The most recent understanding of theevolution 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 aspaleoentomology. It is estimated that the class ofinsects originated on Earth about 480 million years ago, in theOrdovician, at about the same timeterrestrial plants appeared.[1] Insects are thought to have evolved from a group ofcrustaceans.[2] The first insects were landbound, but about 400 million years ago in theDevonian period one lineage of insects evolved flight, the first animals to do so.[1] The oldest insect fossil has been proposed to beRhyniognatha hirsti, estimated to be 400 million years old, but the insect identity of the fossil has been contested.[3] Global climate conditions changed several times during the history of Earth, and along with it thediversity of insects. ThePterygotes (winged insects) underwent a majorradiation in theCarboniferous (358 to 299 million years ago) while theEndopterygota (insects that go through different life stages withmetamorphosis) underwent another major radiation in thePermian (299 to 252 million years ago).
Most extantorders of insects developed during thePermian period. Many of the early groups became extinct during themass extinction at the Permo-Triassic boundary, the largest extinction event in the history of the Earth, around 252 million years ago.[4] The survivors of this event evolved in theTriassic (252 to 201 million years ago) to what are essentially the modern insect orders that persist to this day. Most modern insectfamilies appeared in theJurassic (201 to 145 million years ago).
In an important example ofco-evolution, a number of highly successful insect groups — especially theHymenoptera (wasps, bees and ants) andLepidoptera (butterflies) as well as many types ofDiptera (flies) andColeoptera (beetles) — evolved in conjunction withflowering plants during theCretaceous (145 to 66 million years ago).[5][6]
Many modern insectgenera developed during theCenozoic that began about 66 million years ago; insects from this period onward frequently became preserved inamber, often in perfect condition. Such specimens are easily compared with modern species, and most of them are members of extant genera.
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Due to their external skeleton, the fossil history of insects is not entirely dependent onlagerstätte type preservation as for manysoft-bodied organisms. 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.[7]
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 aresubfossils.[8] 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.[9] Preservation in amber is, however, limited since copious resin production by trees only evolved in the Mesozoic.[10][11]
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 tofootprints andcoprolites.[12]: 42
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 (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 orbrackish 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 Jurassiclimestones fromSolnhofen andEichstätt, Germany, which are marine. These deposits are famous for pterosaurs and the bird-likeArchaeopteryx. The limestones were formed by a very fine mud of calcite that settled within stagnant, hypersaline bays isolated from inland seas. Most organisms in theselimestones, 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.[12]: 42
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 theHolocene. 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]
When an insect is partly or wholly replaced by minerals, usually completely articulated and with three-dimensional fidelity, is calledmineral replication.[12] This is also called petrifaction, as inpetrified 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.
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.[13] The Endopterygota underwent another major radiation in the Permian. Survivors of the mass extinction at theP-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 theTertiary, 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.[7]
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.[14]
Insects, particularlymosquitoes andflies, are also vectors of many pathogens that may even have been responsible for the decimation or extinction of some mammalian species.[15]
Molecular analysis by Gaunt & Miles 2002 suggests that thehexapods diverged from their sister group, theAnostraca (fairy shrimps), at around the start of theSilurian period440 million years ago - coinciding with the appearance ofvascular plants on land.[16]
Misof et. al. suggest that insects could have appeared much earlier, in theEarly Ordovician or evenCambrian. According to this version, the early radiation of insects occurred no later than479 million years ago 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.[17]
TheDevonian (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 asRhyniognatha hirsti[18] orStrudiella devonica[19] were later reconsidered that their affinities as insects are insufficient.[3][20] But based on phylogenic study, the first insects probably appeared earlier, in theSilurian period,[17] from stem-group crustaceans likeTanazios dokeron[21] 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.[9]
TheCarboniferous (359 to 299 million years ago) is famous for its wet, warm climates and extensive swamps ofmosses,ferns,horsetails, andcalamites.[22]:458 Glaciations inGondwana, triggered by Gondwana's southward movement, continued into thePermian and because of the lack of clear markers and breaks, the deposits of this glacial period are often referred to asPermo-Carboniferous in age. The cooling and drying of the climate led to theCarboniferous rainforest collapse (CRC). Tropical rain forests fragmented and then were eventually devastated byclimate change.[23]
Remains of insects are scattered throughout coal deposits, particularly of wings fromstem-dictyopterans (Blattoptera);[24] two deposits in particular are fromMazon Creek, Illinois andCommentry, France.[25] The earliest winged insects are from this time period (Pterygota), including the aforementioned Blattoptera,Caloneurodea, primitive stem-groupEphemeropterans,Orthoptera, andPalaeodictyopteroidea.[22]: 399 In 1940 (in Noble County, Oklahoma), a fossil ofMeganeuropsis americana represented the largest complete insect wing ever found.[26] Juvenile insects are also known from the Carboniferous Period.[27]
Very early Blattopterans had a large, discoid pronotum andcoriaceous 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 anovipositor, although through the Carboniferous, the ovipositor started to diminish. The ordersCaloneurodea andMiomoptera are known, withOrthoptera andBlattodea 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.[22]: 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 distinctivesynapomorphy ofsaltatorial, or adaptive for jumping, hind legs is preserved.
