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1Etymology

2Insects and other bugs

Toggle Insects and other bugs subsection

2.1Distinguishing features

2.2Diversity

2.3Distribution and habitats

3Phylogeny and evolution

Toggle Phylogeny and evolution subsection

3.1External phylogeny

3.2Internal phylogeny

3.3Taxonomy

3.3.1Early

3.3.2Modern

3.4Evolutionary history

4Morphology and physiology

Toggle Morphology and physiology subsection

4.1External

4.1.1Three-part body

4.1.2Segmentation

4.1.3Exoskeleton

4.2Internal systems

4.2.1Nervous

4.2.2Digestive

4.2.3Reproductive

4.2.4Respiratory

4.2.5Circulatory

4.2.6Sensory

5Reproduction and development

Toggle Reproduction and development subsection

5.1Life-cycles

5.2Metamorphosis

5.2.1Incomplete

5.2.2Complete

6Communication

Toggle Communication subsection

6.1Light production

6.2Sound production

6.3Chemical communication

7Social behavior

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7.1Care of young

8Locomotion

Toggle Locomotion subsection

8.1Flight

8.2Walking

8.3Swimming

9Ecology

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9.1Defense

9.2Pollination

9.3Parasitism

10Relationship to humans

Toggle Relationship to humans subsection

10.1As pests

10.2In beneficial roles

10.3Population declines

10.4In research

10.5As food

10.6In other products

10.7In religion and folklore

11See also

12Notes

13References

14Sources

15External links

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Insect

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From Wikipedia, the free encyclopedia

Class of arthropods

For other uses, see Insect (disambiguation).

InsectTemporal range: Carboniferous–Present

PreꞒ

Ꞓ

Insects, such as this scorpionfly, 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

Domain:

Eukaryota

Kingdom:

Animalia

Phylum:

Arthropoda

Clade:

Pancrustacea

Subphylum:

Hexapoda

Class:

InsectaLinnaeus, 1758

Subgroups

Archaeognatha

Dicondylia

Zygentoma

Pterygota

Synonyms

Ectognatha

Entomida

Insects (from Latin insectum) are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and 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 nervous system consists of a brain and a ventral nerve cord. Most insects reproduce by laying eggs. Insects breathe air through a system of paired openings along their sides, connected to small tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open hemocoel. Insect vision is mainly through their compound eyes, with additional small 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 pests, especially those that damage crops, and attempt to control them using insecticides and other techniques. Others are parasitic, and may act as vectors of diseases. Insect pollinators are essential to the reproduction of many flowering plants and so to their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefit. Two species in particular are economically important and were domesticated many centuries ago: 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 arthropods are often called bugs.[a] Entomologists to some extent reserve the name "bugs" for a narrow category of "true bugs", insects of the order Hemiptera, such as cicadas and 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]

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

Spider: eight legs,two-part body

Woodlouse: seven pairs of legs, seven body segments (plus head and tail)

Centipede: many legs,one pair per segment

Millipede: many legs,two pairs per segment

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]

Insects are extremely diverse. Five groups each have over 100,000 described species.

True bugs(Hemiptera)

Butterflies and moths(Lepidoptera)

Flies(Diptera)

Wasps(Hymenoptera)

Beetles(Coleoptera)

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 great diving beetle Dytiscus marginalis larva in a pond

The green orchid bee Euglossa dilemma of Central America

The desert locust Schistocerca gregaria laying eggs in sand

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 clade, a natural group with a common ancestor, among the arthropods.[15] 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

Aristotle was the first to describe the insects as a distinct group. He placed them as the second-lowest level of animals on his scala naturae, above the 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]

Jean-Baptiste de Lamarck, in his 1809 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 systematics have usually given the Hexapoda the rank of 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 orders.[28] Insects can be divided into two groups historically treated as subclasses: wingless insects or Apterygota, and winged insects or Pterygota. The Apterygota traditionally consisted of the primitively wingless orders Archaeognatha (jumping bristletails) and Zygentoma (silverfish). However, Apterygota is not monophyletic, as Archaeognatha are sister to all other insects, based on the arrangement of their mandibles, while the Pterygota, the winged insects, emerged from within the Dicondylia, alongside the Zygentoma.[29]

The Pterygota (Palaeoptera and Neoptera) are winged and have hardened plates on the outside of their body segments; the Neoptera have muscles that allow their wings to fold flat over the abdomen. Neoptera can be divided into groups with incomplete metamorphosis (Polyneoptera and Paraneoptera) and those with complete metamorphosis (Holometabola). The molecular finding that the traditional louse orders Mallophaga and Anoplura are within Psocoptera has led to the new taxon Psocodea.[30] Phasmatodea and 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 Leverhulmia from the Early Devonian 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 Hymenoptera (wasps, bees, and ants) appeared some 200 million years ago in the Triassic period, but achieved their wide diversity more recently in the Cenozoic era, which began 66 million years ago. Some highly successful insect groups evolved in conjunction with flowering plants, a powerful illustration of coevolution. Insects were among the earliest terrestrial herbivores and acted as major selection agents on plants.[37] Plants evolved chemical defenses against this herbivory and the insects, in turn, evolved mechanisms to deal with plant toxins. Many insects make use of these toxins to protect themselves from their predators. Such insects often advertise their toxicity using warning colors.[38]

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]

Beetle Moravocoleus permianus, fossil and reconstruction, from the Early Permian

Hymenoptera such as this Iberomaimetsha from the Early Cretaceous, around 100 million years ago.

