurring in rocks from the Carboniferous to the [[Holocene]]. Impressions are like a cast or mold of a fossil insect, showing its form and even some relief, like pleating in the wings, but usually little or no color from the cuticle. Compressions preserve remains of the cuticle, so color distinguishes structure. In exceptional situations, microscopic features such as microtrichia on sclerites and wing membranes are even visible, but preservation of this scale also requires a matrix of exceptionally fine grain, such as in micritic muds and volcanic tuffs. Because arthropod sclerites are held together by membranes, which readily decompose, many fossil arthropods are known only by isolated sclerites. Far more desirable are complete fossils. Concretions are stones with a fossil at the core whose chemical composition differs from that of the surrounding matrix, usually formed as a result of mineral precipitation from decaying organisms. The most significant deposit consists of various localities of the Late Carboniferous Francis Creek Shale of the Carbondale Formation at Mazon Creek, Illinois, which are composed of shales and coal seams yielding oblong concretions. Within most concretions is a mold of an animal and sometimes a plant that is usually marine in origin.{{citation needed|date=May 2024}}

When an insect is partly or wholly replaced by minerals, usually completely articulated and with three-dimensional fidelity, is called ''mineral replication''.<ref name="Grimaldi 2005"/> This is also called petrifaction, as in [[petrified wood]]. Insects preserved this way are often, but not always, preserved as concretions, or within nodules of minerals that formed around the insect as its nucleus. Such deposits generally form where the sediments and water are laden with minerals, and where there is also quick mineralization of the carcass by coats of bacteria.

==Evolutionary history==
The insect fossil record extends back some 400 million years to the lower Devonian, while the Pterygotes (winged insects) underwent a major radiation in the Carboniferous. The Endopterygota underwent another major radiation in the Permian. Survivors of the mass extinction at the [[P-T boundary]] evolved in the Triassic to what are essentially the modern Insecta Orders that persist to modern times.

Most modern insect families appeared in the Jurassic, and further diversification probably in genera occurred in the Cretaceous. By the [[Tertiary period|Tertiary]], there existed many of what are still modern genera; hence, most insects in amber are, indeed, members of extant genera. Insects diversified in only about 100 million years into essentially modern forms.<ref name="Virtual Museum">{{cite web | url=http://www.fossilmuseum.net/Evolution/evolution-segues/insect_evolution.htm | title=Insect Evolution | publisher=Virtual Fossil Museum | year=2007 | access-date=April 28, 2011}}</ref>

Insect evolution is characterized by rapid adaptation due to selective pressures exerted by the environment and furthered by high fecundity. It appears that rapid radiations and the appearance of new species, a process that continues to this day, result in insects filling all available environmental niches.

The evolution of insects is closely related to the evolution of flowering plants. Insect adaptations include feeding on flowers and related structures, with some 20% of extant insects depending on flowers, nectar or pollen for their food source. This symbiotic relationship is even more paramount in evolution considering that more than 2/3 of flowering plants are insect pollinated.<ref>{{cite journal |last1=Ollerton |first1=J. |last2=Winfree |first2=R. |last3=Tarrant |first3=S. |title=How many flowering plants are pollinated by animals? |journal=Oikos |date=March 2011 |volume=120 |issue=3 |pages=321–326 |doi=10.1111/j.1600-0706.2010.18644.x|bibcode=2011Oikos.120..321O }}</ref>

Insects, particularly [[mosquito]]es and [[flies]], are also vectors of many pathogens that may even have been responsible for the decimation or extinction of some mammalian species.<ref>{{cite journal |last1=Osborn |first1=H.F. |title=The Causes of Extinction in Mammalia |journal=The American Naturalist |date=1906 |volume=XL |issue=480 |pages=829–859 |doi=10.1086/278693 |url=https://zenodo.org/record/2449856 |doi-access=free}}</ref>

===Before the Devonian===
Molecular analysis by Gaunt & Miles 2002 suggests that the [[Hexapoda|hexapod]]s diverged from their sister group, the [[Anostraca]] (fairy shrimps), at around the start of the [[Silurian]] period {{Ma|440}} - coinciding with the appearance of [[vascular plants]] on land.<ref name=Gaunt2002>{{cite journal | author = Gaunt, M.W. | author2 = Miles, M.A. | date = 1 May 2002 | title = An Insect Molecular Clock Dates the Origin of the Insects and Accords with Palaeontological and Biogeographic Landmarks | journal = Molecular Biology and Evolution | volume = 19 | pages = 748–761 | issn = 1537-1719 | url = http://www.mbe.oupjournals.org/cgi/content/abstract/19/5/748 | pmid = 11961108 | issue = 5 | doi = 10.1093/oxfordjournals.molbev.a004133 | url-status = dead | archive-url = https://web.archive.org/web/20050320010953/http://mbe.oupjournals.org/cgi/content/abstract/19/5/748 | archive-date = 20 March 2005 | df = dmy-all | doi-access = free }}</ref>

Misof et. al. suggest that insects could have appeared much earlier, in the [[Early Ordovician]] or even [[Cambrian]]. According to this version, the early radiation of insects occurred no later than {{Ma|479}} in marine or coastal environments. However, the authors emphasize that due to the lack of insect fossils of the Cambrian to the Silurian, this version remains highly controversial.<ref name=Bernhard2014/>

===Devonian===
[[File:Strudiella_interpretation.png|thumb|Reconstruction ''[[Strudiella devonica]]'' as a possible hexapod]]
The [[Devonian]] (419 to 359 million years ago) was a relatively warm period, and probably lacked any glaciers.

The details of early insect fossil records are not well understood. The fossils that were considered as Devonian insects, such as ''[[Rhyniognatha hirsti]]''<ref name="EngelGrim">{{cite journal|bibcode=2004Natur.427..627E|title=New light shed on the oldest insect|last1=Engel|first1=Michael S.|last2=Grimaldi|volume=427|year=2004|pages=627–30|journal=Nature |doi=10.1038/nature02291|pmid=14961119|first2=DA|issue=6975|s2cid=4431205}}</ref> or ''[[Strudiella devonica]]''<ref name="garrouste2012">{{cite journal|last1=Garrouste|first1=Romain|last2=Clément|first2=G|last3=Nel|first3=P|last4=Engel|first4=MS|last5=Grandcolas|first5=P|last6=d'Haese|first6=C|last7=Lagebro|first7=L|last8=Denayer|first8=J|last9=Gueriau|first9=P|last10=Lafaite|first10=P|last11=Olive|first11=Sébastien|year=2012|title=A complete insect from the Late Devonian period|journal=Nature|volume=488|issue=7409|pages=82–5|bibcode=2012Natur.488...82G|doi=10.1038/nature11281|pmid=22859205|first13=A|first12=C|last12=Prestianni|last13=Nel|s2cid=205229663}}
*{{cite web |author=PZ Myers |date=August 2, 2012 |title=A Devonian hexapod |website=Free Thought Blogs |url=http://freethoughtblogs.com/pharyngula/2012/08/02/a-devonian-hexapod/}}</ref> were later reconsidered that their affinities as insects are insufficient.<ref name="Haug C., Haug J. T. (2017)." /><ref>{{Cite journal|last1=Hörnschemeyer|first1=Thomas|last2=Haug|first2=Joachim T.|last3=Bethoux|first3=Olivier|last4=Beutel|first4=Rolf G.|last5=Charbonnier|first5=Sylvain|last6=Hegna|first6=Thomas A.|last7=Koch|first7=Markus|last8=Rust|first8=Jes|last9=Wedmann|first9=Sonja|last10=Bradler|first10=Sven|last11=Willmann|first11=Rainer|date=2013-02-20|title=Is Strudiella a Devonian insect?|url=https://www.nature.com/articles/nature11887|journal=Nature|language=en|volume=494|issue=7437|pages=E3–E4|doi=10.1038/nature11887|pmid=23426326|bibcode=2013Natur.494E...3H |s2cid=205232661|issn=1476-4687}}</ref> But based on phylogenic study, the first insects probably appeared earlier, in the [[Silurian]] period,<ref name=Bernhard2014/> from stem-group crustaceans like ''[[Tanazios|Tanazios dokeron]]''<ref>The origin and evolution of Anthropods, Graham E. Budd & Maximilian J. Telford; Nature 2009</ref> that had lost the second antenna. The first winged insect likely evolved in the Devonian given the appearance of large numbers of insects with wings in the Carboniferous.<ref name=":1" />

===Carboniferous===
[[File:Mazothairos1.jpg|thumb|right|[[Mazothairos]], a Carboniferous member of the now extinct order [[Palaeodictyoptera]].]]
The [[Carboniferous]] ({{Ma|359|299}}) is famous for its wet, warm climates and extensive swamps of [[moss]]es, [[fern]]s, [[horsetail]]s, and [[calamite]]s.<ref name="Resh and Carde">{{cite book|last1=Resh|first1=Vincent H.|url=https://books.google.com/books?id=Jk0Hym1yF0cC|title=Encyclopedia of Insects|last2=Carde|first2=Ring T.|date=July 1, 2009|publisher=Academic Press|isbn=978-0-12-374144-8|edition=2}}{{page needed|date=September 2013}}</ref> Glaciations in [[Gondwana]], triggered by Gondwana's southward movement, continued into the [[Permian]] and because of the lack of clear markers and breaks, the deposits of this glacial period are often referred to as [[Permo-Carboniferous]] in age. The cooling and drying of the climate led to the [[Carboniferous rainforest collapse]] (CRC). Tropical rain forests fragmented and then were eventually devastated by [[climate change]].<ref name="SahneyBentonFerry2010RainforestCollapse">{{cite journal |doi=10.1130/G31182.1 |title=Rainforest collapse triggered Carboniferous tetrapod diversification in Euramerica |year=2010 |last1=Sahney |first1=S. |last2=Benton |first2=M. J. |last3=Falcon-Lang |first3=H. J. |journal=Geology |volume=38 |issue=12 |pages=1079–82|bibcode=2010Geo....38.1079S}}</ref>

Remains of insects are scattered throughout the coal deposits, particularly of wings from [[Blattoptera|stem-dictyopterans]] (Blattoptera);<ref>{{cite journal|title=X-ray micro-tomography of Carboniferous stem-Dictyoptera: New insights into early insects|first1=Russell J.|last1=Garwood|first2=Mark D.|last2=Sutton|year=2010|journal=Biology Letters|volume=6|issue=5|pages=699–702|doi=10.1098/rsbl.2010.0199|pmid=20392720|pmc=2936155}}</ref> two deposits in particular are from [[Mazon Creek fossil beds|Mazon Creek, Illinois]] and [[Commentry]], France.<ref>{{cite book |chapter-url=http://palaeoentomolog.ru/New/dictyo.html |chapter=SUPERORDER DICTYONEURIDEA Handlirsch, 1906 |author=Nina D. Sinitchenkova |title=History of Insects |publisher=[[Kluwer Academic Publishers]] |isbn=978-1-4020-0026-3 |editor1=A. P. Rasnitsyn |editor2=D. L. J. Quicke |year=2002}}</ref> The earliest winged insects are from this time period ([[Pterygota]]), including the aforementioned Blattoptera, [[Caloneurodea]], primitive stem-group [[Ephemeroptera]]ns, [[Orthoptera]], [[Palaeodictyopteroidea]].<ref name="Resh and Carde"/>{{rp|399}} In 1940 (in Noble County, Oklahoma), a fossil of ''Meganeuropsis americana'' represented the largest complete insect wing ever found.<ref>{{cite journal |title=Dragonfly: the largest complete insect wing ever found |journal=Harvard Magazine |page=112 |date=November–December 2007 |url=http://harvardmagazine.com/2007/11/dragonfly}}</ref> Juvenile insects are also known from the Carboniferous Period.<ref>{{cite journal|title=Tomographic Reconstruction of Neopterous Carboniferous Insect Nymphs|first=Russell J.|last=Garwood|display-authors=etal|year=2012|journal=PLOS ONE|volume=7|issue=9|pages=e45779|doi=10.1371/journal.pone.0045779|bibcode = 2012PLoSO...745779G|pmid=23049858|pmc=3458060|doi-access=free}}</ref>