Palaeodictyopteroidea is a large and diverse group that includes 50% of all known Paleozoic insects.[12]:168 Containing many of the primitive features of the time: very longcerci, anovipositor, and wings with little or noanal lobe.Protodonata, as its name implies, is a primitive paraphyletic group similar toOdonata; although lacks distinct features such as anodus, apterostigma and anarculus. Most were only slightly larger than modern dragonflies, but the group does include the largest known insects, such asgriffinflies like the late CarboniferousMeganeura monyi, and the even larger later PermianMeganeuropsis permiana, with wingspans of up to 71 cm (2 ft 4 in). They were probably the top predators for some 100 million years[22]: 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 ofpterygotes,millipedes andscorpions to exist, making the newly arrivedtetrapods remain small until theCarboniferous Rainforest Collapse. However, a large griffinfly with a wingspan about 43–47 cm (1 ft 5 in – 1 ft 7 in) is known from the late Permian, when the oxygen level was much lower.[28] In addition, griffinflies probably lived in open habitats, as evidenced byMeganeurites 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.[29]
ThePermian (299 to 252 million years ago) was a relatively short time period, during which all theEarth's major land masses were collected into a single supercontinent known asPangaea. Pangaea straddled theequator 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. TheCimmeria continentrifted away fromGondwana and drifted north toLaurasia, causing thePaleo-Tethys to shrink.[22]: 400 At the end of the Permian, the biggest mass extinction in history occurred, collectively called thePermian–Triassic extinction event: 30% of all insect species became extinct; this is one of three known mass insect extinctions in Earth's history.[30]
A 2007 study based onDNA of living beetles and maps of likely beetle evolution indicated that beetles may have originated during the LowerPermian, up to299 million years ago.[31] In 2009, a fossil beetle was described from thePennsylvanian ofMazon Creek, Illinois, pushing the origin of the beetles to an earlier date,318 to 299 million years ago.[32] Fossils from this time have been found in Asia and Europe, for instance in the red slate fossil beds of Niedermoschel near Mainz, Germany.[33] Further fossils have been found in Obora, Czech Republic and Tshekarda in the Ural mountains, Russia.[34] More discoveries from North America were made in theWellington Formation of Oklahoma and were published in 2005 and 2008.[30][35] 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).[22]: 400
During this time, many of the species from the Carboniferous diversified, and many new orders developed, including:Protelytroptera, primitive relatives ofPlecoptera (Paraplecoptera),Psocoptera,Mecoptera,Coleoptera,Raphidioptera, andNeuroptera, the last four being the first definitive records of theHolometabola.[22]: 400 By thePennsylvanian and well into the Permian, by far the most successful were primitiveBlattoptera, 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, achitin 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").[36] ThedragonfliesOdonata were the dominant aerial predator and probably dominated terrestrial insect predation as well. True Odonata appeared in the Permian[37][38] and all areamphibian. Their prototypes are the oldest winged fossils,[39] go back to theDevonian, and are different from other wings in every way.[40] Their prototypes may have had the beginnings of many modern attributes even by lateCarboniferous and it is possible that they even captured small vertebrates, for some species had a wing span of 71 cm.[38]
The oldest known insect that resembles species of Coleoptera date back to theLower Permian (270 million years ago), though they instead have 13-segmentedantennae,elytra with more fully developed venation and more irregular longitudinal ribbing, and an abdomen andovipositor 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 havinggenitalia that are internal.[30] The earliest beetle-like species had pointed, leather like forewings with cells and pits.Hemiptera, or true bugs had appeared in the form ofActinoscytina andParaknightia. The later had expanded paranotal lobes, a large ovipositor, and forewings with unusual venation, possibly diverging fromBlattoptera.[41] The orders Raphidioptera and Neuroptera are grouped together asNeuropterida. 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.[22]: 186 Psocoptera was first appeared in thePermian period, they are often regarded as the most primitive of thehemipteroids.[42]
TheTriassic (252 to 201 million years ago) was a period when arid and semiarid savannas developed and when the firstmammals,dinosaurs, andpterosaurs 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 asPanthalassa. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated.[30]
