Insects have a three-part body: head with largecompound eyes andantennae, athorax with three pairs of legs, and a segmented abdomen. Many groups also have two pairs of wings.
The insectnervous system consists of abrain and aventral nerve cord. Most insects reproduceby laying eggs. Insectsbreathe air through a system ofpaired openings along their sides, connected tosmall tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an openhemocoel. Insect vision is mainly through theircompound eyes, with additional smallocelli. Many insects can hear, usingtympanal organs, which may be on the legs or other parts of the body. Theirsense of smell is via receptors, usually on the antennae and the mouthparts.
Nearly all insects hatch fromeggs. Insect growth is constrained by the inelastic exoskeleton, so development involves a series ofmolts. The immature stages often differ from the adults in structure, habit and habitat. Groups that undergofour-stage metamorphosis often have a nearly immobilepupa. Insects that undergothree-stage metamorphosis lack a pupa, developing through a series of increasingly adult-likenymphal stages. The higher level relationship of theinsects is unclear. Fossilized insects of enormous size have been found from thePaleozoic Era, includinggiant dragonfly-like insects with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to havecoevolved withflowering plants.
Adult insects typically move about by walking and flying; some can swim. Insects are the only invertebrates that can achieve sustained powered flight;insect flight evolved just once. Many insects are at least partlyaquatic, and havelarvae with gills; in some species, the adults too are aquatic. Some species, such aswater striders, can walk on the surface of water. Insects are mostly solitary, but some, such asbees,ants andtermites, aresocial and live in large, well-organizedcolonies. Others, such asearwigs, provide maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Malemoths can sense thepheromones of female moths over great distances. Other species communicate with sounds:cricketsstridulate, or rub their wings together, to attract a mate and repel other males.Lampyridbeetles communicate with light.
Humans regard many insects aspests, especially those that damage crops, and attempt to control them usinginsecticides and other techniques. Others areparasitic, and may act asvectors ofdiseases. Insectpollinators are essential to the reproduction of many flowering plants and so to their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefit. Two species in particular are economically important and were domesticated many centuries ago:silkworms forsilk andhoney bees forhoney. Insects are consumed as food in 80% of the world's nations, by people in roughly 3,000 ethnic groups. Human activities are having serious effects oninsect biodiversity.
In common speech, insects and other terrestrialarthropods are often calledbugs.[a] Entomologists to some extent reserve the name "bugs" for a narrow category of "true bugs", insects of the orderHemiptera, such ascicadas andshield bugs.[6] Other terrestrial arthropods, such ascentipedes,millipedes,woodlice,spiders,mites andscorpions, are sometimes confused with insects, since they have a jointed exoskeleton.[7] Adult insects are the only arthropods that ever have wings, with up to two pairs on the thorax. Whether winged or not, adult insects can be distinguished by their three-part body plan, with head, thorax, and abdomen; they have three pairs of legs on the thorax.[8]
Insects and other bugs that could be confused with them
Insect: Six legs, three-part body (head, thorax, abdomen), up to two pairs of wings
About half of alleukaryotes are insects (left side of diagram).
Estimates of the total number of insect species vary considerably, suggesting that there are perhaps some 5.5 million insect species in existence, of which about one million have been described and named.[9] These constitute around half of alleukaryote species, includinganimals,plants, andfungi.[10] The most diverse insectorders are the Hemiptera (true bugs), Lepidoptera (butterflies and moths), Diptera (true flies), Hymenoptera (wasps, ants, and bees), and Coleoptera (beetles), each with more than 100,000 described species.[9]
Insects are extremely diverse. Five groups each have over 100,000 described species.