Very early Blattopterans had a large, discoid pronotum and [[coriaceous]] forewings with a distinct CuP vein (a unbranched wing vein, lying near the claval fold and reaching the wing posterior margin). These were not true cockroaches, as they had an [[ovipositor]], although through the Carboniferous, the ovipositor started to diminish. The orders Caloneurodea and Miomoptera are known, with Orthoptera and Blattodea to be among the earliest Neoptera; developing from the upper Carboniferous to the Permian. These insects had wings with similar form and structure: small anal lobes.<ref name="Resh and Carde"/>{{rp|399}} Species of Orthoptera, or grasshoppers and related kin, is an ancient order that still exist till today extending from this time period. From which time even the distinctive [[synapomorphy]] of [[saltatorial]], or adaptive for jumping, hind legs is preserved.

Palaeodictyopteroidea is a large and diverse group that includes 50% of all known Paleozoic insects.<ref name="Grimaldi 2005"/> Containing many of the primitive features of the time: very long [[Cercus|cerci]], an [[ovipositor]], and wings with little or no [[anal lobe]]. [[Protodonata]], as its name implies, is a primitive paraphyletic group similar to [[Odonata]]; although lacks distinct features such as a [[nodus]], a [[pterostigma]] and an [[Glossary of entomology terms#A–C|arculus]]. Most were only slightly larger than modern dragonflies, but the group does include the largest known insects, such as [[griffinflies]] like the late Carboniferous ''[[Meganeura monyi]]'', and the even larger later Permian ''[[Meganeuropsis permiana]]'', with wingspans of up to {{cvt|71|cm|ftin}}. They were probably the top predators for some 100 million years<ref name="Resh and Carde" />{{rp|400}} and far larger than any present-day insects. Their nymphs must also have reached a very impressive size. This gigantism may have been due to higher atmospheric oxygen-levels (up to 80% above modern levels during the Carboniferous) that allowed increased respiratory efficiency relative to today. This allowed giant forms of [[pterygotes]], [[millipede]]s and [[scorpion]]s to exist, making the newly arrived [[tetrapods]] remain small until the [[Carboniferous Rainforest Collapse]]. However, a large griffinfly with a wingspan about {{cvt|43-47|cm|ftin}} is known from the late Permian, when the oxygen level was much lower.<ref>{{Cite journal |last1=Gand |first1=G. |last2=Nel |first2=A. N. |last3=Fleck |first3=G. |last4=Garrouste |first4=R. |date=2008-01-01 |title=The Odonatoptera of the Late Permian Lodève Basin (Insecta) |url=https://revistas.ucm.es/index.php/JIGE/article/view/JIGE0808120115A |journal=Journal of Iberian Geology |language=es |volume=34 |issue=1 |pages=115–122 |issn=1886-7995}}</ref> In addition, griffinflies probably lived in open habitats, as evidenced by ''Meganeurites gracilipes.'' ''M. gracilipes'' exhibited elongate wings that did not befit densely forested habitats, and had dorsally enlarged compound eyes much like modern dragonflies that hunt in open habitats.<ref>{{Cite journal |last1=Nel |first1=André |last2=Prokop |first2=Jakub |last3=Pecharová |first3=Martina |last4=Engel |first4=Michael S. |last5=Garrouste |first5=Romain |date=2018-08-14 |title=Palaeozoic giant dragonflies were hawker predators |journal=Scientific Reports |volume=8 |issue=1 |pages=12141 |doi=10.1038/s41598-018-30629-w |issn=2045-2322 |pmc=6092361 |pmid=30108284|bibcode=2018NatSR...812141N }}</ref>

===Permian===
The [[Permian]] ({{Ma|299|252}}) was a relatively short time period, during which all the [[Earth]]'s major land masses were collected into a single supercontinent known as [[Pangaea]]. Pangaea straddled the [[equator]] and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean ("[[Panthalassa]]", the "universal sea"), and the Paleo-Tethys Ocean, a large ocean that was between Asia and Gondwana. The [[Cimmeria plate|Cimmeria]] continent [[rift]]ed away from [[Gondwana]] and drifted north to [[Laurasia]], causing the [[Paleo-Tethys]] to shrink.<ref name="Resh and Carde"/>{{rp|400}} At the end of the Permian, the biggest mass extinction in history occurred, collectively called the [[Permian–Triassic extinction event]]: 30% of all insect species became extinct; this is one of three known mass insect extinctions in Earth's history.<ref name="evo">{{cite web |url=http://www.kerbtier.de/Pages/Themenseiten/enPhylogenie.html#L9|title=Phylogeny of the beetles |author=Benisch, Christoph|year=2010 |work=The beetle fauna of Germany |publisher=Kerbtier |access-date=March 16, 2011}}</ref>

A 2007 study based on [[DNA]] of living beetles and maps of likely beetle evolution indicated that beetles may have originated during the Lower [[Permian]], up to {{Ma|299}}.<ref name=predatedino>{{cite news |url=http://www.livescience.com/animals/071226-tough-beetles.html |title=Modern beetles predate dinosaurs |author=Dave Mosher |publisher=[[Live Science]] |date=December 26, 2007 |access-date=June 24, 2010}}</ref> In 2009, a fossil beetle was described from the [[Pennsylvanian (geology)|Pennsylvanian]] of [[Mazon Creek]], Illinois, pushing the origin of the beetles to an earlier date, {{Ma|318|299}}.<ref>{{cite journal |author=Oliver Béthoux |year=2009 |title=The earliest beetle identified |journal=[[Journal of Paleontology]] |volume=83 |issue=6 |pages=931–937 |doi=10.1666/08-158.1|bibcode=2009JPal...83..931B |s2cid=85796546}}</ref> Fossils from this time have been found in Asia and Europe, for instance in the red slate fossil beds of Niedermoschel near Mainz, Germany.<ref>{{cite journal | title = Die Insektentaphozönose von Niedermoschel (Asselian, unt. Perm; Deutschland) | journal = Schriften der Alfred-Wegener-Stiftung | first = T. | last = Hörnschemeyer |author2=H. Stapf |author3=Terra Nostra | issue = 99/8 | pages = 98| language=de}}</ref> Further fossils have been found in Obora, Czech Republic and Tshekarda in the Ural mountains, Russia.<ref>{{cite journal | title = On the systematic position of the supposed Permian beetles, Tshecardocoleidae [sic], with a description of a new collection | journal = Palaeontology | year = 1969 | first = J | last = Moravia | author2 = Kukalová, Sb. Geol. Ved. Rada. P. | issue = 11 | pages = 139–161}}</ref> More discoveries from North America were made in the [[Wellington Formation]] of Oklahoma and were published in 2005 and 2008.<ref name="evo"/><ref>{{cite journal | title = A Second Specimen of Permocoleus (Coleoptera) from the Lower Permian Wellington Formation of Noble County, Oklahoma | journal = Journal of the Kansas Entomological Society | year = 2008 | first = R. J. | last = Beckemeyer |author2=M. S. Engel | volume = 81 | issue = 1 | pages = 4–7 | url = http://fossilinsects.net/pdfs/beckemeyer_engel_2008_JKansEntSoc_PermocoleusPermOklahoma.pdf | access-date = 2011-03-17 | doi = 10.2317/JKES-708.01.1| s2cid = 86835593}}</ref> Some of the most important fossil deposits from this era are from Elmo, Kansas (260 mya); others include New South Wales, Australia (240 mya) and central Eurasia (250 mya).<ref name="Resh and Carde"/>{{rp|400}}

During this time, many of the species from the Carboniferous diversified, and many new orders developed, including: [[Protelytroptera]], primitive relatives of [[Plecoptera]] (Paraplecoptera), [[Psocoptera]], [[Mecoptera]], [[Coleoptera]], [[Raphidioptera]], and [[Neuroptera]], the last four being the first definitive records of the [[Holometabola]].<ref name="Resh and Carde"/>{{rp|400}} By the [[Pennsylvanian (geology)|Pennsylvanian]] and well into the Permian, by far the most successful were primitive [[Blattoptera]], or relatives of cockroaches. Six fast legs, two well-developed folding wings, fairly good eyes, long, well-developed antennae (olfactory), an omnivorous digestive system, a receptacle for storing sperm, a [[chitin]] skeleton that could support and protect, as well as a form of gizzard and efficient mouth parts, gave it formidable advantages over other herbivorous animals. About 90% of insects were cockroach-like insects ("Blattopterans").<ref>{{cite book |first=Elwood Curtin |last=Zimmerman |title=Insects of Hawaii: a manual of the insects of the Hawaiian Islands, including an enumeration of the species and notes on their origin, distribution, hosts, parasites, etc |url=https://books.google.com/books?id=VdsQAQAAMAAJ |year=1948 |publisher=University of Hawaii Press |volume=2}}</ref> The [[dragonflies]] ''[[Odonata]]'' were the dominant aerial predator and probably dominated terrestrial insect predation as well. True Odonata appeared in the Permian<ref>Grzimek HC Bernhard (1975) Grzimek's Animal Life Encyclopedia Vol 22 Insects. Van Nostrand Reinhold Co. NY.</ref><ref name=Riek84>{{cite journal |vauthors=Riek EF, Kukalova-Peck J |title=A new interpretation of dragonfly wing venation based on early Upper Carboniferous fossils from Argentina (Insecta: Odonatoida and basic character states in Pterygote wings) |journal=Can. J. Zool. |volume=62 |issue=6 |pages=1150–60 |year=1984 |doi=10.1139/z84-166}}</ref> and all are [[Amphibious insect|amphibian]]. Their prototypes are the oldest winged fossils,<ref>{{cite journal |author=Wakeling J, Ellington C |title=Dragonfly flight. III. Lift and power requirements |journal=J. Exp. Biol. |volume=200 |issue=Pt 3 |pages=583–600 |date=February 1997 |pmid=9318294 |url=http://jeb.biologists.org/cgi/pmidlookup?view=long&pmid=9318294|last2=Ellington|doi=10.1242/jeb.200.3.583 |url-access=subscription }}</ref> go back to the [[Devonian]], and are different from other wings in every way.<ref>{{cite journal |author=Matsuda R |title=Morphology and evolution of the insect thorax |journal=Mem Entomol Soc Can |volume=102 |issue=S76 |pages=5–431 |date=January 1970 |doi=10.4039/entm10276fv |url=http://journals.cambridge.org/action/displayIssue?decade=1970&jid=MCE&volumeId=102&issueId=S76&iid=8560836|url-access=subscription }}</ref> Their prototypes may have had the beginnings of many modern attributes even by late [[Carboniferous]] and it is possible that they even captured small vertebrates, for some species had a wing span of 71&nbsp;cm.<ref name=Riek84/>

