Insect

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For other uses, see Insect (disambiguation).

Insect
Temporal range: Carboniferous–Present
Common scorpionflyBlue emperorCoffee locustEuropean earwigVinegar flyGerman waspMarch brown mayflyDouble drummerDog fleaOld World swallowtailEuropean mantisPhyllium philippinicumHead louseSilverfishChrysopa perlaEuropean stag beetleNorthern harvester termiteDichrostigma flavipes
A wide diversity of insects from various orders. Insects have a three-part body: head with large compound eyes and antennae, a thorax with three pairs of legs and often wings, and a segmented abdomen.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Clade: Pancrustacea
Subphylum: Hexapoda
Class: Insecta
Linnaeus, 1758
Subgroups
Synonyms
  • Ectognatha
  • Entomida

Insects (from

abdomen), three pairs of jointed legs, compound eyes, and a pair of antennae. Insects are the most diverse group of animals, with more than a million described species
; they represent more than half of all animal species.

The insect

ocelli. Many insects can hear, using tympanal organs, which may be on the legs or other parts of the body. Their sense of smell
is via receptors, usually on the antennae and the mouthparts.

Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton, so development involves a series of molts. The immature stages often differ from the adults in structure, habit and habitat. Groups that undergo four-stage metamorphosis often have a nearly immobile pupa. Insects that undergo three-stage metamorphosis lack a pupa, developing through a series of increasingly adult-like nymphal stages. The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonfly-like insects with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to have coevolved with flowering plants.

Adult insects typically move about by walking and flying; some can swim. Insects are the only invertebrates that can achieve sustained powered flight; insect flight evolved just once. Many insects are at least partly aquatic, and have larvae with gills; in some species, the adults too are aquatic. Some species, such as water striders, can walk on the surface of water. Insects are mostly solitary, but some, such as bees, ants and termites, are social and live in large, well-organized colonies. Others, such as earwigs, provide maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light.

Humans regard many insects as

silkworms for silk and honey bees for honey. Insects are consumed as food in 80% of the world's nations, by people in roughly 3000 ethnic groups. Human activities are having serious effects on insect biodiversity
.

Etymology

The word insect comes from the

Latin word inseco, from in, "to cut up",[1] as insects appear to be cut into three parts. The Latin word was introduced by Pliny the Elder who calqued the Ancient Greek word ἔντομον éntomon "insect" (as in entomology) from ἔντομος éntomos "cut in pieces";[2] this was Aristotle's term for this class of life in his biology, also in reference to their notched bodies. The English word insect first appears in 1601 in Philemon Holland's translation of Pliny.[3][4]

Insects and other bugs

Distinguishing features

In common speech, insects and other terrestrial

shield bugs.[6] Other terrestrial arthropods, such as centipedes, millipedes, woodlice, spiders, mites and scorpions, 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]

Diversity

Main article: Insect biodiversity
About half of all eukaryotes 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 all eukaryote species, including animals, plants, and fungi.[10] The most diverse insect orders 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]

Distribution and habitats

  • Insects occur in habitats as varied as snow, freshwater, the tropics, desert, and even the sea.
  • The snow scorpionfly Boreus hyemalis on snow
    The snow scorpionfly Boreus hyemalis on snow
  • The great diving beetle Dytiscus marginalis larva in a pond
    The great diving beetle
    Dytiscus marginalis
     larva in a pond
  • The green orchid bee Euglossa dilemma of Central America
    The green orchid bee Euglossa dilemma of Central America
  • The desert locust Schistocerca gregaria laying eggs in sand
    The desert locust
    Schistocerca gregaria
     laying eggs in sand
  • Sea skater Halobates on a Hawaii beach
    Sea skater Halobates on a Hawaii beach

Insects are distributed over every continent and almost every terrestrial habitat. There are many more species in the tropics, especially in rainforests, 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 3000 species of the American Arctic must be broadly accurate. In contrast, a large majority of the insect species of the tropics and the southern hemisphere are probably undescribed.[11] Some 30–40,000 species inhabit freshwater; very few insects, perhaps a hundred species, are marine.[12] Insects such as snow scorpionflies flourish in cold habitats including the Arctic and at high altitude.[13] Insects such as desert locusts, ants, beetles, and termites are adapted to some of the hottest and driest environments on earth, such as the Sonoran Desert.[14]