The climate of the Triassic was generally hot and dry, forming typicalred bedsandstones andevaporites. There is no evidence ofglaciation at or near either pole; in fact, the polar regions were apparently moist andtemperate, a climate suitable for reptile-like creatures. Pangaea's large size limited the moderating effect of the global ocean; itscontinental climate was highly seasonal, with very hot summers and cold winters. It probably had strong,cross-equatorialmonsoons.[43]
As a consequence of theP-Tr Mass Extinction at the border of Permian andTriassic, there is only little fossil record of insects including beetles from the Lower Triassic.[44] However, there are a few exemptions, like in Eastern Europe: At the Babiy Kamen site in theKuznetsk Basin numerous beetle fossils were discovered, even entire specimen of the infraordersArchostemata (i.e., Ademosynidae, Schizocoleidae),Adephaga (i.e., Triaplidae, Trachypachidae) andPolyphaga (i.e., Hydrophilidae, Byrrhidae, Elateroidea) and in nearly a perfectly preserved condition.[45] However, species from the familiesCupedidae andSchizophoridae 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.[30]
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 thealgophagous, or algae feeding species (i.e.,Triaplidae andHydrophilidae) begin to appear, as well as predatory water beetles. The first primitive weevils appear (i.e.,Obrieniidae), as well as the first representatives of the rove beetles (i.e.,Staphylinidae), which show no marked difference in physique compared to recent species.[30] 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 theCercopidae, theCicadellidae, theCixiidae, and theMembracidae.Coleoptera included theCarabidae, theStaphylinidae, and theTrachypachidae.Hymenoptera included theXyelidae.Diptera included theAnisopodidae, theChironomidae, and theTipulidae. The firstThysanoptera appeared as well.
The first true species of Diptera are known from the MiddleTriassic, becoming widespread during the Middle and Late Triassic . A single large wing from a species of Diptera in the Triassic (10 mm instead of usual 2–6 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.[46]
TheJurassic (201 to 145 million years ago) was important in the development of birds, one of the insects' major predators. During the early Jurassic period, thesupercontinent Pangaea broke up into the northern supercontinentLaurasia and the southern supercontinentGondwana; theGulf of Mexico opened in the new rift between North America and what is now Mexico'sYucatan Peninsula. The Jurassic NorthAtlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart.[47]
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.[30]
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 includeSolnhofen in Upper Bavaria, Germany,[48] Karatau in SouthKazakhstan,[49] theYixian Formation inLiaoning, North China[50] as well as theJiulongshan Formation and further fossil sites inMongolia. 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.[30][51] 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 includeDorset, England;Issyk-Kul, Kirghizstan; and the most productive site of all,Karatau, Kazakhstan.[citation needed]
During the Jurassic there was a dramatic increase in the known diversity offamily-level Coleoptera[clarify].[30] This includes the development and growth of carnivorous and herbivorous species. Species of the superfamilyChrysomeloidea are believed to have developed around the same time, which include a wide array of plant host ranging fromcycads andconifers, toangiosperms.[52]: 186 Close to the Upper Jurassic, the portion of theCupedidae 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.
TheCretaceous (145 to 66 million years ago) had much of the same insect fauna as the Jurassic until much later on. During the Cretaceous, the late-Paleozoic-to-early-Mesozoicsupercontinent ofPangaea completed itstectonic breakup into present daycontinents, although their positions were substantially different at the time. As theAtlantic Ocean widened, the convergent-marginorogenies that had begun during theJurassic continued in theNorth American Cordillera, as theNevadan orogeny was followed by theSevier andLaramide orogenies. ThoughGondwana was still intact in the beginning of the Cretaceous, it broke up asSouth America,Antarctica andAustralia rifted away fromAfrica (thoughIndia andMadagascar remained attached to each other); thus, the South Atlantic andIndian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raisingeustatic sea levels worldwide. To the north of Africa theTethys Sea continued to narrow. Broad shallow seas advanced across centralNorth America (theWestern Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched betweencoal beds.