Morphology and physiology

Main article: Insect morphology

External

Insect morphology A- Head B- Thorax C- Abdomen antennaocellus (lower)ocellus (upper)compound eyebrain (cerebral ganglia)prothoraxdorsal blood vesseltracheal tubes (trunk with spiracle)mesothoraxmetathoraxforewinghindwingmidgut (stomach)dorsal tube (heart)ovaryhindgut (intestine, rectum, anus)anusoviductnerve cord (abdominal ganglia)Malpighian tubulestarsal padsclawstarsustibiafemurtrochanterforegut (crop, gizzard)thoracic ganglioncoxasalivary glandsubesophageal ganglionmouthparts

Three-part body

Insects have a segmented body supported by an exoskeleton, the hard outer covering made mostly of chitin. The body is organized into three interconnected units: the head, thorax and abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, zero to three simple eyes (or 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 sclerotized, unsegmented 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 nervous system of an insect consists of a brain and a ventral nerve cord. The head capsule is made up of six fused segments, each with either a pair of ganglia, or a cluster of nerve cells outside of the brain. The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect's esophagus, called the subesophageal ganglion.[44] 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.[47] There is extensive variation among different orders, life stages, and even castes in the digestive system of insects.[48] The gut runs lengthwise through the body. It has three sections, with paired salivary glands and salivary reservoirs.[49] By moving its mouthparts the insect mixes its food with saliva.[50][51] Some insects, like flies, expel digestive enzymes onto their food to break it down, but most insects digest their food in the gut.[52] The foregut is lined with cuticule as protection from tough food. It includes the 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 reproductive system of female insects consist of a pair of ovaries, accessory glands, one or more spermathecae to store sperm, and ducts connecting these parts. The ovaries are made up of a variable number of egg tubes, ovarioles. Female insects make eggs, receive and store sperm, manipulate sperm from different males, and lay eggs. Accessory glands produce substances to maintain sperm and to protect the eggs. They can produce glue and protective substances for coating eggs, or tough coverings for a batch of eggs called oothecae.[57]

For males, the reproductive system consists of one or two testes, suspended in the body cavity by 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

The tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depicts cell nuclei.

Insect respiration is accomplished without lungs. Instead, insects have a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need it via their 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 veins or arteries, and instead consists of little more than a single, perforated dorsal tube that pulses peristaltically. This dorsal blood vessel is divided into two sections: the heart and aorta. The dorsal blood vessel circulates the hemolymph, arthropods' fluid analog of blood, from the rear of the body cavity forward.[64][65] Hemolymph is composed of plasma in which hemocytes are suspended. Nutrients, hormones, wastes, and other substances are transported throughout the insect body in the hemolymph. Hemocytes include many types of cells that are important for immune responses, wound healing, and other functions. Hemolymph pressure may be increased by muscle contractions or by swallowing air into the digestive system to aid in molting.[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 sensory organs able to detect stimuli including limb position (proprioception) by campaniform sensilla, light, water, chemicals (senses of taste and smell), sound, and heat.[67] Some insects such as bees can perceive ultraviolet wavelengths, or detect 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 cave crickets, are able to perceive light and dark. Many have acute vision capable of detecting small and rapid movements. The eyes may include simple eyes or ocelli as well as larger compound eyes. Many species can detect light in the infrared, ultraviolet and visible light wavelengths, with color vision. Phylogenetic analysis suggests that UV-green-blue trichromacy existed from at least the Devonian period, some 400 million years ago.[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 eggs. The fertilization and development takes place inside the egg, enclosed by a shell (chorion) that consists of maternal tissue. In contrast to eggs of other arthropods, most insect eggs are drought resistant. This is because inside the chorion two additional membranes develop from embryonic tissue, the amnion and the serosa. This serosa secretes a cuticle rich in chitin that protects the embryo against desiccation.[77] Some species of insects, like aphids and tsetse flies, are ovoviviparous: their eggs develop entirely inside the female, and then hatch immediately upon being laid.[78] Some other species, such as in the cockroach genus Diploptera, are 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 haplodiploidy, polymorphism, paedomorphosis or peramorphosis, sexual dimorphism, parthenogenesis, and more rarely 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 parthenogenetic, meaning that the female can reproduce and give birth without having the eggs fertilized by a male. Many aphids undergo a cyclical form of parthenogenesis in which they alternate between one or many generations of asexual and sexual reproduction.[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 their haplodiploid system, diploid females spawn many females and a few 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.

Hemimetabolous insects, those with incomplete metamorphosis, change gradually after hatching from the egg by undergoing a series of molts through stages called instars, until the final, adult, stage is reached. An insect molts when it outgrows its exoskeleton, which does not stretch and would otherwise restrict the insect's growth. The molting process begins as the insect's epidermis secretes a new epicuticle inside the old one. After this new epicuticle is secreted, the epidermis releases a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. When this stage is complete, the insect makes its body swell by taking in a large quantity of water or air, 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

Holometabolism, or complete metamorphosis, is where the insect changes in four stages, an egg or embryo, a larva, a pupa and the adult or imago. In these species, an egg hatches to produce a larva, which is generally worm-like in form. This can be eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform (elongated, flattened and active), elateriform (wireworm-like) or vermiform (maggot-like). The larva grows and eventually becomes a pupa, a stage marked by reduced movement. There are three types of pupae: obtect, exarate or coarctate. Obtect pupae are compact, with the legs and other appendages enclosed. Exarate pupae have their legs and other appendages free and extended. Coarctate pupae develop inside the larval skin.[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 to hypermetamorphosis. Complete metamorphosis is a trait of the most diverse insect group, the 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 Mycetophilidae (Diptera) and the beetle families Lampyridae, Phengodidae, Elateridae and Staphylinidae are bioluminescent. The most familiar group are the fireflies, beetles of the family Lampyridae. Some species are able to control this light generation to produce flashes. The function varies with some species using them to attract mates, while others use them to lure prey. Cave dwelling larvae of Arachnocampa (Mycetophilidae, fungus gnats) glow to lure small flying insects into sticky strands of silk.[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 grasshoppers and crickets, this is achieved by stridulation. Cicadas make the loudest sounds among the insects by producing and amplifying sounds with special modifications to their body to form tymbals and associated musculature. The African cicada Brevisana brevis has been measured at 106.7 decibels at a distance of 50 cm (20 in).[95] Some insects, such as the Helicoverpa zea moths, 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

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Very low sounds are produced in various species of Coleoptera, Hymenoptera, Lepidoptera, Mantodea and Neuroptera. These low sounds are produced by the insect's movement, amplified by stridulatory structures on the insect's muscles and joints; these sounds can be used to warn or communicate with other insects. Most sound-making insects also have tympanal 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.[102] Insects cannot effectively produce low-frequency sounds, and high-frequency sounds tend to disperse more in a dense environment (such as foliage), so insects living in such environments communicate primarily using substrate-borne vibrations.[103]

Some species use vibrations for communicating, such as to attract mates as in the songs of the shield bug Nezara viridula.[104] Vibrations can also be used to communicate between species; lycaenid caterpillars, which 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 chemical means for communication. These semiochemicals 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 aggregating 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.