Phylogeny and evolution

External phylogeny

Insects form a

phylogenetic analysis by Kjer et al. (2016) places the insects among the Hexapoda, six-legged animals with segmented bodies; their closest relatives are the Diplura (bristletails).[16]

Hexapoda

Collembola (springtails)

Protura (coneheads)

Diplura (two-pronged bristletails)

Insecta (=Ectognatha)

Internal phylogeny

The internal phylogeny is based on the works of Wipfler et al. 2019 for the Polyneoptera,[17] Johnson et al. 2018 for the Paraneoptera,[18] and Kjer et al. 2016 for the Holometabola.[19] The numbers of described extant species (boldface for groups with over 100,000 species) are from Stork 2018.[9]

Insecta
Monocondylia

Archaeognatha (hump-backed/jumping bristletails, 513 spp)

Dicondylia

Zygentoma (silverfish, firebrats, fishmoths, 560 spp)

Pterygota
Palaeoptera

Odonata (dragonflies and damselflies, 5,899 spp)

Ephemeroptera (mayflies, 3,240 spp)

Neoptera
Polyneoptera

Zoraptera (angel insects, 37 spp)

Dermaptera (earwigs, 1,978 spp)

Plecoptera (stoneflies, 3,743 spp)

Orthoptera (grasshoppers, crickets, katydids, 23,855 spp)

Grylloblattodea (ice crawlers, 34 spp)

Mantophasmatodea (gladiators, 15 spp)

Phasmatodea (stick insects, 3,014 spp)

Embioptera (webspinners, 463 spp)

Dictyoptera

Mantodea (mantises, 2,400 spp)

Blattodea (cockroaches and termites, 7,314 spp)

Eumetabola
Paraneoptera

Psocodea (book lice, barklice and sucking lice, 11,000 spp)

Hemiptera (true bugs, 103,590 spp)

Thysanoptera (thrips, 5,864 spp)

Holometabola

Hymenoptera (sawflies, wasps, bees, ants, 116,861 spp)

Neuropteroidea
Coleopterida

Strepsiptera (twisted-wing flies, 609 spp)

Coleoptera (beetles, 386,500 spp)

Neuropterida

Raphidioptera (snakeflies, 254 spp)

Neuroptera (lacewings, 5,868 spp)

Megaloptera (alderflies and dobsonflies, 354 spp)

Panorpida
Amphiesmenoptera

Lepidoptera (butterflies and moths, 157,338 spp)

Trichoptera (caddisflies, 14,391 spp)

Antliophora

Diptera (true flies, 155,477 spp)

Mecoptera (scorpionflies, 757 spp)

Siphonaptera (fleas, 2,075 spp)

larvae, pupae
wings flex over abdomen
wings

Taxonomy

Early

Further information: Aristotle's biology § Classification, and Insecta in the 10th edition of Systema Naturae
Diagram of Linnaeus's key to his seven orders of insect, 1758[20]
Aptera

wingless
Diptera

2‑winged
Coleoptera

forewings fully hardened
Hemiptera

forewings partly hardened
dissimilar pairs
Lepidoptera

wings scaly
Neuroptera

no sting
Hymenoptera

sting
wings membranous
similar pairs
4‑winged
winged
Insecta

scala naturae, above the spontaneously generating sponges and worms, but below the hard-shelled marine snails. His classification remained in use for many centuries.[21]

In 1758, in his Systema Naturae,[22] Carl Linnaeus divided the animal kingdom into six classes including Insecta. He created seven orders of insect according to the structure of their wings. These were the wingless Aptera, the 2-winged Diptera, and five 4-winged orders: the Coleoptera with fully-hardened forewings; the Hemiptera with partly-hardened forewings; the Lepidoptera with scaly wings; the Neuroptera with membranous wings but no sting; and the Hymenoptera, with membranous wings and a sting.[20]

Philosophie Zoologique, treated the insects as one of nine invertebrate phyla.[23] In his 1817 Le Règne Animal, Georges Cuvier grouped all animals into four embranchements ("branches" with different body plans), one of which was the articulated animals, containing arthropods and annelids.[24] This arrangement was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.[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]

Modern

See also: Category:Insect orders and Category:Insect families

Traditional morphology-based

superclass,[27] and identified four groups within it: insects (Ectognatha), Collembola, Protura, and Diplura, the latter three being grouped together as the Entognatha on the basis of internalized mouth parts.[28]