The oldest known insect that resembles species of Coleoptera date back to the [[Permian|Lower Permian]] ({{Ma|270}}), though they instead have 13-segmented [[Antenna (biology)|antenna]]e, [[elytra]] with more fully developed venation and more irregular longitudinal ribbing, and an abdomen and [[ovipositor]] extending beyond the apex of the elytra. The oldest true beetle would have features that include 11-segmented antennae, regular longitudinal ribbing on the elytra, and having [[genitalia]] that are internal.<ref name="evo"/> The earliest beetle-like species had pointed, leather like forewings with cells and pits. [[Hemiptera]], or true bugs had appeared in the form of [[Arctiniscytina]] and [[Paraknightia]]. The later had expanded parapronotal lobes, a large ovipositor, and forewings with unusual venation, possibly diverging from [[Blattoptera]]. The orders Raphidioptera and Neuroptera are grouped together as [[Neuropterida]]. The one family of putative Raphidiopteran clade (Sojanoraphidiidae) has been controversially placed as so. Although the group had a long ovipositor distinctive to this order and a series of short crossveins, however with a primitive wing venation. Early families of Plecoptera had wing venation consistent with the order and its recent descendants.<ref name="Resh and Carde"/>{{rp|186}} [[Psocoptera]] was first appeared in the [[Permian]] period, they are often regarded as the most primitive of the [[hemiptera|hemipteroid]]s.<ref>{{cite book |title=Firefly Encyclopedia of Insects and Spiders |author=Christopher O'Toole |isbn=978-1-55297-612-8 |year=2002 |publisher=Firefly Books |location=Toronto |url-access=registration |url=https://archive.org/details/fireflyencyclope0000unse_k0d2}}</ref>

===Triassic===
The [[Triassic]] ({{Ma|252|201}}) was a period when arid and semiarid savannas developed and when the first [[mammal]]s, [[dinosaur]]s, and [[pterosaur]]s appeared. During the Triassic, almost all the Earth's land mass was still concentrated into Pangaea. From the east a vast gulf entered Pangaea, the Tethys sea. The remaining shores were surrounded by the world-ocean known as [[Panthalassa]]. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated.<ref name="evo"/>

The climate of the Triassic was generally hot and dry, forming typical [[red bed]] [[sandstone]]s and [[evaporite]]s. There is no evidence of [[glacier|glaciation]] at or near either pole; in fact, the polar regions were apparently moist and [[temperate]], a climate suitable for reptile-like creatures. Pangaea's large size limited the moderating effect of the global ocean; its [[continental climate]] was highly seasonal, with very hot summers and cold winters. It probably had strong, [[cross]]-[[equator]]ial [[monsoons]].<ref>{{cite journal | title=Late Triassic brachiopods from the Luning Formation, Nevada, and their palaeobiogeographical significance | journal=Palaeontology | date=14 July 1994 | first=George D. | last=Stanley |author2=Michael R. Sandy |volume=36 | issue=2 | pages=439–480 | url=https://www.palass.org/sites/default/files/media/publications/palaeontology/volume_36/vol36_part2_pp439-480.pdf | access-date=2019-10-31}}</ref>

As a consequence of the [[Permian–Triassic extinction event|P-Tr Mass Extinction]] at the border of Permian and [[Triassic]], there is only little fossil record of insects including beetles from the Lower Triassic.<ref>{{cite journal | title=On Permian and Triassic Insect Faunas in Relation to Biogeography and the Permian-Triassic Crisis | journal=Paleontological Journal | year=2008 | first=D. E. | last=Shcherbakov | volume=42 | issue=1 | pages=15–31| doi=10.1134/S0031030108010036 | s2cid=128919393 }}</ref> However, there are a few exemptions, like in Eastern Europe: At the Babiy Kamen site in the [[Kuznetsk Basin]] numerous beetle fossils were discovered, even entire specimen of the infraorders [[Archostemata]] (i.e., Ademosynidae, Schizocoleidae), [[Adephaga]] (i.e., Triaplidae, Trachypachidae) and [[Polyphaga]] (i.e., Hydrophilidae, Byrrhidae, Elateroidea) and in nearly a perfectly preserved condition.<ref>{{cite journal|title=Beetles (Insecta, Coleoptera) of the Late Permian and Early Triassic |journal=Paleontological Journal |year=2004 |first=A. G. |last=Ponomarenko |volume=38 |issue=Suppl. 2 |pages=S185–96 |url=http://palaeoentomolog.ru/Publ/PALS185.pdf |access-date=2011-03-17 |url-status=dead |archive-url=https://web.archive.org/web/20131111123220/http://palaeoentomolog.ru/Publ/PALS185.pdf |archive-date=2013-11-11}}</ref> However, species from the families [[Cupedidae]] and [[Schizophoroidae]] are not present at this site, whereas they dominate at other fossil sites from the Lower Triassic. Further records are known from Khey-Yaga, Russia in the Korotaikha Basin.<ref name="evo"/>

Around this time, during the Late Triassic, [[mycetophagous]], or fungus feeding species of beetle (i.e., [[Cupedidae]]) appear in the fossil record. In the stages of the Upper Triassic representatives of the [[algophagous]], or algae feeding species (i.e., [[Triaplidae]] and [[Hydrophilidae]]) begin to appear, as well as predatory water beetles. The first primitive weevils appear (i.e., [[Obrienidae]]), as well as the first representatives of the rove beetles (i.e., [[Staphylinidae]]), which show no marked difference in physique compared to recent species.<ref name="evo"/> This was also around the first time evidence of diverse freshwater insect fauna appeared.

Some of the oldest living families also appear around during the Triassic. [[Hemiptera]] included the [[Cercopidae]], the [[Cicadellidae]], the [[Cixiidae]], and the [[Membracidae]]. [[Coleoptera]] included the [[Carabidae]], the [[Staphylinidae]], and the [[Trachypachidae]]. [[Hymenoptera]] included the [[Xyelidae]]. [[Diptera]] included the [[Anisopodidae]], the [[Chironomidae]], and the [[Tipulidae]]. The first [[Thysanoptera]] appeared as well.

The first true species of Diptera are known from the Middle [[Triassic]], becoming widespread during the Middle and Late Triassic&nbsp;. A single large wing from a species of Diptera in the Triassic (10&nbsp;mm instead of usual 2–6&nbsp;mm) was found in Australia (Mt. Crosby). This family Tilliardipteridae, despite the numerous 'tipuloid' features, should be included in Psychodomorpha sensu Hennig on account of loss of the convex distal 1A reaching wing margin and formation of the anal loop.<ref>{{cite book |chapter-url=http://palaeoentomolog.ru/New/diptera.html |chapter=Order Diptera Linné, 1758. The true flies |author1=V. A. Blagoderov |author2=E. D. Lukashevich |author3=M. B. Mostovski |title=History of Insects |publisher=[[Kluwer Academic Publishers]] |isbn=978-1-4020-0026-3 |editor1=A. P. Rasnitsyn |editor2=D. L. J. Quicke |year=2002}}</ref>

===Jurassic===
The [[Jurassic]] ({{Ma|201|145}}) was important in the development of birds, one of the insects' major predators. During the early Jurassic period, the [[supercontinent]] Pangaea broke up into the northern supercontinent [[Laurasia]] and the southern supercontinent [[Gondwana]]; the [[Gulf of Mexico]] opened in the new rift between North America and what is now Mexico's [[Yucatan Peninsula]]. The Jurassic North [[Atlantic Ocean]] was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart.<ref>{{cite web | url = http://www.scotese.com/late1.htm | title = Late Jurassic | access-date = 2011-03-18 | date = February 2, 2003 | publisher = PALEOMAP Project}}</ref>

The global climate during the Jurassic was warm and humid. Similar to the Triassic, there were no larger landmasses situated near the polar caps and consequently, no inland ice sheets existed during the Jurassic. Although some areas of North and South America and Africa stayed arid, large parts of the continental landmasses were lush. The laurasian and the gondwanian fauna differed considerably in the Early Jurassic. Later it became more intercontinental and many species started to spread globally.<ref name="evo"/>

There are many important sites from the Jurassic, with more than 150 important sites with beetle fossils, the majority being situated in Eastern Europe and North Asia. In North America and especially in South America and Africa the number of sites from that time period is smaller and the sites have not been exhaustively investigated yet. Outstanding fossil sites include [[Solnhofen]] in Upper Bavaria, Germany,<ref name="Vienna 1985 135–144">{{cite journal | title = Fossil insects from the Tithonian "Solnhofener Plattenkalke" in the Museum of Natural History, Ponomarenko | journal = Ann. Naturhist. Mus. Wien | year = 1985 | first = A. G | last = Vienna | volume = 87 | issue = 1 | pages = 135–144 | url = http://www.landesmuseum.at/pdf_frei_remote/ANNA_87A_0135-0144.pdf | access-date = 2011-03-17}}</ref> Karatau in South [[Kazakhstan]],<ref name="Yan 2009 78–82">{{cite journal | title = A New Genus of Elateriform Beetles (Coleoptera, Polyphaga) from the Middle-Late Jurassic of Karatau | journal = Paleontological Journal | year = 2009 | first = E. V. | last = Yan | volume = 43 | issue = 1 | pages = 78–82 | url = http://fossilinsects.net/pdfs/Yan_2009_PalJ_ElateriformJurassicKaratau.pdf | access-date = 2011-03-17 | doi = 10.1134/S0031030109010080 | bibcode = 2009PalJ...43...78Y | s2cid = 84621777 | url-status = dead | archive-url = https://web.archive.org/web/20110718202329/http://fossilinsects.net/pdfs/Yan_2009_PalJ_ElateriformJurassicKaratau.pdf | archive-date = 2011-07-18}}</ref> the [[Yixian Formation]] in [[Liaoning]], North China<ref name="liaoning">{{cite journal | title = New Ommatids from the Late Jurassic of western Liaoning, China (Coleoptera: Archostemata) | journal = Insect Science | year = 2005 | first = J.-J. | last = Tan | author2 = D. Ren, M. Liu | volume = 12 | pages = 207–216 | url = http://fossilinsects.net/pdfs/tan_etal_2005.pdf | access-date = 2011-03-17 | doi = 10.1111/j.1005-295X.2005.00026.x | issue = 3 | bibcode = 2005InsSc..12..207T | s2cid = 83733980 | url-status = dead | archive-url = https://web.archive.org/web/20110718202354/http://fossilinsects.net/pdfs/tan_etal_2005.pdf | archive-date = 2011-07-18}}</ref> as well as the [[Jiulongshan Formation]] and further fossil sites in [[Mongolia]]. In North America there are only a few sites with fossil records of insects from the Jurassic, namely the shell limestone deposits in the Hartford basin, the Deerfield basin and the Newark basin.<ref name="evo"/><ref name="Ponomarenko 1997 389–399">{{cite journal | title = New Beetles of the Family Cupedidae from the Mesozoic of Mongolia. Ommatini, Mesocupedini, Priacmini | journal = Paleontological Journal | year = 1997 | first = A. G. | last = Ponomarenko | volume = 31 | issue = 4 | pages = 389–399 | url = http://palaeoentomolog.ru/Publ/PALJ389.pdf | access-date = 2011-03-17}}</ref> Numerous deposits of other insects occur in Europe and Asia. Including Grimmen and Solnhofen, German; Solnhofen being famous for findings of the earliest bird-like theropods (i.e. [[Archaeopteryx]]). Others include [[Dorset]], England; [[Issyk-Kul]], Kirghizstan; and the most productive site of all, [[Karatau]], Kazakhstan.{{Citation needed|date=August 2018}}