Social insects, such as termites, ants and many bees and wasps, are eusocial.[110] They live together in such large well-organized colonies of genetically similar individuals that they are sometimes considered superorganisms. In particular, reproduction is largely limited to a queen caste; other females are 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

Insects such as hoverflies are capable of rapid and agile flight.

Insects are the only group of invertebrates to have developed flight. The ancient groups of insects in the Palaeoptera, the dragonflies, damselflies and mayflies, operate their wings directly by paired muscles attached to points on each wing base that raise and lower them. This can only be done at a relatively slow rate. All other insects, the Neoptera, have indirect flight, in which the flight muscles cause rapid oscillation of the thorax: there can be more wingbeats than nerve impulses commanding the muscles. One pair of flight muscles is aligned vertically, contracting to pull the top of the thorax down, and the wings up. The other pair runs longitudinally, contracting to force the top of the thorax up and the wings down.[116][117] Most insects gain aerodynamic lift by creating a spiralling vortex at the leading edge of the wings.[118] Small insects like thrips with tiny feathery wings gain lift using the clap and fling mechanism; the wings are clapped together and pulled apart, flinging vortices into the air at the leading edges and at the wingtips.[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 tripod gait. This allows for rapid walking with a stable stance; it has been studied extensively in cockroaches and ants. 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 as 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

The backswimmer Notonecta glauca underwater, showing its paddle-like hindleg adaptation

A large number of insects live either part or the whole of their lives underwater. In many of the more primitive orders of insect, the immature stages are aquatic. In some groups, such as water 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 naiads use jet propulsion, forcibly expelling water out of their rectal chamber.[130] Other insects such as the 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 insects rely on camouflage to avoid being noticed by their predators or prey.[139] It is common among leaf beetles and weevils that feed on wood or vegetation.[138] Stick insects mimic the forms of sticks and leaves.[140]

Many insects use mimicry to deceive predators into avoiding them. In Batesian mimicry, edible species, such as of hoverflies (the mimics), gain a survival advantage by resembling inedible species (the models).[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 parasitic. The largest group, with over 100,000 species[147] and perhaps over a million,[148] consists of a single clade of parasitoid 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 either micropredators or external parasites;[151][152] for example, many hemipteran bugs have piercing and sucking mouthparts, adapted for feeding on plant sap,[153][154] while species in groups such as fleas, lice, and mosquitoes are hematophagous, feeding on the blood of animals.[152]

A parasitoid wasp ovipositing into an aphid[155]

Plant parasite or micropredator: a coreid bug sucking plant sap

Human head-lice are directly transmitted obligate ectoparasites.

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 pests by humans. These include parasites of people and livestock, such as lice and 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.

Pollination of flowering plants by insects including bees, butterflies, 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 honey,[164] wax,[165][166] lacquer[167] and silk.[168] Honey bees have been cultured by humans for thousands of years for honey.[169] Beekeeping in pottery vessels began about 9,000 years ago in North Africa.[170] The 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, aphids feed on crops, causing economic loss, but 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 fecundity, the common fruit fly Drosophila melanogaster is a model organism for studies in the genetics of eukaryotes, including genetic linkage, interactions between genes, chromosomal genetics, development, behavior and evolution. Because genetic systems are well conserved among eukaryotes, understanding basic cellular processes like DNA replication or 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.[192][193] In Africa, locally abundant species of locusts and termites are a common traditional human food source.[194] Some, especially 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

Black soldier fly larvae can provide protein and fats for use in cosmetics.[198] Insect cooking oil, insect butter and fatty alcohols can be made from such insects as the superworm (Zophobas morio).[199] Insect species including the black soldier fly or the housefly in their maggot forms, and beetle larvae such as 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

Scarab beetles held religious and cultural symbolism in ancient Egypt, Greece and some shamanistic Old World cultures. The ancient Chinese regarded cicadas as symbols of rebirth or immortality. In Mesopotamian literature, the epic poem of Gilgamesh has allusions to Odonata that signify the impossibility of immortality. Among the Aborigines of Australia of the Arrernte language groups, honey ants and witchetty grubs served as personal clan totems. In the case of the 'San' bush-men of the Kalahari, it is the praying mantis that holds much cultural significance including creation and zen-like patience in waiting.[202]

See also

Entomology

Ethnoentomology

Flying and gliding animals

Insect-borne diseases

Notes

^ The Museum of New Zealand notes that "in everyday conversation", bug "refers to land arthropods with at least six legs, such as insects, spiders, and centipedes".[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 and pillbugs).[6]

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^ a b "Insects as Food for Humans". Retrieved 14 September 2022.

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Sources

Gullan, P. J.; Cranston, P. S. (2005). The Insects: An Outline of Entomology (3rd ed.). Oxford: Blackwell Publishing. ISBN 978-1-4051-1113-3.

Gullan, P. J.; Cranston, P. S. (2014). The Insects: An Outline of Entomology (5th ed.). Oxford: Wiley Blackwell. ISBN 978-1-118-84616-2.

Nation, James L. (2001). Insect Physiology and Biochemistry (1st ed.). CRC Press. ISBN 978-0-8493-1181-9.

Resh, Vincent H.; Carde, Ring T. (2009). Encyclopedia of Insects (2 ed.). Academic Press. ISBN 978-0-12-374144-8.

Schowalter, Timothy Duane (2006). Insect Ecology: An Ecosystem Epproach (2nd (illustrated) ed.). Academic Press. ISBN 978-0-12-088772-9. Archived from the original on 3 June 2016. Retrieved 27 October 2015.