The use of phylogenetic data has brought about numerous changes in relationships above the level of

monophyletic, as Archaeognatha are sister to all other insects, based on the arrangement of their mandibles, while the Pterygota, the winged insects, emerged from within the Dicondylia, alongside the Zygentoma.[29]

The Pterygota (

Embiidina 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]

Evolutionary history

Main article: Evolution of insects

The oldest fossil that may be a primitive wingless insect is

Windyfield chert.[34] The oldest known flying insects are from the mid-Carboniferous, around 328–324 million years ago. The group subsequently underwent a rapid explosive diversification. Claims that they originated substantially earlier, during the Silurian or Devonian (some 400 million years ago) based on molecular clock estimates, are unlikely to be correct, given the fossil record.[35]

Four large-scale radiations of insects have occurred: beetles (from about 300 million years ago), flies (from about 250 million years ago), moths and wasps (both from about 150 million years ago).[36]

The remarkably successful

warning colors.[38]

Morphology and physiology

Main article: Insect morphology

External

subesophageal ganglion
  • mouthparts

    • Three-part body

    Insects have a

    ocelli) and three sets of variously modified appendages that form the mouthparts. The thorax carries the three pairs of legs and up to two pairs of wings. The abdomen contains most of the digestive, respiratory, excretory and reproductive structures.[8]

    Segmentation

    Further information: Insect morphology

    The head is enclosed in a hard, heavily

    head 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) the prothorax, mesothorax and metathorax. The prothorax carries the first pair of legs. The mesothorax carries the second pair of legs and the front wings. The metathorax carries the third pair of legs and the hind wings.[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 of spiracles.[40]

    Exoskeleton

    Main article:
    Arthropod cuticle

    The outer skeleton, the

    cuticle, is made up of two layers: the epicuticle, a thin and waxy water-resistant outer layer without chitin, and a lower layer, the thick chitinous procuticle. The procuticle has two layers: an outer exocuticle and an inner endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each other in a sandwich pattern, while the exocuticle is rigid and sclerotized.[41][42] As an adaptation to life on land, insects have an enzyme 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]

    Internal systems

    Main article: Insect physiology

    Nervous

    The

    thoracic segments have one ganglion on each side, connected into a pair per segment. This arrangement is also seen in the first eight segments of the abdomen. Many insects have fewer ganglia than this.[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.

    mouth, pharynx, and crop 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 the xylem and phloem transport vessels of plants.[54] Once food leaves the crop, it passes to the midgut, where the majority of digestion takes place. Microscopic projections, microvilli, increase the surface area of the wall to absorb nutrients.[55] In the hindgut, undigested food particles are joined by uric acid to form fecal pellets; most of the water is absorbed, leaving a dry pellet to be eliminated. Insects may have one to hundreds of Malpighian 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]

    Reproductive

    Main article: Insect reproductive system

    The

    ovarioles. Female insects make eggs, receive and store sperm, manipulate sperm from different males, and lay eggs. Accessory glands produce substances to maintain sperm and to protect the eggs. They can produce glue and protective substances for coating eggs, or tough coverings for a batch of eggs called oothecae.[57]

    For males, the reproductive system consists of one or two

    tracheae. The testes contain sperm tubes or follicles in a membranous sac. These connect to a duct that leads to the outside. The terminal portion of the duct may be sclerotized to form the intromittent organ, the aedeagus.[58]

    Respiratory

    Main article: Respiratory system of insects
    cell nuclei
    .

    tracheae and tracheoles. In most insects, air is taken in through paired spiracles, 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 late Paleozoic, 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 and diffusive ventilation, to discontinuous.[60][61][62][63]

    Circulatory

    Further information: Insect physiology § Circulatory system

    Because oxygen is delivered directly to tissues via tracheoles, the circulatory system is not used to carry oxygen, and is therefore greatly reduced. The insect circulatory system is open; it has no

    hemocytes are suspended. Nutrients, hormones, wastes, and other substances are transported throughout the insect body in the hemolymph. Hemocytes include many types of cells that are important for immune responses, wound healing, and other functions. Hemolymph pressure may be increased by muscle contractions or by swallowing air into the digestive system to aid in molting.[66]

    Sensory

    Further information: Insect physiology § Sensory organs
    Most insects have a pair of large compound eyes and other sensory organs such as antennae able to detect movements and chemical stimuli on their heads.