At the peak of the Cretaceoustransgression, one-third of Earth's present land area was submerged.[53] TheBerriasian 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.[54] Glaciation was however restricted to alpineglaciers 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.[55][56]
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).[50] Further important sites from the Lower Cretaceous include the Crato Fossil Beds in the Araripe basin in theCeará, 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. InSpain there are important sites nearMontsec andLas Hoyas. In Australia theKoonwarra fossil beds of the Korumburra group,South Gippsland, Victoria is noteworthy. Important fossil sites from the Upper Cretaceous areKzyl-Dzhar in SouthKazakhstan andArkagala in Russia.[30]
During the Cretaceous the diversity of Cupedidae andArchostemata decreased considerably. Predatoryground beetles (Carabidae) androve beetles (Staphylinidae) began to distribute into different patterns: whereas theCarabidae predominantly occurred in the warm regions, theStaphylinidae andclick beetles (Elateridae) preferred many areas with temperate climate. Likewise, predatory species ofCleroidea andCucujoidea, hunted their prey under the bark of trees together with thejewel beetles (Buprestidae). The jewel beetles diversity increased rapidly during the Cretaceous, as they were the primary consumers of wood,[57] whilelonghorn beetles (Cerambycidae) were rather rare and their diversity increased only towards the end of the Upper Cretaceous.[30] The firstcoprophagous beetles have been recorded from the Upper Cretaceous,[58] 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.[59] Also, the first species with an adaption of both larvae and adults to the aquatic lifestyle are found.Whirligig beetles (Gyrinidae) were moderately diverse, although other early beetles (i.e.,Dytiscidae) were less, with the most widespread being the species ofCoptoclavidae, which preyed on aquatic fly larvae.[30]
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.[30] 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).[22]: 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.[22]: 402
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.[60] In the Willershausen clay pit so far 35 genera from 18 beetle families have been recorded, of which six genera are extinct.[61] 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.[62][63]
A report in November 2014 unambiguously places the insects in one clade, with theremipedes as the nearest sister clade.[64] This study resolved insect phylogeny of all extant insect orders, and provides "a robust phylogenetic backbone tree and reliable time estimates of insect evolution."[64] 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.[65]
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| Aphylogenetic tree of the arthropods and related groups[66] |
In 2008, researchers atTufts 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 theCarboniferous Period.[67]DevonianRhyniognatha hirsti, from the 396 million year oldRhynie 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, ifRhyniognatha is actual flying insect, the first insects probably appeared earlier, in theSilurian period.[18][68] However, this species is also considered asmyriapod in later study.[3] There have been four super radiations of insects:beetles (evolved around300 million years ago),flies (evolved around250 million years ago),moths andwasps (evolved around150 million years ago).[12] These four groups account for the majority of described species. The flies and moths along with thefleas evolved from theMecoptera. The origins ofinsect 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.[12]
Insects are prey for a variety of organisms, including terrestrial vertebrates. The earliest vertebrates on land existed350 million years ago and were large amphibiouspiscivores. Through gradual evolutionary change,insectivory was the next diet type to evolve.[23] Insects were among the earliest terrestrialherbivores and acted as major selection agents on plants.[5] Plants evolved chemicaldefenses against this herbivory and the insects in turn evolved mechanisms to deal with plant toxins.[5] 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. Bothpleiotropy andepistasis 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.[69]
Many insects also make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity using warning colors.[5] This successful evolutionary pattern has also been utilized bymimics. Over time, this has led to complex groups of coevolved species. Conversely, some interactions between plants and insects, likepollination, are beneficial to both organisms. Coevolution has led to the development of very specificmutualisms in such systems.
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| Cladogram of living insect groups,[70] with numbers of species in each group.[71] Note thatApterygota,Palaeoptera andExopterygota are possiblyparaphyletic groups. | ||||||||||||||||||||||||||||||||||||||||||||
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| Phylogenetic relationship of some common insect orders:Zygentoma,Odonata,Orthoptera,Phasmatodea,Blattodea,Isoptera,Hemiptera,Hymenoptera,Coleoptera,Lepidoptera,Diptera. No information should be inferred from branch length. |
Traditional morphology-based or appearance-basedsystematics has usually givenHexapoda the rank ofsuperclass,[72] and identified four groups within it: insects (Ectognatha), springtails (Collembola),Protura andDiplura, the latter three being grouped together asEntognatha 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 ispolyphyletic (where the last common ancestor was not a member of the group), with the entognath classes having separate evolutionary histories from Insecta.[73] Many of the traditional appearance-basedtaxa have been shown to be paraphyletic, so rather than using ranks likesubclass,superorder andinfraorder, it has proved better to usemonophyletic 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 asApterygota, and winged insects, known asPterygota. The Apterygota consist of the primitively wingless order of the silverfish (Thysanura). Archaeognatha make up the Monocondylia based on the shape of theirmandibles, while Thysanura and Pterygota are grouped together as Dicondylia. It is possible that the Thysanura themselves are notmonophyletic, with the familyLepidotrichidae being asister group to theDicondylia (Pterygota and the remaining Thysanura).[74][75]
Paleoptera and Neoptera are the winged orders of insects differentiated by the presence of hardened body parts calledsclerites; also, in Neoptera, muscles that allow their wings to fold flatly over the abdomen. Neoptera can further be divided into incomplete metamorphosis-based (Polyneoptera andParaneoptera) 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 ordersMallophaga andAnoplura are derived from withinPsocoptera has led to the new taxonPsocodea.[76]Phasmatodea andEmbiidina have been suggested to form Eukinolabia.[77] Mantodea, Blattodea and Isoptera are thought to form a monophyletic group termedDictyoptera.[78]
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.[79] The Neuropterida are often lumped or split on the whims of the taxonomist. Fleas are now thought to be closely related toboreid mecopterans.[80] 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 calledsystematic 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 examplesystematic dipterology.