External links

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Insect species and observations on iNaturalist

Overview of Orders of Insects

"Insect" at the Encyclopedia of Life

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Tree of Life Project – Insecta, Insecta Movies

Fossil Insect Database: Holotypes at the International Palaeoentological Society

UF Book of Insect Records

InsectImages.org 24,000 high resolution insect photographs

vteExtant Arthropoda classes by subphylum

Kingdom Animalia

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(unranked) Bilateria

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Chelicerata

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Insecta (insects)

italic are paraphyletic groups

vteInsect orders

Kingdom: Animalia

Phylum: Arthropoda

(unranked): Pancrustacea

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ExtantMonocondylia

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Four most speciose orders are marked in bold

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Based on Sasaki et al. (2013)

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Wikidata: Q1390

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ITIS: 99208

NCBI: 50557

NZOR: dd35a426-1309-4fef-b664-68c32e849434

Open Tree of Life: 1062253

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Insects Pictures & Facts

Skip to contentNewslettersSubscribeMenuInsects Pictures & FactsFeaturedAbout InsectsAll insects belong to the phylum Arthropoda. But unlike other arthropods—like lobsters, spiders, or millipedes—insects have three pairs of jointed legs, segmented bodies, an exoskeleton, one pair of antennae, and (usually) one or two pairs of wings.

Insects live in nearly every habitat, and it’s estimated that there are currently 10 quintillion insects on the globe. So far scientists who study bugs, called entomologists, have named one million insect species but studies estimate that four million are still uncategorized.

The oldest insect fossil—a mandible (or jaw) found in Scotland—is between 408 and 438 million years old. The oldest winged fossil dates back 330 million years ago, suggesting that insects were among the first animals to leave the oceans for land during the Devonian period some 400 million years ago.

Insects are vital to every ecosystem. They pollinate plants, decompose plant and animal matter, and are themselves a source of food. Birds alone are estimated to eat 400 to 500 million tons of insects per year.

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Beginning

1Insect bodies

2Physiology

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2.1Respiratory and circulatory systems

2.2How insects grow

3Evolutionary history

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3.1Origin of insects

3.2Origin of wings

4Kinds of insects

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6.1Pesticides

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Insect

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InsectaTemporal range: Ordovician ~479 mya to present [1]

Clockwise from top left: dance fly, long-nosed weevil, mole cricket, wasp, emperor gum moth, assassin bug

Scientific classification

Domain:

Eukaryota

Kingdom:

Animalia

Phylum:

Arthropoda

Clade:

Pancrustacea

Subphylum:

Hexapoda

Class:

InsectaLinnaeus

Subgroups

See text.

Synonyms

Ectognatha

Entomida

Insects are a class in the phylum Arthropoda.[2] They are small terrestrial invertebrates which have a hard exoskeleton.

Insects are the largest group of animals on Earth by far: about 926,400 different species have been described.[3] They are more than half of all known living species.[4][4][5][6][7] They may be over 90% of animal species on Earth.[8]

New species of insects are continually being found.[9] Estimates of the total number of species range from 2 million to 30 million.[3]

All adult insects have six legs; and most have wings. Insects were the first animals capable of flight.

As they develop from eggs, insects undergo metamorphosis. Insects live all over the planet: almost all are terrestrial (live on land). Few insects live in the oceans or in very cold places, such as Antarctica. The most species live in tropical areas.

Some people call all insects "bugs", but this is not correct. Only some insects are true bugs, which is a particular order of insects. People who study insects are called entomologists.

Insect bodies[change | change source]

Insect anatomy A- Head B- Thorax C- Abdomen 1. antenna 2. ocelli (lower) 3. ocelli (upper) 4. compound eye 5. brain (cerebral ganglia) 6. prothorax 7. dorsal blood vessel 8. tracheal tubes (trunk with spiracle) 9. mesothorax 10. metathorax 11. forewing 12. hindwing 13. mid-gut (stomach) 14. dorsal tube (Heart) 15. ovary 16. hind-gut (intestine, rectum & anus) 17. anus 18. oviduct 19. nerve chord (abdominal ganglia) 20. Malpighian tubes 21. tarsal pads 22. claws 23. tarsus 24. tibia 25. femur 26. trochanter 27. fore-gut (crop, gizzard) 28. thoracic ganglion 29. coxa 30. salivary gland 31. subesophageal ganglion 32. mouthparts .

Insects have exoskeletons (skeletons on the outside). Their skeletons are made out of thin, hard pieces or plates, like armour, made of chitin. All together, these pieces make a hard layer around the insect's body. The exoskeleton protects the insect.

The body of an insect has three main parts: a head, a thorax, and an abdomen. On the head are an insect's compound eyes, its two antennae (they feel and smell things), and its mouth.

On the thorax, insects have wings and legs. All insects have six legs (three pairs of jointed legs) and usually four wings (two pairs).

The abdomen is the back part of the insect. Inside the abdomen is the stomach, the heart, and the excretory system where body wastes pass out of the insect. Bees also have a stinger at the back of the abdomen.

Physiology[change | change source]

Just like our muscles connect to our bones to make us walk and stand up, the muscles of an insect connect to the exoskeleton to make it walk and move. Their muscles are on the inside of their skeleton.

Insects are cold-blooded, which means that they cannot control their body temperature.[10] This means that insects are not good at surviving the cold, at any rate out in the open. In the winter, many insects go into something called diapause, which is the insect version of hibernation. Some insects, like cockroaches, cannot go into diapause and they will die if it gets too cold outside. This is why cockroaches love living in people's warm houses.

Respiratory and circulatory systems[change | change source]

Tracheal system of a cockroach. The largest tracheae run across the width of the body and are horizontal in this image. Scale bar: 2 mm

The tracheal system branches into ever smaller tubes. here they supply the crop of the cockroach. Scale bar: 2 mm

Insect respiration happens without lungs. There is a system of internal tubes and sacs through which gases diffuse or are actively pumped. Air is taken in through openings on the sides of the abdomen called spiracles. Oxygen gets to tissues that need it through their trachea (element 8 in diagram).

Many insect larvae live in water. Many of those have gills that can extract oxygen dissolved in water. Others must rise to the water surface to get air which may be held or trapped in special parts of their body.[11]

Adult insects use oxygen at a high rate when they fly. They need it for the flight muscles, the most active tissue known in biology.[12] The flight muscles use oxygen at a huge rate: 100 ccs of oxygen for every single cc of tissue per hour.[13] With this system, the greatest diameter a muscle could have (and still consume oxygen at this rate) is about 0.5 cm.[12] Even with special extra arrangements, insects cannot get larger than about 11 cm long. The largest insect bodies are about as big as a mouse.[13]

Some insects also use a molecule called haemocyanin, which does the same job as haemoglobin does in vertebrates (but less efficiently). The insect circulatory system has no veins or arteries. The 'blood' is called haemolymph, and moves around in the space called the haemocoel. The organs sit in the haemocoel and are bathed in the haemolymph. The 'heart' is little more than a single tube which pulses (squeezes).[14]:61–65[15]

How insects grow[change | change source]

A mantis nymph looks just like a mantis adult but much smaller.