Insects are distributed over every continent and almost every terrestrial habitat. There are many more species in thetropics, especially inrainforests, than in temperate zones.[11] The world's regions have received widely differing amounts of attention from entomologists. The British Isles have been thoroughly surveyed, so that Gullan and Cranston 2014 state that the total of around 22,500 species is probably within 5% of the actual number there; they comment that Canada's list of 30,000 described species is surely over half of the actual total. They add that the 3,000 species of the American Arctic must be broadly accurate. In contrast, a large majority of the insect species of the tropics and thesouthern hemisphere are probably undescribed.[11] Some 30–40,000 speciesinhabit freshwater; very few insects, perhaps a hundred species, are marine.[12] Insects such assnow scorpionflies flourish in cold habitats including theArctic and at high altitude.[13] Insects such asdesert locusts, ants, beetles, and termites are adapted to some of the hottest and driest environments on earth, such as theSonoran Desert.[14]
Phylogeny and evolution
External phylogeny
Insects form aclade, a natural group with a common ancestor, among thearthropods.[15] Aphylogenetic analysis by Kjer et al. (2016) places the insects among theHexapoda, six-legged animals with segmented bodies; their closest relatives are theDiplura (bristletails).[16]
The internal phylogeny is based on the works of Wipfler et al. 2019 for thePolyneoptera,[17] Johnson et al. 2018 for theParaneoptera,[18] and Kjer et al. 2016 for theHolometabola.[19] The numbers of describedextant species (boldface for groups with over 100,000 species) are from Stork 2018.[9]
Aristotle was the first to describe the insects as a distinct group. He placed them as the second-lowest level of animals on hisscala naturae, above thespontaneously generating sponges and worms, but below the hard-shelled marine snails. His classification remained in use for many centuries.[21]
In 1758, in hisSystema Naturae,[22]Carl Linnaeus divided the animal kingdom into six classes includingInsecta. He created seven orders of insect according to the structure of their wings. These were the wingless Aptera, the two-winged Diptera, and five four-winged orders: the Coleoptera with fully-hardened forewings; the Hemiptera with partly-hardened forewings; the Lepidoptera with scaly wings; the Neuroptera with membranous wings but nosting; and the Hymenoptera, with membranous wings and a sting.[20]
Jean-Baptiste de Lamarck, in his 1809Philosophie Zoologique, treated the insects as one of nine invertebratephyla.[23] In his 1817Le Règne Animal,Georges Cuvier grouped all animals into fourembranchements ("branches" with different body plans), one of which was the articulated animals, containing arthropods and annelids.[24] This arrangement was followed by the embryologistKarl Ernst von Baer in 1828, the zoologistLouis Agassiz in 1857, and the comparative anatomistRichard Owen in 1860.[25] In 1874,Ernst Haeckel divided the animal kingdom into two subkingdoms, one of which was Metazoa for the multicellular animals. It had five phyla, including the articulates.[26][25]
Traditional morphology-basedsystematics have usually given theHexapoda the rank ofsuperclass,[27] and identified four groups within it: insects (Ectognatha),Collembola,Protura, andDiplura, the latter three being grouped together as theEntognatha on the basis of internalized mouth parts.[28]
The use of phylogenetic data has brought about numerous changes in relationships above the level oforders.[28] Insects can be divided into two groups historically treated as subclasses: wingless insects orApterygota, and winged insects orPterygota. The Apterygota traditionally consisted of the primitively wingless ordersArchaeognatha (jumping bristletails) andZygentoma (silverfish). However, Apterygota is notmonophyletic, as Archaeognatha are sister to all other insects, based on the arrangement of theirmandibles, while the Pterygota, the winged insects, emerged from within theDicondylia, alongside the Zygentoma.[29]
The Pterygota (Palaeoptera andNeoptera) are winged and havehardened plates on the outside of their body segments; the Neoptera have muscles that allow their wings to fold flat over the abdomen. Neoptera can be divided into groups with incomplete metamorphosis (Polyneoptera andParaneoptera) and those with complete metamorphosis (Holometabola). The molecular finding that the traditionallouse ordersMallophaga andAnoplura are withinPsocoptera has led to the new taxonPsocodea.[30]Phasmatodea andEmbiidina have been suggested to form the Eukinolabia.[31] Mantodea, Blattodea, and Isoptera form a monophyletic group,Dictyoptera.[32] Fleas are now thought to be closely related to boreid mecopterans.[33]
The oldest fossil that may be a primitive wingless insect isLeverhulmia from the EarlyDevonianWindyfield chert.[34] The oldest known flying insects are from the mid-Carboniferous, around 328–324 million years ago. The group subsequently underwent a rapidexplosive diversification. Claims that they originated substantially earlier, during theSilurian orDevonian (some 400 million years ago) based onmolecular clock estimates, are unlikely to be correct, given the fossil record.[35]
Fourlarge-scale radiations of insects have occurred:beetles (from about 300 million years ago),flies (from about 250 million years ago),moths andwasps (both from about 150 million years ago).[36]