According to phylogenic estimation, first insects possibly appeared in theSilurian period and got wings in Devonian.[17][81]
The subclassApterygota (wingless insects) is now considered artificial as thesilverfish (orderThysanura) are more closely related toPterygota (winged insects) than to bristletails (orderArchaeognatha). 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 mayflynymphs resemble aquatic thysanurans.
Modern Archaeognatha and Thysanura still have rudimentary appendages on theirabdomen called styli, while more primitive and extinct insects known asMonura had much more developed abdominal appendages. The abdominal andthoracic 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-developedcompound eyes and longantennae. 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 coastalniches could also have a secondary origin, just as could their jumpinglocomotion, as it is the crawling Thysanura who are considered to be most original (plesiomorphic). By looking at how primitivecheliceratanbook gills (still seen inhorseshoe crabs) evolved intobook lungs in primitivespiders and finally intotracheae 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 ofwings.[82]
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TheOdonata (dragonflies) are also a good candidate as the oldest living member of thePterygota.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 theNeoptera, 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.
Dragonflynymphs have a unique labial "mask" used for catching prey, and theimago 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 inscorpions. 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 andThysanura, 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 laidspermatophores 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 adecomposer.
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 arthropodtrachea can only arise in anatmosphere 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 andPlecoptera), 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.
The origin ofinsect flight remains obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects (e.g. thePalaeodictyoptera) had an additional pair of winglets attached to the first segment of thethorax, for a total of three pairs.
Thewings themselves are sometimes said to be highly modified (tracheal) gills.[83] 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.[84][85] Specifically, genetic research on mayflies has revealed that the gills and insect wings both may have originated from insect legs.[86] 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.
When the first forests arose on Earth, newniches 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 theirexoskeleton and used them for gliding, one pair on each segment. Further evolution in this direction would give bigger gliding structures on theirthorax and gradually smaller ones on theirabdomen. Their bodies would have become stiffer whilethysanurans, which never evolved flight, kept their flexible abdomen.
Mayflynymphs 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 andarboreal insects seem like a good explanation for this evolutionary process for several reasons. Because early winged insects were lacking the sophisticatedwing folding mechanism ofneopterous 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 treecanopies 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-dayvertebrates such as somerodents andmarsupials, which have grown wide, flat expansions of skin for this purpose. The flying dragons (genusDraco) ofIndonesia has modified its ribs into gliders, and even somesnakes can glide through the air by spreading their ribs. The main difference is that while vertebrates have an innerskeleton, primitive insects had a flexible and adaptive exoskeleton.
Some animals would be living in the trees, as animals are always taking advantage of all availableniches, 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 thegliding ants 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/preyco-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 theneopterous 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.[87] This theory is based on the fact that what may be the first fossil insects, the DevonianRhyniognatha 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.
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Another primitive trait of the mayflies are thesubimago; no other insects have this winged yet sexually immature stage.[88] 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 enoughselection 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 howApterygota 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 withincomplete 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 andcomplete metamorphosis (Endopterygota) have gone even further. An interesting theory is that thepupal 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.
The distant ancestor of flying insects, a species with primitive proto-wings, had a more or lessametabolous life-cycle andinstars of basically the same type asthysanurans with no defined nymphal, subimago or adult stages as the individual became older. Individuals developed gradually as they grew and moulted, 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 earlierametabolous 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 arehemimetabolous orholometabolous), 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 whereimago 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] made a more complex adult body structure. These strategies obviously became very successful with time.