During the Jurassic there was a dramatic increase in the known diversity of {{clarify span|text=family-level Coleoptera|date=November 2021}}.<ref name="evo"/> This includes the development and growth of carnivorous and herbivorous species. Species of the superfamily [[Chrysomeloidea]] are believed to have developed around the same time, which include a wide array of plant host ranging from [[cycad]]s and [[conifer]]s, to [[angiosperm]]s.<ref name=insenc>{{cite book |last1=Powell |first1=Jerry A. |editor2-last=Cardé|editor2-first=Ring T.|editor1-first=Vincent H. |editor1-last=Resh |title=Encyclopedia of Insects |chapter-url=https://books.google.com/books?id=wrMcPwAACAAJ |access-date=14 November 2010 |edition=2 (illustrated) |year=2009 |publisher=Academic Press |isbn=978-0-12-374144-8 |pages=1132 |chapter=Coleoptera }}</ref>{{rp|186}} Close to the Upper Jurassic, the portion of the [[Cupedidae]] decreased, however at the same time the diversity of the early plant eating, or phytophagous species increased. Most of the recent phytophagous species of Coleoptera feed on flowering plants or angiosperms.

===Cretaceous===

The [[Cretaceous]] ({{Ma|145|66}}) had much of the same insect fauna as the Jurassic until much later on. During the Cretaceous, the late-[[Paleozoic]]-to-early-Mesozoic [[supercontinent]] of [[Pangaea]] completed its [[Plate tectonics|tectonic]] breakup into present day [[continent]]s, although their positions were substantially different at the time. As the [[Atlantic Ocean]] widened, the convergent-margin [[orogenies]] that had begun during the [[Jurassic]] continued in the [[American cordillera|North American Cordillera]], as the [[Nevadan orogeny]] was followed by the [[Sevier orogeny|Sevier]] and [[Laramide orogeny|Laramide orogenies]]. Though [[Gondwana]] was still intact in the beginning of the Cretaceous, it broke up as [[South America]], [[Antarctica]] and [[Australia]] rifted away from [[Africa]] (though [[India]] and [[Madagascar]] remained attached to each other); thus, the South Atlantic and [[Indian Ocean]]s were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising [[Sea level#Dry land|eustatic sea levels]] worldwide. To the north of Africa the [[Tethys Sea]] continued to narrow. Broad shallow seas advanced across central [[North America]] (the [[Western Interior Seaway]]) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between [[coal]] beds.

At the peak of the Cretaceous [[transgression (geology)|transgression]], one-third of Earth's present land area was submerged.<ref>{{cite book |first1=Dougal |last1=Dixon |first2=Michael J. |last2=Benton |first3=Ayala |last3=Kingsley |first4=Julian |last4=Baker |title=Atlas of Life on Earth |publisher=Barnes & Noble |year=2001 |isbn=978-0760719572 |page=215}}</ref> The [[Berriasian]] epoch showed a cooling trend that had been seen in the last epoch of the Jurassic. There is evidence that snowfalls were common in the higher latitudes and the tropics became wetter than during the Triassic and Jurassic.<ref name="The Berriasian Age">[http://www.palaeos.com/Mesozoic/Cretaceous/Berriasian.html The Berriasian Age] {{webarchive|url=https://web.archive.org/web/20101220223930/http://palaeos.com/Mesozoic/Cretaceous/Berriasian.html |date=2010-12-20}}</ref> Glaciation was however restricted to alpine [[glacier]]s on some high-[[latitude]] mountains, though seasonal snow may have existed farther south. Rafting by ice of stones into marine environments occurred during much of the Cretaceous but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia.<ref>{{cite journal | author = Alley N.F., Frakes L.A. | year = 2003 | title = First known Cretaceous glaciation: Livingston Tillite, South Australia | journal = Australian Journal of Earth Sciences | volume = 50 | pages = 134–150 | doi = 10.1046/j.1440-0952.2003.00984.x | issue = 2 |bibcode = 2003AuJES..50..139A | last2 = Frakes | s2cid = 128739024}}</ref><ref>{{cite journal | author = Frakes L.A., Francis J. E. | year = 1988 | title = A guide to Phanerozoic cold climates from high latitude ice rafting in the Cretaceous | journal = Nature | volume = 333 | issue = 6173| pages = 547–9 | doi = 10.1038/333547a0 |bibcode = 1988Natur.333..547F | last2 = Francis | s2cid = 4344903}}</ref>

There are a large number of important fossil sites worldwide containing beetles from the Cretaceous. Most of them are located in Europe and Asia and belong to the temperate climate zone during the Cretaceous. A few of the fossil sites mentioned in the chapter Jurassic also shed some light on the early cretaceous beetle fauna (e.g. the Yixian formation in Liaoning, North China).<ref name="liaoning"/> Further important sites from the Lower Cretaceous include the Crato Fossil Beds in the Araripe basin in the [[Ceará]], North Brazil as well as overlying Santana formation, with the latter was situated near the paleoequator, or the position of the earth's equator in the geologic past as defined for a specific geologic period. In [[Spain]] there are important sites near [[Montsec]] and [[Las Hoyas]]. In Australia the [[Koonwarra]] fossil beds of the Korumburra group, [[South Gippsland]], Victoria is noteworthy. Important fossil sites from the Upper Cretaceous are [[Kzyl-Dzhar]] in South [[Kazakhstan]] and [[Arkagala]] in Russia.<ref name="evo"/>

During the Cretaceous the diversity of Cupedidae and [[Archostemata]] decreased considerably. Predatory [[ground beetle]]s (Carabidae) and [[rove beetle]]s (Staphylinidae) began to distribute into different patterns: whereas the [[Carabidae]] predominantly occurred in the warm regions, the [[Staphylinidae]] and [[click beetle]]s (Elateridae) preferred many areas with temperate climate. Likewise, predatory species of [[Cleroidea]] and [[Cucujoidea]], hunted their prey under the bark of trees together with the [[jewel beetle]]s (Buprestidae). The jewel beetles diversity increased rapidly during the Cretaceous, as they were the primary consumers of wood,<ref>{{cite journal |title=New Jewel Beetles (Coleoptera: Buprestidae) from the Cretaceous of Russia, Kazakhstan, and Mongolia |journal=Paleontological Journal |date=May 2009 |issue=3 |volume=43 |pages=277–281 |url=http://fossilinsects.net/pdfs/Alexeev_2009_PalJ_BuprestidaeCretaceousRussiaKazakhstanMongolia.pdf |doi=10.1134/S0031030109030058 |last1=Alexeev |first1=A. V. |bibcode=2009PalJ...43..277A |s2cid=129618839 |url-status=dead |archive-url=https://web.archive.org/web/20110718202416/http://fossilinsects.net/pdfs/Alexeev_2009_PalJ_BuprestidaeCretaceousRussiaKazakhstanMongolia.pdf |archive-date=2011-07-18}}</ref> while [[longhorn beetle]]s (Cerambycidae) were rather rare and their diversity increased only towards the end of the Upper Cretaceous.<ref name="evo"/> The first [[coprophagous]] beetles have been recorded from the Upper Cretaceous,<ref>{{cite journal |author1=Chin, K. |author2= Gill, B.D. |title=Dinosaurs, dung beetles, and conifers; participants in a Cretaceous food web |journal=PALAIOS |date=June 1996 |issue=3 |volume=11 |pages=280–5 |jstor=3515235 |doi=10.2307/3515235|bibcode= 1996Palai..11..280C}}</ref> and are believed to have lived on the excrement of herbivorous dinosaurs, however there is still a discussion, whether the beetles were always tied to mammals during its development.<ref>{{cite journal |title=Did dinosaurs have any relation with dung-beetles? (The origin of coprophagy) |author=Antonio Arillo, Vicente M. Ortuño |journal=[[Journal of Natural History]] |year=2008 |volume=42 |issue=19–20 |pages=1405–8 |doi=10.1080/00222930802105130|last2=Ortuño |bibcode=2008JNatH..42.1405A |s2cid=83643794}}</ref> Also, the first species with an adaption of both larvae and adults to the aquatic lifestyle are found. [[Whirligig beetle]]s (Gyrinidae) were moderately diverse, although other early beetles (i.e., [[Dytiscidae]]) were less, with the most widespread being the species of [[Coptoclavidae]], which preyed on aquatic fly larvae.<ref name="evo"/>

===Paleogene===
There are many fossils of beetles known from this era, though the beetle fauna of the Paleocene is comparatively poorly investigated. In contrast, the knowledge on the Eocene beetle fauna is very good. The reason is the occurrence of fossil insects in amber and clay slate sediments. Amber is fossilized tree resin, that means it consists of fossilized organic compounds, not minerals. Different amber is distinguished by location, age and species of the resin producing plant. For the research on the Oligocene beetle fauna, Baltic and Dominican amber is most important.<ref name="evo"/> Even with the insect fossils record in general lacking, the most diverse deposit being from the Fur Formation, Denmark; including giant ants and primitive moths ([[Noctuidae]]).<ref name="Resh and Carde"/>{{rp|402}}

The first butterflies are from the Upper Paleogene, while most, like beetles, already had recent genera and species already existed during the Miocene, however, their distribution differed considerably from today's.<ref name="Resh and Carde"/>{{rp|402}}