Insects start life as an egg. Usually a female (mother) insect lays eggs, but a few species have live birth (the eggs develop inside the mother). The eggs are small; but they can usually be seen with the naked eye.

Although the adults are larger, they do need a magnifying glass or a binocular microscope to see the details. A professional entomologist uses a binocular microscope to identify insects, plus a printed reference work.[16] There are far too many insects for anyone to remember them all, and most entomologists specialise in just one or two orders.

After the eggs hatch, two kinds of development may occur. Some insects have what is called 'incomplete metamorphosis'. This means that a small insect, called a nymph comes out of the egg, and the nymph looks almost the same as the adult insect. As the nymph grows, it does not change the way it looks, but only how big it is. It goes through a number of stages, called 'instars'. Grasshoppers grow in this way.

Other insects have complete metamorphosis, which means that the small larva which comes out of the egg looks very different from the adult insect. Insects that have complete metamorphosis usually come out of the egg as a larva, which usually looks like a worm. The larva eats food and gets bigger until it turns into a pupa. Butterfly pupae (plural for pupa) are often inside cocoons. Inside the cocoon the insect changes the way it looks and often grows wings. When the cocoon opens, the adult insect comes out. Many insects have complete metamorphosis, for example beetles, butterflies and moths, and flies. The adult stage of development is called the imago.

Evolutionary history[change | change source]

Origin of insects[change | change source]

The oldest known insect fossil is the Devonian Rhyniognatha, from the 411 million year old Rhynie chert. It may have superficially resembled a modern-day silverfish insect. This species already possessed mandibles of a type associated with winged insects, suggesting that wings may already have evolved at this time. Thus, anatomical records suggest the first insects may have appeared earlier, in the Silurian period.[17][18] Genomic analysis puts their origin even further back in the Ordovician period.[1]

If Rhyniognatha is not an insect, then Rhyniella from the same place is the first known insect. Also 411 mya.

Origin of wings[change | change source]

In 2008, researchers uncovered what they believe is the world's oldest known full-body impression of a primitive flying insect, a 300 million-year-old specimen from the Carboniferous period.[19]

The origin of insect flight is unclear, since the earliest known winged insects 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. It seems the insects were not a particularly successful group of animals before they evolved wings.[3]

Upper Carboniferous and Lower Permian insect orders include both living groups and a number of Palaeozoic groups, now extinct. During this era, some giant dragonfly-like forms reached wingspans of 55 to 70 cm (22 to 28 in) making them far larger than any living insect.

This gigantism may have been due to higher atmospheric oxygen levels, which allowed increased respiratory efficiency. The lack of flying vertebrates could have been another factor. Many of the early groups became extinct during the Permian–Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago.[20]

Kinds of insects[change | change source]

A beetle (ladybird or ladybug). The red part is the hard front pair of wings, or elytra.

Different kinds of insects are put into groups called orders. There are about 29 insect orders. The biggest insect orders are listed below:

Beetles (order Coleoptera) have the front pair of wings changed into a hard shell to protect the back wings.

Butterflies and moths (order Lepidoptera) have large, often colourful wings.

Flies (order Diptera) have only two wings.

Ants, bees, and wasps (order Hymenoptera) sometimes have stingers and sometimes live in large colonies (like ant hills).

True bugs (order Hemiptera) have a mouth that is long and narrow, like a drinking straw. This kind of mouth is called a beak.

Grasshoppers (order Orthoptera) can usually jump with their legs. Eat grass and grain plants.

Odonata, dragonflies and damselflies are top predators of other insects. Both aquatic nymphs and flying adults are carnivorous.

Phasmatodea, the stick and leaf insects, is an order which is entirely based on camouflage. It includes the world's longest insect, Chan's megastick.

All these groups except one (Odonata) are strongly connected with plants as a source of food.[21]

Spiders, scorpions, and similar animals are not insects; they are arachnids. Arachnids are arthropods that have four pairs of legs. Centipedes are also arthropods, but not insects: they are in a subphylum called the Myriapoda.

Taxonomy[change | change source]

This taxonomy lists some of the better known groups of insects.

Archaeognatha (jumping bristletails)

Thysanura (silverfish or bristletails)

Palaeoptera (insects that cannot flex their wings over their abdomen)

Ephemeroptera (Mayflies)

Odonata

Anisoptera (dragonflies)

Zygoptera (damselflies)

Neoptera (insects that can flex their wings over their abdomen)

Exopterygota sensu stricto

Caloneurodea (extinct)

Titanoptera (extinct)

Protorthoptera (extinct)

Plecoptera (stone flies, about 1700 species)

Embioptera (webspinners, about 300 species)

Orthoptera (grasshoppers, crickets and locusts)

Zoraptera (one genus, about 30 species, resemble termites)

Dermaptera (earwigs)

Dictyoptera

Notoptera ~tentative~

Grylloblattidae (ice crawlers)

Mantophasmatidae (discovered in 2001, (gladiators)

Phasmatodea (stick insects, about 2500 species) ~tentative~

Blattaria (cockroaches)

Isoptera (termites)

Mantodea (mantids)

Paraneoptera

Psocoptera (booklice)

Thysanoptera (thrips)

Phthiraptera (lice)

Hemiptera (true bugs, 80.000 species)

Endopterygota or Holometabola (850,000 living species in eleven orders) [22]

Hymenoptera (ants, bees, wasps, sawflies)

Coleoptera (beetles)

Strepsiptera (parasites that mostly live inside other insects)

Raphidioptera (snakeflies)

Megaloptera

Neuroptera (net-winged insects, contains antlions for example)

Mecoptera (scorpionflies, may include fleas)

Siphonaptera (fleas)

Diptera (true flies)

Trichoptera (moth-like)

Lepidoptera (moths and butterflies)

Insects and people[change | change source]

Some insects can be pests to people in different ways. Some are parasites, such as lice and bed bugs. Some of these parasite insects spread diseases, for example mosquitoes spread malaria.