The remarkably successfulHymenoptera (wasps, bees, and ants) appeared some 200 million years ago in theTriassic period, but achieved their wide diversity more recently in theCenozoic era, which began 66 million years ago. Some highly successful insect groups evolved in conjunction withflowering plants, a powerful illustration ofcoevolution. Insects were among the earliest terrestrialherbivores and acted as major selection agents on plants.[37] Plants evolved chemicaldefenses against this herbivory and the insects, in turn, evolved mechanisms to deal with plant toxins. Many insects make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity usingwarning colors.[38]
The giant dragonfly-like insectMeganeura monyi grew to wingspans of 75 cm (2 ft 6 in) in the lateCarboniferous, around 300 million years ago.[39]
Insects have asegmented body supported by anexoskeleton, the hard outer covering made mostly ofchitin. The body is organized into threeinterconnected units: thehead,thorax andabdomen. The head supports a pair of sensoryantennae, a pair ofcompound eyes, zero to three simple eyes (orocelli) and three sets of variously modified appendages that form themouthparts. The thorax carries the three pairs of legs and up to two pairs ofwings. The abdomen contains most of the digestive, respiratory, excretory and reproductive structures.[8]
The head is enclosed in a hard, heavilysclerotized, unsegmentedhead capsule, which contains most of the sensing organs, including the antennae, compound eyes, ocelli, and mouthparts.[40] The thorax is composed of three sections named (from front to back) theprothorax,mesothorax andmetathorax. The prothorax carries the first pair of legs. The mesothorax carries the second pair of legs and the front wings. The metathorax carries the third pair of legs and the hind wings.[8][40] The abdomen is the largest part of the insect, typically with 11–12 segments, and is less strongly sclerotized than the head or thorax. Each segment of the abdomen has sclerotized upper and lower plates (the tergum and sternum), connected to adjacent sclerotized parts by membranes. Each segment carries a pair ofspiracles.[40]
The outer skeleton, thecuticle, is made up of two layers: the epicuticle, a thin and waxy water-resistant outer layer withoutchitin, and a lower layer, the thick chitinous procuticle. The procuticle has two layers: an outer exocuticle and an inner endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each other in a sandwich pattern, while the exocuticle is rigid and sclerotized.[41][42] As an adaptation to life on land, insects have anenzyme that uses atmospheric oxygen to harden their cuticle, unlike crustaceans which use heavy calcium compounds for the same purpose. This makes the insect exoskeleton a lightweight material.[43]
Thenervous system of an insect consists of abrain and aventral nerve cord. The head capsule is made up of six fused segments, each with either a pair ofganglia, or a cluster of nerve cells outside of the brain. The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect'sesophagus, called thesubesophageal ganglion.[44] Thethoracic segments have one ganglion on each side, connected into a pair per segment. This arrangement is also seen in the first eight segments of the abdomen. Many insects have fewer ganglia than this.[45] Insects are capable of learning.[46]
Digestive
An insect uses its digestive system to extract nutrients and other substances from the food it consumes.[47] There is extensive variation among differentorders,life stages, and evencastes in the digestive system of insects.[48] Thegut runs lengthwise through the body. It has three sections, with pairedsalivary glands and salivary reservoirs.[49] By moving its mouthparts the insect mixes its food with saliva.[50][51] Some insects, likeflies, expeldigestive enzymes onto their food to break it down, but most insects digest their food in the gut.[52] Theforegut is lined with cuticule as protection from tough food. It includes themouth, pharynx, andcrop which stores food.[53] Digestion starts in the mouth with enzymes in the saliva. Strong muscles in the pharynx pump fluid into the mouth, lubricating the food, and enabling certain insects to feed on blood or from thexylem andphloem transport vessels of plants.[54] Once food leaves the crop, it passes to themidgut, where the majority of digestion takes place. Microscopic projections,microvilli, increase the surface area of the wall to absorb nutrients.[55] In thehindgut, undigested food particles are joined byuric acid to form fecal pellets; most of the water is absorbed, leaving a dry pellet to be eliminated. Insects may have one to hundreds ofMalpighian tubules. These remove nitrogenous wastes from the hemolymph of the insect and regulate osmotic balance. Wastes and solutes are emptied directly into the alimentary canal, at the junction between the midgut and hindgut.[56]
Thereproductive system of female insects consist of a pair ofovaries, accessory glands, one or morespermathecae to store sperm, and ducts connecting these parts. The ovaries are made up of a variable number of egg tubes,ovarioles. Female insects make eggs, receive and store sperm, manipulate sperm from different males, and lay eggs. Accessory glands produce substances to maintain sperm and to protect the eggs. They can produce glue and protective substances for coating eggs, or tough coverings for a batch of eggs calledoothecae.[57]
For males, the reproductive system consists of one or twotestes, suspended in the body cavity bytracheae. The testes contain sperm tubes or follicles in a membranous sac. These connect to a duct that leads to the outside. The terminal portion of the duct may be sclerotized to form theintromittent organ, theaedeagus.[58]