===Neogene===
The most important sites for beetle fossils of the Neogene are situated in the warm temperate and to subtropical zones. Many recent genera and species already existed during the Miocene, however, their distribution differed considerably from today's. One of the most important fossil sites for insects of the Pliocene is Willershausen near Göttingen, Germany with excellently preserved beetle fossils of various families (longhorn beetles, weevils, ladybugs and others) as well as representatives of other orders of insects.<ref>{{cite journal | title=Dritter Beitrag über Käfer (Coleoptera) aus dem Jungtertiär von Willershausen | author=Gersdorf, Geol | journal=Bl. Northeim | year=1976 | volume=4226.E. | issue=36 | pages=103–145 | language=de}}</ref> In the Willershausen clay pit so far 35 genera from 18 beetle families have been recorded, of which six genera are extinct.<ref>{{cite journal | title=Late Pleistocene and Holocene Seasonal Temperatures Reconstructed from Fossil Beetle Assemblages in the Rocky Mountains | author=Elias, S.A. | journal=Quaternary Research | year=1996 | volume=46 | issue=3 | pages=311–8 | doi=10.1006/qres.1996.0069|bibcode = 1996QuRes..46..311E| s2cid=140554913 }}</ref> The Pleistocene beetle fauna is relatively well known, since the composition of the beetle fauna has been used to reconstruct climate conditions in the Rocky Mountains and on Beringia, the former land bridge between Asia and North America.<ref>{{cite journal | title=Late Pleistocene Climates of Beringia, Based on Analysis of Fossil Beetles | author=Elias, S. A. | journal=Quaternary Research | year=2000 | volume=53 | issue=2 | pages=229–235 | doi=10.1006/qres.1999.2093 |bibcode = 2000QuRes..53..229E | s2cid=140168723 }}</ref><ref>{{cite journal | title=Climatic Tolerances and Zoogeography of the Late Pleistocene Beetle Fauna of Beringia | author=Elias, S.A. | journal=Géographie Physique et Quaternaire | year=2000 | volume=54 | issue=2 | pages=143–155 | url=http://www.erudit.org/revue/gpq/2000/v54/n2/004813ar.pdf | doi=10.7202/004813ar| doi-access=free }}</ref>

==Phylogeny==
[[File:Rhyniognatha specimen.png|left|thumb|265x265px|Mandibles of ''Rhyniognatha hirsti'', it may be an oldest insect, but also possible to be a myriapod.]]
A report in November 2014 unambiguously places the insects in one clade, with the [[remipede]]s as the nearest sister clade.<ref name=misof>{{cite journal | display-authors=1 | last1=Misof | first1=Bernhard | first2=Shanlin | last3=Meusemann | first3=K. | last4=Peters | first4=R. S. | last5=Donath | first5=A. | last6=Mayer | first6=C. | last7=Frandsen | first7=P. B. | last8=Ware | first8=J. | last9=Flouri | first9=T. | last10=Beutel | first10=R. G. | last11=Niehuis | first11=O. | last12=Petersen | first12=M. | last13=Izquierdo-Carrasco | first13=F. | last14=Wappler | first14=T. | last15=Rust | first15=J. | last16=Aberer | first16=A. J. | last17=Aspock | first17=U. | last18=Aspock | first18=H. | last19=Bartel | first19=D. | last20=Blanke | first20=A. | last21=Berger | first21=S. | last22=Bohm | first22=A. | last23=Buckley | first23=T. R. | last24=Calcott | first24=B. | last25=Chen | first25=J. | last26=Friedrich | first26=F. | last27=Fukui | first27=M. | last28=Fujita | first28=M. | last29=Greve | first29=C. | last30=Grobe | first30=P. | last2=Liu | 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 | bibcode=2014Sci...346..763M | pmid=25378627| s2cid=36008925}}</ref> This study resolved insect phylogeny of all extant insect orders, and provides "a robust phylogenetic backbone tree and reliable time estimates of insect evolution."<ref name=misof/> Finding strong support for the closest living relatives of the hexapods had proven challenging due to convergent adaptations in a number of arthropod groups for living on land.<ref name="Garwood">{{cite journal |author1=Russell Garwood |author2=Gregory Edgecombe |year=2011 |title=''Early terrestrial animals, evolution and uncertainty'' |journal=[[Evolution: Education and Outreach]] |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |doi-access=free}}</ref>

{{Cladogram|
|clades={{clade| style=line-height:85%;font-size:75%
|1={{clade
|1={{clade| style=line-height:100%
|1=[[Hexapoda]] (Insecta, [[Collembola]], [[Diplura]], [[Protura]])
|2=[[Crustacea]] ([[crab]]s, [[shrimp]], [[isopods]], etc.)
}}
|label2=[[Myriapoda]]
|2={{clade| style=line-height:100%
|1=[[Pauropoda]]
|2=[[Diplopoda]] (millipedes)
|3=[[Chilopoda]] (centipedes)
|4=[[Symphyla]]
}}
|label3=[[Chelicerata]]
|3={{clade| style=line-height:100%
|1=[[Arachnid]]a ([[spider]]s, [[scorpion]]s and allies)
|2=[[Eurypterid]]a (sea scorpions: extinct)
|3=[[Xiphosura]] (horseshoe crabs)
|4=[[Pycnogonida]] (sea spiders)
}}
|4=[[Trilobite]]s (extinct)
}}
}}
|caption=A [[phylogenetic]] tree of the arthropods and related groups<ref>{{cite web | title=Tree of Life Web Project. Version 1 January 1995 (temporary) of Arthropoda | url=http://www.tolweb.org/Arthropoda | publisher=Tree of Life Web Project | year=1995 | access-date=2009-05-09}}</ref>
}}

In 2008, researchers at [[Tufts University]] uncovered what they believe is the world's oldest known full-body impression of a primitive flying insect, a 300 million-year-old specimen from the [[Carboniferous Period]].<ref>{{cite web | title=Researchers Discover Oldest Fossil Impression of a Flying Insect | url=http://newswise.com/articles/view/545296/ | publisher=Newswise | access-date= }}</ref> [[Devonian]] ''[[Rhyniognatha hirsti]]'', from the 396 million year old [[Rhynie chert]] is known only from mandibles, and considered as the oldest insect. This species already possessed dicondylic mandibles (two articulations in the mandible), a feature associated with winged insects, suggesting that wings may already have evolved at this time. Thus, if ''Rhyniognatha'' is actual flying insect, the first insects probably appeared earlier, in the [[Silurian]] period.<ref name="EngelGrim"/><ref>{{cite journal |doi=10.1144/gsjgs.152.2.0229 |title=A Devonian auriferous hot spring system, Rhynie, Scotland |year=1995 |last1=Rice |first1=C. M. |last2=Ashcroft |first2=W. A. |last3=Batten |first3=D. J. |last4=Boyce |first4=A. J. |last5=Caulfield |first5=J. B. D. |last6=Fallick |first6=A. E. |last7=Hole |first7=M.J. |last8=Jones |first8=E. |last9=Pearson |first9=M. J. |last10=Rogers |first10=G. |last11=Saxton |first11=J. M. |last12=Stuart |first12=F. M. |last13=Trewin |first13=N. H. |last14=Turner |first14=G. |journal=Journal of the Geological Society |volume=152 |issue=2 |pages=229–50|bibcode=1995JGSoc.152..229R |s2cid=128977213}}</ref> However, this species is also considered as [[myriapod]] in later study.<ref name="Haug C., Haug J. T. (2017)." /> There have been four super radiations of insects: [[beetle]]s (evolved around {{Ma|300}}), [[flies]] (evolved around {{Ma|250}}), [[moth]]s and [[wasp]]s (evolved around {{Ma|150}}).<ref name="Grimaldi 2005"/> These four groups account for the majority of described species. The flies and moths along with the [[flea]]s evolved from the [[Mecoptera]]. The origins of [[insect flight]] remain obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects had an additional pair of winglets attaching to the first segment of the thorax, for a total of three pairs. There is no evidence that suggests that the insects were a particularly successful group of animals before they evolved to have wings.<ref name="Grimaldi 2005">{{cite book |author-link1=David Grimaldi (entomologist) |first1=David |last1=Grimaldi |author-link2=Michael S. Engel |first2=Michael S. |last2=Engel |title=Evolution of the Insects |year=2005 |publisher=[[Cambridge University Press]] | isbn=978-0-521-82149-0}}{{page needed|date=September 2013}}</ref>

===Evolutionary relationships===
Insects are prey for a variety of organisms, including terrestrial vertebrates. The earliest vertebrates on land existed {{Ma|350}} and were large amphibious [[piscivore]]s, through gradual evolutionary change, [[insectivory]] was the next diet type to evolve.<ref name="SahneyBentonFerry2010RainforestCollapse"/> Insects were among the earliest terrestrial [[herbivore]]s and acted as major selection agents on plants.<ref name="Biology-coevolution"/> Plants evolved chemical [[Plant defense against herbivory|defenses against this herbivory]] and the insects in turn evolved mechanisms to deal with plant toxins.<ref name="Biology-coevolution"/> These toxins limit the diet breadth of herbivores, and evolving mechanisms to nonetheless continue herbivory is an important part of maintaining diet breadth in insects, and so in their evolutionary history as a whole. Both [[pleiotropy]] and [[epistasis]] have complex effects in this regard, with the simulations of Griswold 2006 showing that more genes provide the benefit of more targets for adaptive mutations, while Fisher 1930 showed that a mutation can improve one trait while epistasis causes it to also trigger negative effects - slowing down adaptation.<ref name="Hardy-et-al-2020">{{cite journal | last1=Hardy | first1=Nate B. | last2=Kaczvinsky | first2=Chloe | last3=Bird | first3=Gwendolyn | last4=Normark | first4=Benjamin B. | title=What We Don't Know About Diet-Breadth Evolution in Herbivorous Insects | journal=[[Annual Review of Ecology, Evolution, and Systematics]] | publisher=[[Annual Reviews (publisher)|Annual Reviews]] | volume=51 | issue=1 | date=2020-11-02 | issn=1543-592X | doi=10.1146/annurev-ecolsys-011720-023322 | pages=103–122| s2cid=225521141 | doi-access=free }}</ref>

Many insects also make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity using warning colors.<ref name="Biology-coevolution"/> This successful evolutionary pattern has also been utilized by [[mimic]]s. Over time, this has led to complex groups of coevolved species. Conversely, some interactions between plants and insects, like [[pollination]], are beneficial to both organisms. Coevolution has led to the development of very specific [[Mutualism (biology)|mutualism]]s in such systems.