Many insects eat agricultural products (plants meant for people to eat). Locustss are an example of pest insects that eat plants in agriculture.

Some insects are used by us. Bees make honey. The larvae of some moths make silk, which people use to make clothing. In some parts of the world, people actually eat insects. Eating insects for food is called entomophagy.

Many bees and flies pollinate plants. This means the insects help the plants make seeds by moving pollen from one flower to another. Some good insects eat pest insects, such as lady beetles (or ladybirds or ladybugs) eating aphids. Many insects eat dead plants and animals.

Pesticides[change | change source]

People often use poisons called insecticides to kill pest insects. Insecticides do not always work. Sometimes the pest insects become resistant to the insecticides, which means the insecticides do not hurt them anymore. Both the Colorado potato beetle and the diamondback moth are insects that are resistant to many insecticides.

Insecticides do not only kill pest insects; sometimes many helpful insects are killed too. When helpful insects are killed, such as those that eat pest insects, the pest insects may come back in larger numbers than before because they are not being eaten by helpful insects anymore.

References[change | change source]

↑ 1.0 1.1 Misof B. and others 2014. Phylogenomics resolves the timing and pattern of insect evolution. Science 346 763-767. [1] doi:10.1126/science.1257570

↑ Or, if the Arthropods are regarded as a superphylum, then the Insecta is a phylum.

↑ 3.0 3.1 3.2 Grimaldi D. and Engel M.S. 2005. Evolution of the insects. Cambridge University Press. 11–15: How many species of insects? ISBN 0-521-82149-5

↑ 4.0 4.1 Chapman A.D. (2006). Numbers of living species in Australia and the World. Canberra: Australian Biological Resources Study. ISBN 978-0-642-56850-2. Archived from the original on 2009-06-09. Retrieved 2015-11-08.

↑ Wilson, E.O. "Threats to global diversity". Archived from the original on 20 February 2015. Retrieved 17 May 2009.

↑ Novotny, Vojtech; et al. (2002). "Low host specificity of herbivorous insects in a tropical forest". Nature. 416 (6883): 841–844. Bibcode:2002Natur.416..841N. doi:10.1038/416841a. PMID 11976681. S2CID 74583.

↑ Erwin, Terry L. (1997). Biodiversity at its utmost: tropical forest beetles. pp. 27–40. In: Reaka-Kudla M.L; Wilson D.E. and Wilson E.O. (ed.). Biodiversity II. Joseph Henry Press, Washington, D.C.{{cite book}}: CS1 maint: multiple names: editors list (link)

↑ Erwin, Terry L. (1982). "Tropical forests: their richness in Coleoptera and other arthropod species". Coleopt. Bull. 36: 74–75.

↑ Hall, Derek 2005. Encyclopedia of insects & spiders. Grange Books. ISBN 1-84013-793-2 / 1-84013-793-2

↑ Although most social insects can control the temperature of their hive or nest.

↑ Merritt R.W; KW Cummins K.W. & Berg M.B. (2007). An introduction to the aquatic insects of North America. Kendall Hunt Publishing Company. ISBN 978-0-7575-4128-5.{{cite book}}: CS1 maint: multiple names: authors list (link)

↑ 12.0 12.1 Weis-Foch T. 1964. Diffusion in insect wing-muscles, the most active tissue known. J. Experimental Biology 41, 229–256.

↑ 13.0 13.1 Alexander, R. McNeil 1971. Size and shape. London: Arnold. Institute of Biology's Studies in Biology #29, p21.

↑ Gullan, P.J. & Cranston P.S. 2005. The insects: an outline of entomology. 3rd ed, Oxford: Blackwell. ISBN 1-4051-1113-5

↑ Meyer, John R. (17 February 2006). "Circulatory System". NC State University: Department of Entomology, NC State University. p. 1. Archived from the original on 2009-09-27. Retrieved 2009-10-11.

↑ Either a key (a special book to helps identify insects) such as Richards O.W. 1977. Hymenoptera: Introduction and key to families (Handbooks for the identification of British insects). Royal Entomological Society, London; or a large reference work, such as Carde, Ring T. and Resh, Vincent H. eds 2003. Encyclopedia of Insects. Academic Press N.Y. ISBN 0-12-586990-8

↑ Engel, Michael S.; David A. Grimaldi (2004). "New light shed on the oldest insect". Nature. 427 (6975): 627–630. Bibcode:2004Natur.427..627E. doi:10.1038/nature02291. PMID 14961119. S2CID 4431205.

↑ Rice C.M.; et al. (1995). "A Devonian auriferous hot spring system, Rhynie, Scotland". Journal of the Geological Society, London. 152 (2): 229–250. Bibcode:1995JGSoc.152..229R. doi:10.1144/gsjgs.152.2.0229. S2CID 128977213.

↑ "Researchers discover oldest fossil impression of a flying insect". Newswise. Retrieved 2008-09-20.

↑ Rasnitsyn A.P. and Quicke, D.L.J. (2002). History of insects. Kluwer. ISBN 1-4020-0026-X.

↑ Southwood T.R.E. 1973. The insect-plant relationship – an evolutionary perpective. Symposium Royal Entomological Society London.

↑ Rolf G. Beutel & Hans Pohl (2006). "Endopterygote systematics – where do we stand and what is the goal (Hexapoda, Arthropoda)?". Systematic Entomology. 31 (2): 202–219. doi:10.1111/j.1365-3113.2006.00341.x. S2CID 83714402.

Hoell H.V; Doyen J.T. & Purcell A.H. 1998. Introduction to insect biology and diversity. 2nd ed, Oxford University Press. ISBN 0-19-510033-6

Wikispecies has information on: Insecta.

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Other websites[change | change source]

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Animals & Nature

What Are Insects?

Classifying and Identifying Insects

Thomas J Peterson/ Photographer's Choice RF/ Getty Images

Animals & Nature

Insects

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Ants. Bees, & Wasps

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Debbie Hadley

Entomology Expert

B.A., Political Science, Rutgers University

Debbie Hadley is a science educator with 25 years of experience who has written on science topics for over a decade.

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Updated on December 10, 2019

Insects are the largest group in the animal kingdom. Scientists estimate there are over 1 million insect species on the planet, living in every conceivable environment from volcanoes to glaciers.