    Many insects possess numerous specialized

    polarized light, while the antennae of male moths can detect the pheromones 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 some

    visible light wavelengths, with color vision. Phylogenetic analysis suggests that UV-green-blue trichromacy existed from at least the Devonian period, some 400 million years ago.[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's sense of smell is via chemical receptors, usually on the antennae and the mouthparts. These detect both airborne volatile compounds and odorants on surfaces, including pheromones from other insects and compounds released by food plants. Insects use olfaction to locate mating partners, food, and places to lay eggs, and to avoid predators. It is thus an extremely important sense, enabling insects to discriminate between thousands of volatile compounds.[73]

    Some insects are capable of magnetoreception; ants and bees navigate using it both locally (near their nests) and when migrating.[74] The Brazilian stingless bee detects magnetic fields using the hair-like sensilla on its antennae.[75][76]

    Reproduction and development

    Life-cycles

    Butterflies mating

    The majority of insects hatch from

    viviparous, gestating inside the mother and born alive.[79] Some insects, like parasitoid wasps, are polyembryonic, meaning that a single fertilized egg divides into many separate embryos.[80] Insects may be univoltine, bivoltine or multivoltine, having one, two or many broods in a year.[81]

    Aphid giving birth to live female young by parthenogenesis from unfertilized eggs

    Other developmental and reproductive variations include

    hermaphroditism.[82][83] In haplodiploidy, which is a type of sex-determination system, the offspring's sex is determined by the number of sets of chromosomes an individual receives. This system is typical in bees and wasps.[84]

    Some insects are

    haploid 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.

    Incomplete

    Main article: Hemimetabolism
    Incomplete metamorphosis in a locust with multiple instars. Egg is not shown. The largest specimen is adult.

    epicuticle inside the old one. After this new epicuticle is secreted, the epidermis releases a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. When this stage is complete, the insect makes its body swell by taking in a large quantity of water or air, which makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest.[87][88]

    Complete

    Main article: Holometabolism
    Life-cycle of butterfly, undergoing complete metamorphosis from egg through caterpillar larvae to pupa and adult

    Endopterygota.[82]

    Communication

    Insects that produce sound can generally hear it. Most

    insects can hear only a narrow range of frequencies related to the frequency of the sounds they can produce. Mosquitoes can hear up to 2 kilohertz.[90] Certain predatory and parasitic insects can detect the characteristic sounds made by their prey or hosts, respectively. Likewise, some nocturnal moths can perceive the ultrasonic emissions of bats, which helps them avoid predation.[91]

    Light production

    A few insects, such as

    fireflies, beetles of the family Lampyridae. Some species are able to control this light generation to produce flashes. The function varies with some species using them to attract mates, while others use them to lure prey. Cave dwelling larvae of Arachnocampa (Mycetophilidae, fungus gnats) glow to lure small flying insects into sticky strands of silk.[92] Some fireflies of the genus Photuris mimic the flashing of female Photinus 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. In

    hawk moths and Hedylid butterflies, can hear ultrasound and take 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 (acoustic aposematism),[98] while some palatable moths have evolved to mimic these calls (acoustic Batesian mimicry).[99] The claim that some moths can jam bat sonar has been revisited. Ultrasonic recording and high-speed infrared videography of bat-moth interactions suggest the palatable tiger moth really does defend against attacking big brown bats using ultrasonic clicks that jam bat sonar.[100]

    Grasshopper stridulation
    Several unidentified grasshoppers stridulating
    Problems playing this file? See media help.

    Very low sounds are produced in various species of

    Mantodea and Neuroptera. These low sounds are produced by the insect's movement, amplified by stridulatory structures on the insect's muscles and joints; these sounds can be used to warn or communicate with other insects. Most sound-making insects also have tympanal organs that can perceive airborne sounds. Some hemipterans, such as the water boatmen, communicate via underwater sounds.[101]

    Cricket in garage with familiar call

    Communication using surface-borne vibrational signals is more widespread among insects because of size constraints in producing air-borne sounds.

    foliage), so insects living in such environments communicate primarily using substrate-borne vibrations.[103]