==Taxonomy==
{| class="wikitable" style="float:right; width:20em;"
|-
|
{| style="background:Transparent; border:solid 0 #503df9;"
|-
! colspan="4" style="background:#E6D09D"| Classification
|- 
| rowspan="4" style="background:#ECF4ED"| [[Insecta]]
| colspan="3" style="background:#ECF4ED"| 
{| align="right" class="collapsible collapsed" style="background:Transparent"
! [[Monocondylia]]
|-
| style="border-top: 1px solid black;"| -[[Archaeognatha]] <small>- 470</small>
|}
|-
| rowspan="3" style="background:#ECF4ED"| [[Dicondylia]]
| colspan="2" style="background:#ECF4ED"| 
{| align="right" class="collapsible collapsed" style="background:Transparent;"
! [[Apterygota]]
|-
| style="border-top: 1px solid black;"| -[[Thysanura]]<small><200 </small>
|-
| -[[Monura]]<small> </small>
|}
|-
| rowspan="2" style="background:#ECF4ED"| [[Pterygota]]
| style="background:#ECF4ED"|
{| align="right" class="collapsible collapsed" style="background:Transparent"
! [[Paleoptera]]
|-
| style="border-top: 1px solid black;"| -[[Ephemeroptera]]<small>- 2,500–<3,000</small>
|-
| -[[Odonata]]<small>- 6,500 </small>
|}
|-
| style="background:#ECF4ED"|
{| align="right" class="collapsible collapsed" style="background:Transparent"
! [[Neoptera]]
|-
| style="border-top: 1px solid black;"| -[[Blattodea]]<small> - 3,684–4,000</small>
|-
| -[[Coleoptera]]<small> - 360,000–400,000</small>
|-
| -[[Dermaptera]]<small> - 1,816</small>
|-
| -[[Diptera]]<small> - 152,956</small>
|-
| -[[Embioptera]]<small> - 200–300</small>
|-
| -[[Hymenoptera]]<small> - 115,000</small>
|-
| -[[Lepidoptera]]<small> - 174,250</small>
|-
| -[[Mantodea]]<small> - 2,200</small>
|-
| -[[Mecoptera]]<small> - 481</small>
|-
| -[[Megaloptera]]<small> - 250–300</small>
|-
| -[[Neuroptera]]<small> - 5,000</small>
|-
| -[[Notoptera]]<small> - 55</small>
|-
| -[[Orthoptera]]<small> - 24,380</small>
|-
| -[[Phasmatodea]]<small> - 2,500–3,300</small>
|-
| -[[Phthiraptera]]<small> - 3,000–3,200</small>
|-
| -[[Plecoptera]]<small> - 2,274</small>
|-
| -[[Siphonaptera]]<small> - 2,525</small>
|-
| -[[Strepsiptera]]<small> - 596</small>
|-
| -[[Trichoptera]]<small> - 12,627</small>
|-
| -[[Zoraptera]]<small> - 28</small>
|-
| -[[Zygentoma]]<small> - 370</small>
|-
|}
|}
|-
|colspan="4" style="background:Transparent"|<small>[[Cladogram]] of living insect groups,<ref>{{cite web | title=Insecta | url=http://www.tolweb.org/Insecta/8205 | year=2002 | author=Tree of Life Web Project| access-date=2009-05-12}}</ref> with numbers of species in each group.<ref name="number">{{cite book |last=Erwin |first=Terry L. |editor1-last=Reaka-Kudla |editor1-first=M.L. |editor2-first=D.E. |editor2-last=Wilson |editor3-first=E.O. |editor3-last=Wilson |chapter=Ch. 4: Biodiversity at its utmost: Tropical Forest Beetles |title=Biodiversity II: Understanding and Protecting Our Biological Resources |chapter-url=https://books.google.com/books?id=MPp5RkhBrZEC&pg=PT27 |year=1996 |publisher=Joseph Henry Press |isbn=978-0-309-17656-9 |pages=27–40}}</ref> Note that [[Apterygota]], [[Palaeoptera]] and [[Exopterygota]] are possibly [[paraphyletic]] groups.</small>
|}

{{cladogram<!-- |title=???? -->
|align=right
|caption=Phylogenetic relationship of some common insect orders: [[Thysanura]], [[Odonata]], [[Orthoptera]], [[Phasmatodea]], [[Blattodea]], [[Isoptera]], [[Hemiptera]], [[Coleoptera]], [[Hymenoptera]], [[Lepidoptera]], [[Diptera]]. No information should be inferred from branch length.
|clades=
{{clade|style=font-size:100%;line-height:100%;white-space:nowrap;
|label1=[[Insects]]
|1={{clade 
   |1=[[Thysanura]] (silverfish)
   |2={{clade 
      |1=[[Odonata]] (dragonflies)
      |2={{clade 
         |1={{clade
            |1={{clade
               |1=[[Orthoptera]] (grasshoppers and crickets)
               |2=[[Phasmatodea]] (stick insects)
               }}
            |2={{clade
               |1=[[Blattaria]] (cockroaches)
               |2=[[Isoptera]] (termites)
               }}
            }}
         |2={{clade 
            |1=[[Hemiptera]] (true bugs)
            |2={{clade 
               |1=[[Coleoptera]] (beetles)
               |2={{clade 
                  |1=[[Hymenoptera]] (ants, bees, and wasps)
                  |2={{clade 
                     |1=[[Lepidoptera]] (butterflies and moths)
                     |2=[[Diptera]] (flies)
}} }} }} }} }} }} }} }}
}}

Traditional morphology-based or appearance-based [[systematics]] has usually given [[Hexapoda]] the rank of [[superclass (biology)|superclass]],<ref name="Gullan and Cranston">{{cite book |last1=Gullan |first1=P.J. |first2=P.S. |last2=Cranston |title=The Insects: An Outline of Entomology |publisher=Blackwell Publishing |location=Oxford |year=2005 |edition=3rd |page=[https://archive.org/details/isbn_9781405111133/page/180 180] |isbn=978-1-4051-1113-3 |url-access=registration |url=https://archive.org/details/isbn_9781405111133/page/180}}</ref> and identified four groups within it: insects (Ectognatha), springtails ([[Collembola]]), [[Protura]] and [[Diplura]], the latter three being grouped together as [[Entognatha]] on the basis of internalized mouth parts. Supraordinal relationships have undergone numerous changes with the advent of methods based on evolutionary history and genetic data. A recent theory is that Hexapoda is [[polyphyletic]] (where the last common ancestor was not a member of the group), with the entognath classes having separate evolutionary histories from Insecta.<ref>{{cite web | title=Classification of Insect | author=David A. Kendall | url=http://www.kendall-bioresearch.co.uk/class.htm | year=2009 | access-date=2009-05-09}}</ref> Many of the traditional appearance-based [[taxa]] have been shown to be paraphyletic, so rather than using ranks like [[Class (biology)|subclass]], [[Order (biology)|superorder]] and [[Order (biology)|infraorder]], it has proved better to use [[monophyletic]] groupings (in which the last common ancestor is a member of the group). The following represents the best supported monophyletic groupings for the Insecta.

Insects can be divided into two groups historically treated as subclasses: wingless insects, known as [[Apterygota]], and winged insects, known as [[Pterygota]]. The Apterygota consist of the primitively wingless order of the silverfish (Thysanura). Archaeognatha make up the Monocondylia based on the shape of their [[Mandible (insect mouthpart)|mandible]]s, while Thysanura and Pterygota are grouped together as Dicondylia. It is possible that the Thysanura themselves are not [[monophyletic]], with the family [[Lepidotrichidae]] being a [[sister group]] to the [[Dicondylia]] (Pterygota and the remaining Thysanura).<ref name="Classification of Insects">{{cite book |last=Gilliott |first=Cedric |title=Entomology |publisher=Springer-Verlag |location=New York |date=August 1995 |edition=2nd |page=96 |isbn=978-0-306-44967-3}}</ref><ref>{{cite book |last=Kapoor |first=V.C. C. |title=Principles and Practices of Animal Taxonomy |publisher=Science Publishers |date=January 1998 |edition=1st |volume=1 |page=48 |isbn=978-1-57808-024-3}}</ref>

Paleoptera and Neoptera are the winged orders of insects differentiated by the presence of hardened body parts called [[sclerite]]s; also, in Neoptera, muscles that allow their wings to fold flatly over the abdomen. Neoptera can further be divided into incomplete metamorphosis-based ([[Polyneoptera]] and [[Paraneoptera]]) and complete metamorphosis-based groups. It has proved difficult to clarify the relationships between the orders in Polyneoptera because of constant new findings calling for revision of the taxa. For example, Paraneoptera has turned out to be more closely related to Endopterygota than to the rest of the Exopterygota. The recent molecular finding that the traditional louse orders [[Mallophaga]] and [[Anoplura]] are derived from within [[Psocoptera]] has led to the new taxon [[Psocodea]].<ref>{{cite journal |doi=10.1098/rspb.2004.2798 |title=Multiple origins of parasitism in lice |year=2004 |last1=Johnson |first1=Kevin P. |last2=Yoshizawa |first2=Kazunori |last3=Smith |first3=Vincent S. |journal=Proceedings of the Royal Society B |volume=271 |issue=1550 |pages=1771–6 |jstor=4142860 |pmid=15315891 |pmc=1691793}}</ref> [[Phasmatodea]] and [[Embiidina]] have been suggested to form Eukinolabia.<ref>{{cite journal |doi=10.1111/j.1096-0031.2005.00062.x |title=Mantophasmatodea and phylogeny of the lower neopterous insects |year=2005 |last1=Terry |first1=Matthew D. |last2=Whiting |first2=Michael F. |journal=Cladistics |volume=21 |issue=3 |pages=240–57|s2cid=86259809|doi-access=free }}</ref> Mantodea, Blattodea and Isoptera are thought to form a monophyletic group termed [[Dictyoptera]].<ref>{{cite journal |doi=10.1016/S0960-9822(00)00561-3 |title=Evidence from multiple gene sequences indicates that termites evolved from wood-feeding cockroaches |year=2000 |last1=Lo |first1=Nathan |last2=Tokuda |first2=Gaku |last3=Watanabe |first3=Hirofumi |last4=Rose |first4=Harley |last5=Slaytor |first5=Michael |last6=Maekawa |first6=Kiyoto |last7=Bandi |first7=Claudio |last8=Noda |first8=Hiroaki |journal=Current Biology |volume=10 |issue=13 |pages=801–4 |pmid=10898984|s2cid=14059547|doi-access=free |bibcode=2000CBio...10..801L }}</ref>

It is likely that Exopterygota is paraphyletic in regard to Endopterygota. Matters that have had a lot of controversy include Strepsiptera and Diptera grouped together as Halteria based on a reduction of one of the wing pairs – a position not well-supported in the entomological community.<ref>{{cite journal |doi=10.1111/j.1365-2583.2006.00654.x |title=The rapid divergence of the ecdysone receptor is a synapomorphy for Mecopterida that clarifies the Strepsiptera problem |year=2006 |last1=Bonneton |first1=F. |last2=Brunet |first2=F. G. |last3=Kathirithamby |first3=J. |last4=Laudet |first4=V. |journal=Insect Molecular Biology |volume=15 |issue=3 |pages=351–62 |pmid=16756554|s2cid=25178911 }}</ref> The Neuropterida are often lumped or split on the whims of the taxonomist. Fleas are now thought to be closely related to [[Snow scorpionfly|boreid mecopterans]].<ref>{{cite journal |doi=10.1046/j.0300-3256.2001.00095.x |title=Mecoptera is paraphyletic: Multiple genes and phylogeny of Mecoptera and Siphonaptera |year=2002 |last1=Whiting |first1=Michael F. |journal=Zoologica Scripta |volume=31 |pages=93–104|s2cid=56100681 }}</ref> Many questions remain to be answered when it comes to basal relationships amongst endopterygote orders, particularly Hymenoptera.

The study of the classification or taxonomy of any insect is called [[Entomology|systematic entomology]]. If one works with a more specific order or even a family, the term may also be made specific to that order or family, for example [[Dipterology|systematic dipterology]].