Insect respiration is accomplished withoutlungs. Instead, insects have a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need it via theirtracheae and tracheoles. In most insects, air is taken in through pairedspiracles, openings on the sides of the abdomen and thorax. The respiratory system limits the size of insects. As insects get larger,gas exchange via spiracles becomes less efficient, and thus the heaviest insect currently weighs less than 100 g. However, with increased atmospheric oxygen levels, as were present in the latePaleozoic, larger insects were possible, such as dragonflies with wingspans of more than two feet (60 cm).[59] Gas exchange patterns in insects range from continuous anddiffusive ventilation, todiscontinuous.[60][61][62][63]
Because oxygen is delivered directly to tissues via tracheoles, the circulatory system is not used to carry oxygen, and is therefore greatly reduced. The insect circulatory system is open; it has noveins orarteries, and instead consists of little more than a single, perforated dorsal tube that pulsesperistaltically. This dorsal blood vessel is divided into two sections: the heart and aorta. The dorsal blood vessel circulates thehemolymph, arthropods' fluid analog ofblood, from the rear of the body cavity forward.[64][65] Hemolymph is composed of plasma in whichhemocytes are suspended. Nutrients, hormones, wastes, and other substances are transported throughout the insect body in the hemolymph. Hemocytes include many types of cells that are important for immune responses, wound healing, and other functions. Hemolymph pressure may be increased by muscle contractions or by swallowing air into the digestive system to aid in molting.[66]
Most insects have a pair of largecompound eyes and other sensory organs such as antennae able to detect movements and chemical stimuli on their heads.
Many insects possess numerous specializedsensory organs able to detect stimuli including limb position (proprioception) bycampaniform sensilla, light,water, chemicals (senses oftaste andsmell), sound, and heat.[67] Some insects such asbees can perceiveultraviolet wavelengths, or detectpolarized light, while theantennae of male moths can detect thepheromones of female moths over distances of over a kilometer.[68] There is a trade-off between visual acuity and chemical or tactile acuity, such that most insects with well-developed eyes have reduced or simple antennae, and vice versa. Insects perceive sound by different mechanisms, such as thin vibrating membranes (tympana).[69] Insects were the earliest organisms to produce and sense sounds. Hearing has evolved independently at least 19 times in different insect groups.[70]
Most insects, except somecave crickets, are able to perceive light and dark. Many have acute vision capable of detecting small and rapid movements. The eyes may include simple eyes orocelli as well as largercompound eyes. Many species can detect light in theinfrared,ultraviolet andvisible light wavelengths, with color vision. Phylogenetic analysis suggests that UV-green-bluetrichromacy existed from at least theDevonian period, some 400 million years ago.[71]
The individual lenses in compound eyes are immobile, but fruit flies have photoreceptor cells underneath each lens which move rapidly in and out of focus, in a series of movements called photoreceptor microsaccades. This gives them, and possibly many other insects, a much clearer image of the world than previously assumed.[72]
An insect'ssense of smell is viachemical receptors, usually on the antennae and the mouthparts. These detect both airbornevolatile compounds and odorants on surfaces, including pheromones from other insects and compounds released by food plants. Insects use olfaction to locate mating partners, food, and places to lay eggs, and to avoid predators. It is thus an extremely important sense, enabling insects to discriminate between thousands of volatile compounds.[73]
Some insects are capable ofmagnetoreception; ants and bees navigate using it both locally (near their nests) and when migrating.[74] TheBrazilian stingless bee detects magnetic fields using the hair-likesensilla on its antennae.[75][76]
The majority of insects hatch fromeggs. The fertilization and development takes place inside the egg, enclosed by a shell (chorion) that consists of maternal tissue. In contrast to eggs of other arthropods, most insect eggs are drought resistant. This is because inside the chorion two additional membranes develop from embryonic tissue, theamnion and theserosa. This serosa secretes acuticle rich inchitin that protects the embryo against desiccation.[77] Some species of insects, like aphids and tsetse flies, areovoviviparous: their eggs develop entirely inside the female, and then hatch immediately upon being laid.[78] Some other species, such as in the cockroach genusDiploptera, areviviparous,gestating inside the mother andborn alive.[79] Some insects, likeparasitoid wasps, arepolyembryonic, meaning that a single fertilized egg divides into many separate embryos.[80] Insects may beunivoltine, bivoltine or multivoltine, having one, two or many broods in a year.[81]
Some insects areparthenogenetic, meaning that the female can reproduce and give birth without having the eggsfertilized by amale. Many aphids undergo a cyclical form of parthenogenesis in which they alternate between one or many generations of asexual and sexual reproduction.[85][86] In summer, aphids are generally female and parthenogenetic; in the autumn, males may be produced for sexual reproduction. Other insects produced by parthenogenesis are bees, wasps and ants; in theirhaplodiploid system,diploid females spawn many females and a fewhaploid males.[78]
Metamorphosis
Metamorphosis in insects is the process of development that converts young to adults. There are two forms of metamorphosis: incomplete and complete.