Insects help us by pollinating our food crops, decomposing organic matter, providing researchers with clues to a cancer cure, and even solving crimes. They can also harm us by spreading diseases and damaging plants and structures.

How Insects Are Classified

Insects are arthropods. All animals in the phylum Arthropoda have hard external skeletons called exoskeletons, segmented bodies, and at least three pairs of legs. Other classes that belong to the phylum Arthropoda include:

Arachnida (spiders)

Diplopoda (millipedes)

Chilopoda (centipedes)

The class Insecta encompasses all of the insects on the earth. It is most often divided into 29 orders. These 29 orders use the physical characteristics of the insects to group similar insect families.

Some insect taxonomists organize the insects differently, using evolutionary links instead of physical traits. For the purpose of identifying an insect, it makes more sense to use the system of 29 orders, since you can see the physical similarities and differences between insects you observe.

Here is an example of how an insect, the monarch butterfly, is classified:

Kingdom Animalia: the animal kingdom

Phylum Arthropoda: arthropods

Class Insects: insects

Order Lepidoptera: butterflies and moths

Family Nymphalidae: brush-footed butterflies

Genus Danaus

Species plexippus

The genus and species names are always italicized and used together to give the scientific name of the individual species. An insect species may occur in many regions and may have different common names in other languages and cultures.

The scientific name is a standard name that is used by entomologists around the world. This system of using two names (genus and species) is called binomial nomenclature.

Basic Insect Anatomy

As you may remember from elementary school, the most basic definition of an insect is an organism with three pairs of legs and three body regions: head, thorax, and abdomen.

Entomologists, scientists who study insects, might also add that insects have a pair of antennae and external mouthparts. As you learn more about insects, you will find there are some exceptions to these rules.

The Head Region

The head region is at the front of the insect’s body and contains the mouthparts, antennae, and eyes.

Insects have mouthparts designed to help them feed on different things. Some insects drink nectar and have mouthparts modified into a tube called a proboscis to suck up liquid. Other insects have chewing mouthparts and eat leaves or other plant matter. Some insects bite or pinch, and others pierce and suck blood or plant fluids.

The pair of antennae may have obvious segments or look like a feather. They come in different forms and are a clue to identifying the insect. Antennae are used to perceive sounds, vibrations, and other environmental factors.

Insects can have two types of eyes: compound or simple. Compound eyes are usually large with many lenses, giving the insect a complex image of its surroundings. A simple eye contains just a single lens. Some insects have both kinds of eyes.

The Thorax Region

The thorax, or middle region of an insect’s body, includes the wings and legs. All six legs are attached to the thorax. The thorax also contains the muscles that control movement.

All insect legs have five parts. Legs can be different shapes and have different adaptations to help the insect move within its unique habitat. Grasshoppers have legs designed for jumping, while honey bees have legs with special baskets to hold pollen as the bee moves from flower to flower.

Wings also come in different shapes and sizes and are another important clue to help you identify an insect. Butterflies and moths have wings made of overlapping scales, often in brilliant colors. Some insect wings appear transparent, with just a web of veins to identify their shape. When at rest, insects like beetles and praying mantids keep their wings folded flat against their bodies. Other insects hold their wings vertically, like butterflies and damselflies.

The Abdomen Region

The abdomen is the final region in the insect body and contains the insect’s vital organs. Insects have digestive organs, including a stomach and intestines, to absorb nutrients from their food and separate waste matter. The sexual organs of the insect are also in the abdomen. Glands that secrete pheromones for marking the insect’s trail or attracting a mate are in this region as well.

Take a Closer Look

The next time you observe a lady beetle or a moth in your yard, stop and take a closer look. See if you can distinguish the head, thorax, and abdomen. Look at the shape of the antennae, and watch how the insect holds its wings. These clues will help you identify a mystery insect, and provide information about how the insect lives, feeds and moves.

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A Guide to the 29 Insect Orders

The Hexapods

10 Ways to Identify an Insect

What Are Arachnids?

Insects: The Most Diverse Animal Group in the Planet

The Malacostraca Family: Crabs, Lobsters, and Their Relatives

Overview of the Biggest Bugs That Ever Lived

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Daddy Longlegs: Arachnids, but Not Spiders

Crustaceans: Species, Characteristics, and Diet

Do Insects Have Brains?

Habits and Traits of Centipedes, Class Chilopoda

Types of Insect Fossils

10 Fascinating Facts About Caterpillars

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What dominates life on Earth?InsectsStructure and Function of InsectsInsect FlightInsect ReproductionInsect BehaviorKQED: Ants: The Invisible MajorityKQED: Ladybugs: A Population of MillionsInsects and HumansKQED: Better Bees: Super Bee and Wild BeeSummaryReview

What dominates life on Earth?

Well, by numbers, it's not humans. This may look like a scary creature from your worst nightmare, but it wouldn’t hurt a fly. In fact, it is a fly! The picture shows the charming portrait of a horsefly, up close and personal. Those big, striped, colorful orbs are its eyes. Did you ever look through a kaleidoscope? If so, then you have an idea of what the world looks like to a horsefly.

What other organs do insects like this horsefly have? Besides sensing their environment, what other functions do their organs serve?

Insects

Most members of the subphylum Hexapoda are insects (class Insecta). In fact, more than half of all known organisms are insects. There may be more than 10 million insect species in the world, most of them yet to be identified. It’s clear that insects, and not humans, dominate life on Earth.

Hexapoda. A cricket on green leaf. Can you find the six legs attached to the thorax?

Structure and Function of Insects

Insects range in length from less than a millimeter to about the length of your arm. They can be found in most habitats, but they are mainly terrestrial. Many can fly, so they are also aerial. Like other arthropods, insects have a head, thorax, and abdomen. They have a wide variety of appendages, including six legs attached to the thorax.

Insects have a pair of antennae for “smelling” and “tasting” chemicals. Some insects can also use their antennae to detect sound. Other sensory organs on the head include several simple eyes and a pair of compound eyes. The compound eyes let insects see images. Butterflies and bees can even see in color. For feeding, the head contains one pair of mandibles and two pairs of maxillae. Insects consume a wide range of foods, and their mouthparts have become specialized. Several variations are shown in Figure below.