==Early evidence==
According to phylogenic estimation, first insects possibly appeared in the [[Silurian]] period and got wings in Devonian.<ref name=Bernhard2014>{{Cite journal |last1=Misof |first1=Bernhard |last2=Liu |first2=Shanlin |last3=Meusemann |first3=Karen |last4=Peters |first4=Ralph S. |last5=Donath |first5=Alexander |last6=Mayer |first6=Christoph |last7=Frandsen |first7=Paul B. |last8=Ware |first8=Jessica |last9=Flouri |first9=Tomáš |last10=Beutel |first10=Rolf G. |last11=Niehuis |first11=Oliver |date=2014-11-07 |title=Phylogenomics resolves the timing and pattern of insect evolution |url=https://www.researchgate.net/publication/267916365|journal=Science |volume=346 |issue=6210 |pages=763–767 |language=EN |doi=10.1126/science.1257570|pmid=25378627 |bibcode=2014Sci...346..763M |s2cid=36008925 }}</ref><ref name=":12">{{cite journal |last1=McNamara |first1=M.E. |last2=Briggs |first2=D.EG. |last3=Orr |first3=P.J. |last4=Gupta |first4=N.S. |last5=Locatelli |first5=E.R. |last6=Qiu |first6=L. |last7=Yang |first7=H. |last8=Wang |first8=Z. |last9=Noh |first9=H. |last10=Cao |first10=H. |date=April 2013 |title=The fossil record of insect color illuminated by maturation experiments |journal=Geology |volume=41 |issue=4 |pages=487–490 |bibcode=2013Geo....41..487M |doi=10.1130/G33836.1}}</ref>

The subclass [[Apterygota]] (wingless insects) is now considered artificial as the [[silverfish]] (order [[Thysanura]]) are more closely related to [[Pterygota]] (winged insects) than to bristletails (order [[Archaeognatha]]). For instance, just like flying insects, Thysanura have so-called dicondylic mandibles, while Archaeognatha have monocondylic mandibles. The reason for their resemblance is not due to a particularly close relationship, but rather because they both have kept a primitive and original anatomy in a much higher degree than the winged insects. The most primitive order of flying insects, the mayflies ([[Ephemeroptera]]), are also those who are most morphologically and physiologically similar to these wingless insects. Some mayfly [[nymph (biology)|nymphs]] resemble aquatic thysanurans.

Modern Archaeognatha and Thysanura still have rudimentary appendages on their [[abdomen]] called styli, while more primitive and extinct insects known as [[Monura]] had much more developed abdominal appendages. The abdominal and [[thoracic]] segments in the earliest terrestrial ancestor of the insects would have been more similar to each other than they are today, and the head had well-developed [[compound eye]]s and long [[Antenna (biology)|antennae]]. Their body size is not known yet. As the most primitive group today, Archaeognatha, is most abundant near the coasts, it could mean that this was the kind of habitat where the insect ancestors became terrestrial. But this specialization to coastal [[Ecological niche|niches]] could also have a secondary origin, just as could their jumping [[animal locomotion|locomotion]], as it is the crawling Thysanura who are considered to be most original ([[plesiomorphic]]). By looking at how primitive [[chelicerata]]n [[book gill]]s (still seen in [[horseshoe crab]]s) evolved into [[book lung]]s in primitive [[spider]]s and finally into [[Invertebrate trachea|trachea]]e in more advanced spiders (most of them still have a pair of book lungs intact as well), it is possible the trachea of insects was formed in a similar way, modifying gills at the base of their appendages.

So far, no published research suggests that insects were a particularly successful group prior to their evolution of [[insect wing|wings]].<ref>{{cite journal | last1 = Dudley | first1 = Robert | title = Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance | year = 1998 | journal = The Journal of Experimental Biology | volume = 201 | issue = Pt 8| pages = 1043–050 | doi = 10.1242/jeb.201.8.1043 | pmid = 9510518}}</ref>

===Odonata===
{{Unreferenced section|date=May 2023}}
The [[Odonata]] (dragonflies) are also a good candidate as the oldest living member of the [[Pterygota]]. [[Mayflies]] are morphologically and physiologically more basal, but the derived characteristics of dragonflies could have evolved independently in their own direction for a long time. It seems that orders with aquatic nymphs or larvae become evolutionarily conservative once they had adapted to water. If mayflies made it to the water first, this could partly explain why they are more primitive than dragonflies, even if dragonflies have an older origin. Similarly, [[stoneflies]] retain the most basal traits of the [[Neoptera]], but they were not necessarily the first order to branch off. This also makes it less likely that an aquatic ancestor would have the evolutionary potential to give rise to all the different forms and species of insects that we know today.

Dragonfly [[Nymph (biology)|nymphs]] have a unique labial "mask" used for catching prey, and the [[imago]] has a unique way of copulating, using a secondary male sex organ on the second abdominal segment. It looks like abdominal appendages modified for sperm transfer and direct insemination have occurred at least twice in insect evolution, once in Odonata and once in the other flying insects. If these two different methods are the original ways of copulating for each group, it is a strong indication that it is the dragonflies who are the oldest, not the mayflies. There is still not agreement about this. Another scenario is that abdominal appendages adapted for direct insemination have evolved three times in insects; once Odonata, once in mayflies and once in the Neoptera, both mayflies and Neoptera choosing the same solution. If so, it is still possible that mayflies are the oldest order among the flying insects. The power of flight is assumed to have evolved only once, suggesting sperm was still transferred indirectly in the earliest flying insects.

One possible scenario on how direct insemination evolved in insects is seen in [[scorpion]]s. The male deposits a spermatophore on the ground, locks its claws with the female's claws and then guides her over his packet of sperm, making sure it comes in contact with her genital opening. When the early (male) insects laid their spermatophores on the ground, it seems likely that some of them used the clasping organs at the end of their body to drag the female over the package. The ancestors of Odonata evolved the habit of grabbing the female behind her head, as they still do today. This action, rather than not grasping the female at all, would have increased the male's chances of spreading its genes. The chances would be further increased if they first attached their spermatophore safely on their own abdomen before they placed their abdominal claspers behind the female's head; the male would then not let the female go before her abdomen had made direct contact with his sperm storage, allowing the transfer of all sperm.

This also meant increased freedom in searching for a female mate because the males could now transport the packet of sperm elsewhere if the first female slipped away. This ability would eliminate the need to either wait for another female at the site of the deposited sperm packet or to produce a new packet, wasting energy. Other advantages include the possibility of mating in other, safer places than flat ground, such as in trees or bushes.

If the ancestors of the other flying insects evolved the same habit of clasping the female and dragging her over their spermatophore, but posterior instead of anterior like the Odonata does, their genitals would come very close to each other. And from there on, it would be a very short step to modify the vestigial appendages near the male genital opening to transfer the sperm directly into the female. The same appendages the male Odonata use to transfer their sperm to their secondary sexual organs at the front of their abdomen. All insects with an aquatic nymphal or larval stage seem to have adapted to water secondarily from terrestrial ancestors. Of the most primitive insects with no wings at all, [[Archaeognatha]] and [[Thysanura]], all members live their entire life cycle in terrestrial environments. As mentioned previously, Archaeognatha were the first to split off from the branch that led to the winged insects ([[Pterygota]]), and then the Thysanura branched off. This indicates that these three groups (Archaeognatha, Thysanura and Pterygota) have a common terrestrial ancestor, which probably resembled a primitive model of Apterygota, was an opportunistic generalist and laid [[spermatophore]]s on the ground instead of copulating, like Thysanura still do today. If it had feeding habits similar to the majority of apterygotes of today, it lived mostly as a [[decomposer]].

One should expect that a gill breathing arthropod would modify its gills to breathe air if it were adapting to terrestrial environments, and not evolve new respiration organs from bottom up next to the original and still functioning ones. Then comes the fact that insect (larva and nymph) gills are actually a part of a modified, closed trachea system specially adapted for water, called tracheal gills. The arthropod [[Invertebrate trachea|trachea]] can only arise in an [[atmosphere]] and as a consequence of the adaptations of living on land. This too indicates that insects are descended from a terrestrial ancestor.

And finally when looking at the three most primitive insects with aquatic nymphs (called naiads: [[Ephemeroptera]], [[Odonata]] and [[Plecoptera]]), each order has its own kind of tracheal gills that are so different from one another that they must have separate origins. This would be expected if they evolved from land-dwelling species. This means that one of the most interesting parts of insect evolution is what happened between the Thysanura-Pterygota split and the first flight.

==Origin of insect flight==
The origin of [[insect flight]] remains obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects (e.g. the [[Palaeodictyoptera]]) had an additional pair of winglets attached to the first segment of the [[thorax]], for a total of three pairs.

The [[insect wing|wings]] themselves are sometimes said to be highly modified (tracheal) gills.<ref>{{Cite journal|last=Crampton|first=G.|date=1916|title=The Phylogenetic Origin and the Nature of the Wings of Insects According to the Paranotal Theory|journal=Journal of the New York Entomological Society|volume=24|issue=1|pages=1–39|jstor=25003692}}</ref> By comparing a well-developed pair of gill blades in mayfly naiads and a reduced pair of hind wings on the adults, it is not hard to imagine that the mayfly gills (tergaliae) and insect wings have a common origin, and newer research also supports this.<ref>{{cite journal |last1=Kaiser |first1=Jocelyn |title=A New Theory of Insect Wing Origins Takes Off |journal=Science |date=21 October 1994 |volume=266 |issue=5184 |page=363 |doi=10.1126/science.266.5184.363 |jstor=2885311 |pmid=17816673|bibcode=1994Sci...266..363K }}</ref><ref>{{cite journal |last1=Almudi |first1=Isabel |last2=Vizueta |first2=Joel |last3=Wyatt |first3=Christopher |last4=de Mendoza |first4=Alex |last5=Marlétaz |first5=Ferdinand |last6=Firbas |first6=Panos |last7=Feuda |first7=Roberto |last8=Masiero |first8=Giulio |last9=Medina |first9=Patricia |last10=Alcaina-Caro |first10=Ana |last11=Cruz |first11=Fernando |last12=Gómez-Garrido |first12=Jessica |last13=Gut |first13=Marta |last14=Alioto |first14=Tyler S. |last15=Vargas-Chavez |first15=Carlos |last16=Davie |first16=Kristofer |last17=Misof |first17=Bernhard |last18=González |first18=Josefa |last19=Aerts |first19=Stein |last20=Lister |first20=Ryan |last21=Paps |first21=Jordi |last22=Rozas |first22=Julio |last23=Sánchez-Gracia |first23=Alejandro |last24=Irimia |first24=Manuel |last25=Maeso |first25=Ignacio |last26=Casares |first26=Fernando |title=Genomic adaptations to aquatic and aerial life in mayflies and the origin of insect wings |journal=Nature Communications |date=2020 |volume=11 |issue=1 |page=2631 |doi=10.1038/s41467-020-16284-8 |pmid=32457347 |pmc=7250882|bibcode=2020NatCo..11.2631A }}</ref> Specifically, genetic research on mayflies has revealed that the gills and insect wings both may have originated from insect legs.<ref>{{cite magazine |last1=Pennisi |first1=Elizabeth |author-link=Elizabeth Pennisi |title=Insect wings evolved from legs, mayfly genome suggests |journal=Science |date=2 June 2020 |url=https://www.science.org/content/article/insect-wings-evolved-legs-mayfly-genome-suggests |access-date=1 September 2020}}</ref> The tergaliae are not found in any other order of insects, and they have evolved in different directions with time. In some nymphs/naiads the most anterior pair has become sclerotized and works as a gill cover for the rest of the gills. Others can form a large sucker, be used for swimming or modified into other shapes. But that need not necessarily mean that these structures were originally gills. It could also mean that the tergaliae evolved from the same structures which gave rise to the wings, and that flying insects evolved from a wingless terrestrial species with pairs of plates on its body segments: three on the thorax and nine on the abdomen (mayfly nymphs with nine pairs of tergaliae on the abdomen exist, but so far no living or extinct insects with plates on the last two segments have been found). If these were primary gills, it would be a mystery why they should have waited so long to be modified when we see the different modifications in modern mayfly nymphs.