    Some species use vibrations for communicating, such as to attract mates as in the songs of the

    form a mutualistic association with ants communicate with ants in this way.[105] The Madagascar hissing cockroach has the ability to press air through its spiracles to make a hissing noise as a sign of aggression;[106] the death's-head hawkmoth makes a squeaking noise by forcing air out of their pharynx when agitated, which may also reduce aggressive worker honey bee behavior when the two are close.[107]

    Chemical communication

    Main articles: Chemical communication in insects and Insect olfaction
    Social insects such as ants have multiple types of pheromonal glands, producing different semiochemicals for communication with other insects.[108]

    Many insects have evolved

    conspecific 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 fight desiccation and pathogens. They are important, too, as pheromones, especially in social insects.[109]

    Social behavior

    Main article: Eusociality
    A cathedral mound created by eusocial mound-building termites.
    Honey bee's figure-eight waggle dance. An orientation 45° to the right of ‘up' on the comb indicates food 45° to the right of the sun. The dancer's rapid waggling blurs her abdomen.

    workers, prevented from reproducing by worker 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, called dance language, the angle at which a bee dances represents a direction relative to the sun, and the length of the dance represents the distance to be flown.[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 can navigate unerringly to a single hole a few millimeters in diameter among thousands of similar holes, after a trip of several kilometers. In philopatry, insects that hibernate are able to recall a specific location up to a year after last viewing the area of interest.[113] A few insects seasonally migrate large distances between different geographic regions, as in the continent-wide monarch 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 provide parental care, where they at least guard their eggs, and sometimes guard their offspring until adulthood, possibly even feeding them. Many wasps and bees construct a nest or burrow, store provisions in it, and lay an egg upon those provisions, providing no further care.[115]

    Locomotion

    Flight

    Main article: Insect flight
    hoverflies are capable of rapid and agile flight
    .

    Insects are the only group of

    clap and fling mechanism; the wings are clapped together and pulled apart, flinging vortices into the air at the leading edges and at the wingtips.[119][120]

    The evolution of insect wings has been a subject of debate; it has been suggested they came from modified gills, flaps on the spiracles, or an appendage, the epicoxa, at the base of the legs.[121] More recently, entomologists have favored evolution of wings from lobes of the notum, of the pleuron, or more likely both.[122] In the Carboniferous age, the dragonfly-like Meganeura 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]

    Unlike birds, small insects are swept along by the prevailing winds[125] although many larger insects migrate. Aphids are transported long distances by low-level jet streams.[126]

    Walking

    Further information: Walking § Insects
    Spatial and temporal stepping pattern of walking desert ants performing an alternating tripod gait. Recording rate: 500 fps, Playback rate: 10 fps.

    Many adult insects use six legs for walking, with an alternating

    Water 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 genus Halobates even live on the surface of open oceans, a habitat that has few insect species.[129]

    Swimming

    Main article:
    Aquatic insects
    backswimmer Notonecta glauca
    underwater, showing its paddle-like hindleg adaptation

    A large number of insects live either part or the whole of their lives underwater. In many of the more primitive orders of insect, the immature stages are aquatic. In some groups, such as water 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. Dragonfly

    rove beetle Stenus emit pygidial gland surfactant secretions that reduce surface tension; this enables them to move on the surface of water by Marangoni propulsion.[131][132]

    Ecology

    Main article: Insect ecology

    Insects play many critical roles in ecosystems, 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] many beetles are scavengers; dung beetles recycle biological materials into forms useful to other organisms.[135][136] Insects are responsible for much of the process by which topsoil is created.[137]

    Defense

    Main article: Defense in insects
    Reduvius personatus, the masked hunter bug nymph, camouflages itself with sand grains to avoid predators.

    Insects are mostly small, soft bodied, and fragile compared to larger lifeforms. The immature stages are small, move slowly or are immobile, and so all stages are exposed to predation and parasitism. Insects accordingly employ multiple defensive strategies, including camouflage, mimicry, toxicity and active defense.[138] Many

    Stick insects mimic the forms of sticks and leaves.[140]
    Many insects use
    hoverflies (the mimics), gain a survival advantage by resembling inedible species (the models).[138][141] In Müllerian mimicry, inedible species, such as of wasps and bees, resemble each other so as to reduce the sampling rate by predators who need to learn that those insects are inedible. Heliconius butterflies, many of which are toxic, form Müllerian complexes, advertising their inedibility.[142]
    Chemical defense is common among Coleoptera and Lepidoptera, usually being advertised by bright warning colors (aposematism), as in the monarch butterfly. As larvae, they obtain their toxicity by sequestering chemicals from the plants they eat into their own tissues. Some manufacture their own toxins. Predators that eat poisonous butterflies and moths may vomit violently, learning not to eat insects with similar markings; this is the basis of Müllerian mimicry.[143] Some ground beetles of the family Carabidae actively defend themselves, spraying chemicals from their abdomen with great accuracy, to repel predators.[138]