Hemimetabolous insects, those with incomplete metamorphosis, change gradually after hatching from theegg by undergoing a series ofmolts through stages calledinstars, until the final,adult, stage is reached. An insect molts when it outgrows its exoskeleton, which does not stretch and would otherwise restrict the insect's growth. The molting process begins as the insect'sepidermis secretes a newepicuticle inside the old one. After this new epicuticle is secreted, the epidermis releases a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. When this stage is complete, the insect makes its body swell by taking in a large quantity of water or air; this makes the old cuticle split along predefined weaknesses where it was thinnest.[87][88]
Holometabolism, or complete metamorphosis, is where the insect changes in four stages, an egg orembryo, alarva, apupa and the adult orimago. In these species, an egg hatches to produce a larva, which is generally worm-like in form. This can be eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform (elongated, flattened and active), elateriform (wireworm-like) or vermiform (maggot-like). The larva grows and eventually becomes a pupa, a stage marked by reduced movement. There are threetypes of pupae: obtect, exarate or coarctate. Obtect pupae are compact, with the legs and other appendages enclosed. Exarate pupae have their legs and other appendages free and extended. Coarctate pupae develop inside the larval skin.[89] Insects undergo considerable change in form during the pupal stage, and emerge as adults. Butterflies are well-known for undergoing complete metamorphosis; most insects use this life cycle. Some insects have evolved this system tohypermetamorphosis. Complete metamorphosis is a trait of the most diverse insect group, theEndopterygota.[82]
Communication
Insects that produce sound can generally hear it. Mostinsects can hear only anarrow range offrequencies related to the frequency of the sounds they can produce. Mosquitoes can hear up to 2kilohertz.[90] Certain predatory and parasitic insects can detect the characteristic sounds made by their prey or hosts, respectively. Likewise, some nocturnal moths can perceive theultrasonic emissions ofbats, which helps themavoid predation.[91]
Light production
A few insects, such asMycetophilidae (Diptera) and the beetle familiesLampyridae,Phengodidae,Elateridae andStaphylinidae arebioluminescent. The most familiar group are thefireflies, beetles of the family Lampyridae. Some species are able to control this light generation to produce flashes. The function varies with some species using them to attract mates, while others use them to lure prey. Cave dwelling larvae ofArachnocampa (Mycetophilidae, fungus gnats) glow to lure small flying insects into sticky strands of silk.[92] Some fireflies of the genusPhoturismimic the flashing of femalePhotinus species to attract males of that species, which are then captured and devoured.[93] The colors of emitted light vary from dull blue (Orfelia fultoni, Mycetophilidae) to the familiar greens and the rare reds (Phrixothrix tiemanni, Phengodidae).[94]
Sound production
Insects make sounds mostly by mechanical action of appendages. Ingrasshoppers and crickets, this is achieved bystridulation.Cicadas make the loudest sounds among the insects by producing and amplifying sounds with special modifications to their body to formtymbals and associated musculature. The AfricancicadaBrevisana brevis has been measured at 106.7 decibels at a distance of 50 cm (20 in).[95] Some insects, such as theHelicoverpa zea moths,hawk moths andHedylid butterflies, can hearultrasound andtake evasive action when they sense that they have been detected by bats.[96][97] Some moths produce ultrasonic clicks that warn predatory bats of their unpalatability (acousticaposematism),[98] while some palatable moths have evolved to mimic these calls (acousticBatesian mimicry).[99] The claim that some moths canjam bat sonar has been revisited. Ultrasonic recording and high-speed infrared videography of bat-moth interactions suggest the palatable tiger moth really does defend against attacking big brown bats using ultrasonic clicks that jam bat sonar.[100]
Very low sounds are produced in various species ofColeoptera,Hymenoptera,Lepidoptera,Mantodea andNeuroptera. These low sounds are produced by the insect's movement, amplified by stridulatory structures on the insect's muscles and joints; these sounds can be used to warn or communicate with other insects. Most sound-making insects also havetympanal organs that can perceive airborne sounds. Somehemipterans, such as thewater boatmen, communicate via underwater sounds.[101]
Some species use vibrations for communicating, such as to attract mates as in the songs of theshield bugNezara viridula.[104] Vibrations can also be used to communicate between species;lycaenid caterpillars, whichform a mutualistic association with ants communicate with ants in this way.[105] TheMadagascar hissing cockroach has the ability to press air through its spiracles to make a hissing noise as a sign of aggression;[106] thedeath's-head hawkmoth makes a squeaking noise by forcing air out of their pharynx when agitated, which may also reduce aggressive worker honey bee behavior when the two are close.[107]