===Theories===
When the first forests arose on Earth, new [[ecological niche|niches]] for terrestrial animals were created. [[Spore]]-feeders and others who depended on plants and/or the animals living around them would have to adapt too to make use of them. In a world with no flying animals, it would probably just be a matter of time before some arthropods who were living in the trees evolved paired structures with muscle attachments from their [[exoskeleton]] and used them for gliding, one pair on each segment. Further evolution in this direction would give bigger gliding structures on their [[thorax]] and gradually smaller ones on their [[abdomen]]. Their bodies would have become stiffer while [[thysanura]]ns, which never evolved flight, kept their flexible abdomen.

[[Mayfly]] [[nymph (biology)|nymphs]] must have adapted to water while they still had the "gliders" on their abdomen intact. So far there is no concrete evidence to support this theory either, but it is one that offers an explanation for the problems of why presumably aquatic animals evolved in the direction they did.

Leaping and [[arboreal]] insects seem like a good explanation for this evolutionary process for several reasons. Because early winged insects were lacking the sophisticated [[insect wing|wing]] folding mechanism of [[neoptera|neopterous]] insects, they must have lived in the open and not been able to hide or search for food under leaves, in cracks, under rocks and other such confined spaces. In these old forests there were few open places where insects with huge structures on their back could have lived without experiencing huge disadvantages. If insects got their wings on land and not in water, which clearly seems to be the case, the tree [[canopy (forest)|canopies]] would be the most obvious place where such gliding structures could have emerged, in a time when the air was a new territory.

The question is if the plates used for gliding evolved from "scratch" or by modifying already existing anatomical details. The thorax in Thysanura and Archaeognatha are known to have some structures connected to their trachea which share similarities to the wings of primitive insects. This suggests the origin of the wings and the spiracles are related.

Gliding requires universal body modifications, as seen in present-day [[vertebrate]]s such as some [[rodent]]s and [[marsupial]]s, which have grown wide, flat expansions of skin for this purpose. The flying dragons (genus ''[[Draco (genus)|Draco]]'') of [[Indonesia]] has modified its ribs into gliders, and even some [[snake]]s can glide through the air by spreading their ribs. The main difference is that while vertebrates have an inner [[skeleton]], primitive insects had a flexible and adaptive exoskeleton.

Some animals would be living in the trees, as animals are always taking advantage of all available [[ecological niche|niches]], both for feeding and protection. At the time, the reproductive organs were by far the most nutritious part of the plant, and these early plants show signs of arthropod consumption and adaptations to protect themselves, for example by placing their reproductive organs as high up as possible. But there will always be some species who will be able to cope with that by following their food source up the trees. Knowing that insects were terrestrial at that time and that some arthropods (like primitive insects) were living in the tree crowns, it seems less likely that they would have developed their wings down on the ground or in the water.

In a three dimensional environment such as trees, the ability to glide would increase the insects' chances to survive a fall, as well as saving energy. This trait has repeated itself in modern wingless species such as the [[gliding ant]]s who are living an arboreal life. When the gliding ability first had originated, gliding and leaping behavior would be a logical next step, which would eventually be reflected in their anatomical design. The need to navigate through vegetation and to land safely would mean good muscle control over the proto-wings, and further improvements would eventually lead to true (but primitive) wings. While the thorax got the wings, a long abdomen could have served as a stabilizer in flight.

Some of the earliest flying insects were large predators: it was a new ecological niche. Some of the prey were no doubt other insects, as insects with proto-wings would have radiated into other species even before the wings were fully evolved. From this point on, the arms race could continue: the same predator/prey [[co-evolution]] which has existed as long as there have been predators and prey on earth; both the hunters and the hunted were in need of improving and extending their flight skills even further to keep up with the other.

Insects that had evolved their proto-wings in a world without flying predators could afford to be exposed openly without risk, but this changed when carnivorous flying insects evolved. It is unknown when they first evolved, but once these predators had emerged they put a strong selection pressure on their victims and themselves. Those of the prey who came up with a good solution about how to fold their wings over their backs in a way that made it possible for them to live in narrow spaces would not only be able to hide from flying predators (and terrestrial predators if they were on the ground) but also to exploit a wide variety of niches that were closed to those unable to fold their wings in this way. And today the [[neoptera|neopterous]] insects (those that can fold their wings back over the abdomen) are by far the most dominant group of insects.

The water-skimming theory suggests that skimming on the water surface is the origin of insect flight.<ref>{{cite journal |doi=10.1126/science.266.5184.427 |bibcode=1994Sci...266..427M |title=Surface-Skimming Stoneflies: A Possible Intermediate Stage in Insect Flight Evolution |year=1994 |last1=Marden |first1=James H. |last2=Kramer |first2=Melissa G. |journal=Science |volume=266 |issue=5184 |pages=427–30 |pmid=17816688|s2cid=1906483}}</ref> This theory is based on the fact that what may be the first fossil insects, the Devonian ''[[Rhyniognatha hirsti]]'', are thought to have possessed wings. However, they may be closer to the myriapods, whereas insects' closest evolutionary ties are with crustaceans, which are aquatic.

==Life cycle==
{{Unreferenced section|date=September 2013}}

===Mayflies===
Another primitive trait of the mayflies are the [[subimago]]; no other insects have this winged yet sexually immature stage. A few specialized species have females with no subimago, but retain the subimago stage for males.

The reasons the subimago still exists in this order could be that there has never been enough [[selection pressure]] to get rid of it; it also seems specially adapted to do the transition from water to air.

The male genitalia are not fully functional at this point. One reason for this could be that the modification of the abdominal appendages into male copulation organs emerged later than the evolution of flight. This is indicated by the fact that dragonflies have a different copulation organ than other insects.

As we know, in mayflies the nymphs and the adults are specialized for two different ways of living; in the water and in the air. The only stage ([[instar]]) between these two is the subimago. In more primitive fossil forms, the preadult individuals had not just one instar but numerous ones (while the modern subimago do not eat, older and more primitive species with a subimagos were probably feeding in this phase of life too as the lines between the instars were much more diffuse and gradual than today). Adult form was reached several moults before maturity. They probably did not have more instars after becoming fully mature. This way of maturing is how [[Apterygota]] do it, which moult even when mature, but not winged insects.

Modern mayflies have eliminated all the instars between imago and nymph, except the single instar called subimago, which is still not (at least not in the males) fully sexually mature. The other flying insects with [[incomplete metamorphosis]] ([[Exopterygota]]) have gone a little further and completed the trend; here all the immature structures of the animal from the last nymphal stage are completed at once in a single final moult. The more advanced insects with larvae and [[complete metamorphosis]] ([[Endopterygota]]) have gone even further. An interesting theory is that the [[pupa]]l stage is actually a strongly modified and extended stage of subimago, but so far it is nothing more than a theory. There are some insects within the Exopterygota, [[thrips]] and whiteflies ([[Aleyrodidae]]), who have evolved pupae-like stages too.

===Distant ancestors===
The distant ancestor of flying insects, a species with primitive proto-wings, had a more or less [[ametabolous]] life-cycle and [[instar]]s of basically the same type as [[thysanura]]ns with no defined nymphal, subimago or adult stages as the individual became older. Individuals developed gradually as they were grew and moulting, but probably without major changes inbetween instars.

Modern mayfly nymphs do not acquire gills until after their first moult. Before this stage they are so small that they need no gills to extract oxygen from the water. This could be a trait from the common ancestor of all flyers. An early terrestrial insect would have no need for paired outgrowths from the body before it started to live in the trees (or in the water, for that matter), so it would not have any.

This would also affect the way their offspring looked like in the early instars, resembling earlier [[ametabolous]] generations even after they had started to adapt to a new way of living, in a habitat where they actually could have some good use for flaps along their body. Since they matured in the same way as thysanurans with plenty of moultings as they were growing and very little difference between the adults and much younger individuals (unlike modern insects, which are [[hemimetabolous]] or [[holometabolous]]), there probably was not as much room for adaptation into different niches depending on age and stage. Also, it would have been difficult for an animal already adapted to a niche to make a switch to a new niche later in life based on age or size differences alone when these differences were not significant.

So proto-insects had to specialize and focus their whole existence on improving a single lifestyle in a particular niche. The older the species and the single individuals became, the more would they differ from their original form as they adapted to their new lifestyles better than the generations before. The final body-structure was no longer achieved while still inside the egg, but continued to develop for most of a lifetime, causing a bigger difference between the youngest and oldest individuals. Assuming that mature individuals most likely mastered their new element better than did the nymphs who had the same lifestyle, it would appear to be an advantage if the immature members of the species reached adult shape and form as soon as possible. This may explain why they evolved fewer but more intense instars and a stronger focus on the adult body, and with greater differences between the adults and the first instars, instead of just gradually growing bigger as earlier generations had done. This evolutionary trend explains how they went from ametabolous to hemimetabolous insects.

Reaching maturity and a fully-grown body became only a part of the development process; gradually a new anatomy and new abilities - only possible in the later stages of life - emerged. The anatomy insects were born and grew up with had limitations which the adults who had learned to fly did not suffer from. If they were unable to live their early life the way adults did, immature individuals had to adapt to the best way of living and surviving despite their limitations till the moment came when they could leave them behind. This would be a starting point in the evolution where [[imago]] and nymphs started to live in different niches, some more clearly defined than others. Also, a final anatomy, size and maturity reached at once with a single final nymphal stage meant less waste of time and energy, and also{{citation needed|date=January 2014}} made a more complex adult body structure. These strategies obviously became very successful with time.

==See also==
*[[Evolution of arthropods]]
*[[Evolution of butterflies]]
*[[Evolution of spiders]]

==References==
{{Reflist|32em}}

==External links==
*[http://www.pnas.org/cgi/content/full/101/11/3723 What arthropod brains say about arthropod phylogeny]
*[http://palaeoentomolog.ru/New/ecology.html Ecological history of the terrestrial insects]
*[http://palaeoentomolog.ru/New/geography.html Geographical history of the insects]
*[https://web.archive.org/web/20051208033451/http://www.famu.org/mayfly/china/prim_chars.html The Primitive Characters of Extant Mayflies (Ephemeroptera)]
*[https://web.archive.org/web/20051215001146/http://www.agctr.lsu.edu/arthropodmuseum/morphlect%209.htm The insect abdomen and terminalia]
*[https://web.archive.org/web/20060706160231/http://www.famu.edu/acad/research/mayfly/kluge/morph.html Morphology of Ephemeroptera]
*[https://archive.today/20070419222754/http://fossilinsects.net/index.html International Palaeoentomological Society]

{{Evolution}}
{{Orders of Insects}}
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[[Category:Evolution of insects| ]]