    Pollination

    Main article: Entomophily
    European honey bee carrying pollen in a pollen basket
    back to the hive

    Pollination is the process by which pollen is transferred in the reproduction of plants, thereby enabling fertilisation and sexual reproduction.[144] Most flowering plants require an animal to do the transportation. The majority of pollination is by insects.[145] Because insects usually receive benefit for the pollination in the form of energy rich nectar it is a mutualism. The various flower traits, such as bright colors and pheromones that coevolved with their pollinators, have been called pollination syndromes, though around one third of flowers cannot be assigned to a single syndrome.[146]

    Parasitism

    Further information: Parasitism and Parasitoid wasp

    Many insects are

    lice, and mosquitoes are hematophagous, feeding on the blood of animals.[152]

    Relationship to humans

    Main article: Human interactions with insects

    As pests

    Aedes aegypti, the yellow fever mosquito, is a vector of several diseases.
    Main article:
    Pest insect

    Many insects are considered

    bed bugs; mosquitoes act as vectors of several diseases. Other pests include insects like termites that damage wooden structures; herbivorous insects such as locusts, aphids, and thrips that destroy agricultural crops, or like wheat weevils damage stored agricultural produce. Farmers have often attempted to control insects with chemical insecticides, but increasingly rely on biological pest control. This uses one organism to reduce the population density of a pest organism; it is a key element of integrated pest management.[156][157] Biological control is favored because insecticides can cause harm to ecosystems far beyond the intended pest targets.[158][159]

    In beneficial roles

    See also: Economic entomology § Beneficial insects
    Silkworms were domesticated for silk for over 5000 years.[160][161]
    Here, silk cocoons are being unrolled.

    flies, and beetles, 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 produce useful substances such as

    silkworm has greatly affected human history, as silk-driven trade established relationships between China and the rest of the world.[171][172]

    Insects that feed on or parasitise other insects are beneficial to humans if they thereby reduce damage to agriculture and human structures. For example,

    ladybugs feed on aphids, and can be used to control them. Insects account for the vast majority of insect consumption.[173][174][175]

    Fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene, as they would only consume dead flesh. This treatment is finding modern usage in some hospitals. Insects have gained attention as potential sources of drugs and other medicinal substances.[176] Adult insects, such as crickets and insect larvae of various kinds, are commonly used as fishing bait.[177]

    Population declines

    Main article: Decline in insect populations

    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]

    In research

    The fruit fly Drosophila melanogaster is a widely used model organism.

    Insects play important roles in biological research. For example, because of its small size, short generation time and high

    transcription in fruit flies can help to understand those processes in other eukaryotes, including humans.[189] The genome of D. melanogaster was sequenced in 2000, reflecting the organism's important role in biological research. It was found that 70% of the fly genome is similar to the human genome, supporting the theory of evolution.[190]

    As food

    Main article: Insects as food
    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 3000 ethnic groups.

    deep-fried cicadas, are considered to be delicacies. Insects have a high protein content for their mass, and some authors suggest their potential as a major source of protein in human nutrition.[195] In most first-world countries, however, entomophagy (the eating of insects), is taboo.[196] They are also recommended by armed forces as a survival food for troops in adversity.[194] Because of the abundance of insects and a worldwide concern of food shortages, the Food and Agriculture Organization of the United Nations considers that people throughout the world may have to eat insects as a food staple. Insects are noted for their nutrients, having a high content of protein, minerals and fats and are already regularly eaten by one-third of the world's population.[197]

    In other products

    mealworms, can be processed and used as feed for farmed animals including chicken, fish and pigs.[200] Many species of insects are sold and kept as pets.[201]

    In religion and folklore

    Further information: Insects in mythology
    Ancient Egyptian scarab with separate wings, c. 712-342 BC

    praying mantis that holds much cultural significance including creation and zen-like patience in waiting.[202]

    See also

    Notes

    1. pillbugs).[6]

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    Sources

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