Many insects have evolvedchemical means for communication. Thesesemiochemicals are often derived from plant metabolites including those meant to attract, repel and provide other kinds of information.Pheromones are used for attracting mates of the opposite sex, for aggregatingconspecific individuals of both sexes, for deterring other individuals from approaching, to mark a trail, and to trigger aggression in nearby individuals.Allomones benefit their producer by the effect they have upon the receiver.Kairomones benefit their receiver instead of their producer. Synomones benefit the producer and the receiver. While some chemicals are targeted at individuals of the same species, others are used for communication across species. The use of scents is especially well-developed in social insects.[108]Cuticular hydrocarbons are nonstructural materials produced and secreted to the cuticle surface to fightdesiccation andpathogens. They are important, too, as pheromones, especially in social insects.[109]
Honey bee's figure-eightwaggle dance. An orientation 45° to the right of ‘up' on the comb indicates food 45° to the right of the sun. The dancer's rapid waggling blurs her abdomen.
Social insects, such astermites,ants and manybees andwasps, areeusocial.[110] They live together in such large well-organized colonies of genetically similar individuals that they are sometimes consideredsuperorganisms. In particular, reproduction is largely limited to aqueen caste; other females areworkers, prevented from reproducing byworker policing.Honey bees have evolved a system of abstract symbolic communication where a behavior is used to represent and convey specific information about the environment. In this communication system, calleddance language, the angle at which a bee dances represents a direction relative to the sun, and the length of the dance represents the distance to be flown.[111]Bumblebees too have some social communication behaviors.Bombus terrestris, for example, more rapidly learns about visiting unfamiliar, yet rewarding flowers, when they can see a conspecific foraging on the same species.[112]
Only insects that live in nests or colonies possess fine-scale spatial orientation. Some cannavigate unerringly to a single hole a few millimeters in diameter among thousands of similar holes, after a trip of several kilometers. Inphilopatry, insects thathibernate are able to recall a specific location up to a year after last viewing the area of interest.[113] A few insects seasonallymigrate large distances between different geographic regions, as in the continent-widemonarch butterfly migration.[114]
Care of young
Eusocial insects build nests, guard eggs, and provide food for offspring full-time. Most insects, however, lead short lives as adults, and rarely interact with one another except to mate or compete for mates. A small number provideparental care, where they at least guard their eggs, and sometimes guard their offspring until adulthood, possibly even feeding them. Many wasps and bees construct a nest or burrow,store provisions in it, and lay an egg upon those provisions, providing no further care.[115]
Insects are the only group ofinvertebrates to have developed flight. The ancient groups of insects in the Palaeoptera, the dragonflies, damselflies and mayflies, operate their wings directly by paired muscles attached to points on each wing base that raise and lower them. This can only be done at a relatively slow rate. All other insects, the Neoptera, haveindirect flight, in which the flight muscles cause rapid oscillation of the thorax: there can be more wingbeats than nerve impulses commanding the muscles. One pair of flight muscles is aligned vertically, contracting to pull the top of the thorax down, and the wings up. The other pair runs longitudinally, contracting to force the top of the thorax up and the wings down.[116][117] Most insects gainaerodynamic lift by creating a spirallingvortex at theleading edge of the wings.[118] Small insects like thrips with tiny feathery wings gain lift using theclap and fling mechanism; the wings are clapped together and pulled apart, flinging vortices into the air at the leading edges and at the wingtips.[119][120]
The evolution ofinsect wings has beena subject of debate; it has been suggested they came from modified gills, flaps on the spiracles, or an appendage, the epicoxa, at the base of the legs.[121] More recently, entomologists have favored evolution of wings from lobes of thenotum, of thepleuron, or more likely both.[122] In theCarboniferous age, the dragonfly-likeMeganeura had as much as a 50 cm (20 in) wide wingspan. The appearance of gigantic insects is consistent with high atmospheric oxygen at that time, as the respiratory system of insects constrains their size.[123] The largest flying insects today are much smaller, with the largest wingspan belonging to the white witch moth (Thysania agrippina), at approximately 28 cm (11 in).[124]
Spatial and temporal stepping pattern of walking desert ants performing an alternating tripod gait. Recording rate: 500 fps, Playback rate: 10 fps.
Many adult insects use six legs for walking, with an alternatingtripod gait. This allows for rapid walking with a stable stance; it has been studied extensively incockroaches andants. For the first step, the middle right leg and the front and rear left legs are in contact with the ground and move the insect forward, while the front and rear right leg and the middle left leg are lifted and moved forward to a new position. When they touch the ground to form a new stable triangle, the other legs can be lifted and brought forward in turn.[127] The purest form of the tripedal gait is seen in insects moving at high speeds. However, this type of locomotion is not rigid and insects can adapt a variety of gaits. For example, when moving slowly, turning, avoiding obstacles, climbing or slippery surfaces, four (tetrapodal) or more feet (wave-gait) may be touching the ground.[128] Cockroaches are among the fastest insect runners and, at full speed, adopt a bipedal run. More sedate locomotion is seen in the well-camouflaged stick insects (Phasmatodea). A small number of species such asWater striders can move on the surface of water; their claws are recessed in a special groove, preventing the claws from piercing the water's surface film.[62] The ocean-skaters in the genusHalobates even live on the surface of open oceans, a habitat that has few insect species.[129]
A large number of insects live either part or the whole of their lives underwater. In many of the more primitive orders of insect, the immature stages are aquatic. In some groups, such aswater beetles, the adults too are aquatic.[62]
Many of these species are adapted for under-water locomotion. Water beetles and water bugs have legs adapted into paddle-like structures. Dragonflynaiads use jet propulsion, forcibly expelling water out of their rectal chamber.[130] Other insects such as therove beetleStenus emitpygidial glandsurfactant secretions that reduce surface tension; this enables them to move on the surface of water byMarangoni propulsion.[131][132]
Insects play many critical roles inecosystems, including soil turning and aeration, dung burial, pest control, pollination and wildlife nutrition.[133] For instance, termites modify the environment around their nests, encouraging grass growth;[134] manybeetles arescavengers; dung beetlesrecycle biological materials into forms useful to otherorganisms.[135][136] Insects are responsible for much of the process by whichtopsoil is created.[137]
Insects are mostly small, soft bodied, and fragile compared to larger lifeforms. The immature stages are small, move slowly or are immobile, and so all stages are exposed topredation andparasitism. Insects accordingly employ multipledefensive strategies, includingcamouflage,mimicry, toxicity and active defense.[138] Manyinsects rely on camouflage to avoid being noticed by their predators or prey.[139] It is common amongleaf beetles andweevils that feed on wood or vegetation.[138]Stick insects mimic the forms of sticks and leaves.[140]Many insects usemimicry to deceive predators into avoiding them. InBatesian mimicry, edible species, such as ofhoverflies (the mimics), gain a survival advantage by resembling inedible species (the models).[138][141] InMüllerian mimicry, inedible species, such as of wasps and bees, resemble each other so as to reduce the sampling rate by predators who need to learn that those insects are inedible.Heliconius butterflies, many of which are toxic, form Müllerian complexes, advertising their inedibility.[142]Chemical defense is common among Coleoptera and Lepidoptera, usually being advertised by bright warning colors (aposematism), as in themonarch butterfly. As larvae, they obtain theirtoxicity by sequestering chemicals from the plants they eat into their own tissues. Some manufacture their own toxins. Predators that eat poisonous butterflies and moths may vomit violently, learning not to eat insects with similar markings; this is the basis of Müllerian mimicry.[143] Someground beetles of the family Carabidae actively defend themselves, spraying chemicals from their abdomen with great accuracy, to repel predators.[138]
Pollination is the process by whichpollen is transferred in the reproduction of plants, thereby enablingfertilisation andsexual reproduction.[144] Most flowering plants require an animal to do the transportation. The majority ofpollination is by insects.[145] Because insects usually receive benefit for the pollination in the form of energy rich nectar it is amutualism. The various flower traits, such as bright colors andpheromones thatcoevolved with their pollinators, have been calledpollination syndromes, though around one third of flowers cannot be assigned to a single syndrome.[146]
Many insects areparasitic. The largest group, with over 100,000 species[147] and perhaps over a million,[148] consists of a singleclade ofparasitoid wasps among the Hymenoptera.[149] These are parasites of other insects, eventually killing their hosts.[147] Some are hyper-parasites, as their hosts are other parasitoid wasps.[147][150] Several groups of insects can be considered as eithermicropredators orexternal parasites;[151][152] for example, manyhemipteran bugs have piercing and sucking mouthparts, adapted for feeding on plant sap,[153][154] while species in groups such asfleas,lice, andmosquitoes arehematophagous, feeding on theblood of animals.[152]
Many insects are consideredpests by humans. These include parasites of people and livestock, such aslice andbed bugs;mosquitoes act asvectors ofseveral diseases. Other pests include insects liketermites that damage wooden structures; herbivorous insects such aslocusts, aphids, andthrips that destroy agricultural crops, or likewheat weevils damage stored agricultural produce. Farmers have often attempted to control insects with chemicalinsecticides, but increasingly rely onbiological pest control. This uses one organism to reduce the population density of a pest organism; it is a key element ofintegrated pest management.[156][157] Biological control is favored because insecticides can cause harm to ecosystems far beyond the intended pest targets.[158][159]
Pollination of flowering plants by insects includingbees,butterflies,flies, andbeetles, is economically important.[162] The value of insect pollination of crops and fruit trees was estimated in 2021 to be about $34 billion in the US alone.[163]
Insects that feed on or parasitise other insects are beneficial to humans if they thereby reduce damage to agriculture and human structures. For example,aphids feed on crops, causing economic loss, butladybugs feed on aphids, and can be usedto control them. Insects account for the vast majority of insect consumption.[173][174][175]
Fly larvae (maggots) were formerlyused to treat wounds to prevent or stopgangrene, as they would only consume dead flesh. This treatment is finding modern usage in some hospitals. Insects have gained attention as potential sources of drugs and other medicinal substances.[176] Adult insects, such as crickets and insect larvae of various kinds, are commonly used as fishing bait.[177]
At least 66 insect species extinctions have been recorded since 1500, many of them on oceanic islands.[178]Declines in insect abundance have been attributed to human activity in the form of artificial lighting,[179] land use changes such as urbanization or farming,[180][181] pesticide use,[182] and invasive species.[183][184] A 2019 research review suggested that a large proportion of insect species is threatened with extinction in the 21st century,[185] though the details have been disputed.[186] A larger 2020 meta-study, analyzing data from 166 long-term surveys, suggested that populations of terrestrial insects are indeed decreasing rapidly, by about 9% per decade.[187][188]
Witchetty grubs are prized as high-protein foods by Aboriginal Australians.[191]
Insects are consumed as food in 80% of the world's nations, by people in roughly 3,000 ethnic groups.[192][193] In Africa, locally abundant species oflocusts andtermites are a common traditional human food source.[194] Some, especiallydeep-friedcicadas, are considered to bedelicacies. Insects have a high protein content for their mass, and some authors suggest their potential as a major source ofprotein in humannutrition.[195] In most first-world countries, however,entomophagy (the eating of insects), istaboo.[196] They are also recommended byarmed forces as asurvival food for troops in adversity.[194] Because of the abundance of insects and a worldwide concern of food shortages, theFood and Agriculture Organization of theUnited Nations considers that people throughout the world may have to eat insects as a food staple. Insects are noted for their nutrients, having a high content of protein, minerals and fats and are already regularly eaten by one-third of the world's population.[197]
^TheMuseum of New Zealand notes that "in everyday conversation",bug "refers to land arthropods with at least six legs, such as insects, spiders, and centipedes".[5] In a chapter on "Bugs That Are Not Insects", entomologist Gilbert Walbauer specifies centipedes, millipedes, arachnids (spiders,daddy longlegs, scorpions,mites,chiggers and ticks) as well as the few terrestrial crustaceans (sowbugs andpillbugs).[6]
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![The giant dragonfly-like insect Meganeura monyi grew to wingspans of 75 cm (2 ft 6 in) in the late Carboniferous, around 300 million years ago.[39]](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aMeganeura_monyi_au_Museum_de_Toulouse.jpg)
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![A parasitoid wasp ovipositing into an aphid[155]](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aCSIRO_ScienceImage_2357_Spotted_alfalfa_aphid_being_attacked_by_parasitic_wasp.jpg)
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