Mouthpart Specialization in Insects. The mouthparts of insects are adapted for different food sources. How do you think the different mouthparts evolved? (CC BY-NC 3.0; Christopher Auyeung - CK-12 Foundation).

An insect’s abdomen contains most of the internal organs. Like other arthropods, insects have a complete digestive system. They also have an open circulatory system and central nervous system. Like other terrestrial arthropods, they have trachea for breathing air and Malpighian tubules for excretion.

Insect Flight

The main reason that insects have been so successful is their ability to fly. Insects are the only invertebrates that can fly and were the first animals to evolve flight. Flight has important advantages. It’s a guaranteed means of escape from nonflying predators. It also aids in the search for food and mates.

Insects generally have two pairs of wings for flight. Wings are part of the exoskeleton and attached to the thorax. Insect wings show a lot of variation. As you can see in Figure below, butterfly wings are paper-thin, whereas beetle wings are like armor. Not all insect wings work the same way, either. They differ in how the muscles are attached and whether the two pairs of wings work independently or together. Besides flight, wings serve other functions. They may protect the body (beetles), communicate visually with other insects (butterflies), or produce sounds to attract mates (katydids).

Form and Function in Insect Wings. Beetles, butterflies, and katydids all have two pairs of wings that they use for flight. However, the wings are very different because they have other functions as well.

Insect Reproduction

Nearly all insects reproduce sexually. Some can also reproduce asexually. An example of an insect life cycle is shown in Figure below.

Insect Life Cycle. This diagram represents the life cycle of a mosquito. Most insects have a similar life cycle.

When an insect egg hatches, a larva emerges. The larva eats and grows and then enters the pupa stage. The pupa is immobile and may be encased in a cocoon. During the pupa stage, the insect goes through metamorphosis. Tissues and appendages of the larva break down and reorganize into the adult form. How did such an incredible transformation evolve? Metamorphosis is actually very advantageous. It allows functions to be divided between life stages. Each stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stage.

Insect Behavior

Insects are capable of a surprising range of behaviors. Most of their behaviors, such as flying and mating, are instinctive. These are behaviors that don’t need to be learned. They are largely controlled by genes. However, some insect behaviors are learned. For example, ants and bees can learn where food is located and keep going back for more.

Many species of insects have evolved complex social behaviors. They live together in large, organized colonies (see Figure below). This is true of ants, termites, bees, and wasps. Colonies may include millions of individual insects. Colony members divide up the labor of the colony. Different insects are specialized for different jobs. Some reproduce, while others care for the young. Still others get food or defend the nest.

Termite Nest. This cathedral-like structure is the nest of a huge colony of termites in Australia. In fact, it is the world’s largest known termite nest. It towers 7.5 meters (25 feet) above the ground and houses millions of termites.

Living in a large colony requires good communication. Ants communicate with chemicals called pheromones. For example, an ant deposits pheromones on the ground as it returns to the nest from a food source. It is marking the path so other ants can find the food. Honeybees communicate by doing a “waggle dance.”

KQED: Ants: The Invisible Majority

Most of us think ants are just pests. But not Brian Fisher. Known as “The Ant Guy,” he's on a mission to show the world just how important and amazing these little creatures are. In the process, he hopes to catalog all of the world's 30,000 ant species before they become casualties of habitat loss.

KQED: Ladybugs: A Population of Millions

Ladybugs, also known as ladybird beetles, have a life cycle of four to six weeks. In one year as many as six generations of ladybird beetles may hatch. In the spring, each adult female lays up to 300 eggs in small clusters on plants where aphids are present. After a week the wingless larvae hatch. Both the ladybird beetle larvae and adults are active predators, eating only aphids, scales, mites and other plant-eating insects. The ladybugs live on the vegetation where their prey is found, which includes roses, oleander, milkweed and broccoli. Adult ladybugs don’t taste very good. A bird careless enough to try to eat one will not swallow it.

By late May to early June, when the larvae have depleted the food supply, the adults migrate to the mountains. There, they eat mainly pollen. The ladybugs gain fat from eating the pollen and this tides them over during their nine-month hibernation. Thousands of adults hibernate overwinter in tight clusters, called aggregates, under fallen leaves and ground litter near streams. In the clear, warmer days of early spring, the ladybugs break up the aggregates and begin several days of mating.

Insects and Humans

Most humans interact with insects every day. Many of these interactions are harmless and often go unnoticed. However, insects cause humans a lot of harm. They spread human diseases. For example, the deadly bubonic plague of the middle ages was spread by fleas. Today, millions of people die each year from malaria, which is spread by mosquitoes. Insects also eat our crops. Sometimes they travel in huge swarms that completely strip the land of all plant material (see Figure below). On the other hand, we depend on insects for the very food we eat. Without insects to pollinate them, flowering plants—including many food crops—could not reproduce.

Locust Swarm. A swarm of locusts in the African country of Mauritania darkens the mid-day sky. The hungry insects will eat virtually all the plants in their path.

KQED: Better Bees: Super Bee and Wild Bee

Honeybees are one of the most well-known insects on the planet. Bees are naturalized on every continent except Antarctica. Honeybees have a highly developed social structure and depend on their community, or colony, for survival, with a colony containing up to 20,000 bees. When bees search plants for nectar, pollen sticks to the fuzzy hairs that cover their hind legs. At the next flower, some of the pollen rubs off and fertilizes that flower. In this way, bees help improve fruit production. Bees pollinate an estimated 130 different varieties of fruit, flowers, nuts and vegetables in the United States alone. Farmers obviously depend on bees to pollinate crops, such as fruit and nuts, but in recent years thousands of bee colonies have disappeared. This could be a devastating issue for farmers. Can anything be done? Meet two Northern California researchers looking for ways to make sure we always have bees to pollinate crops.

Summary

Insects are arthropods in the class Hexapoda. They are the most numerous organisms in the world.

Most insects are terrestrial, and many are aerial.

Insects have six legs and a pair of antennae for sensing chemicals. They also have several eyes and specialized mouthparts for feeding.

Insects are the only invertebrates than can fly. Flight is the main reason for their success.

Insects may live in large colonies and have complex social behaviors.

Insects spread disease and destroy crops. However, they are essential for pollinating flowering plants.

Review

List two traits that characterize insects.

State two important advantages of flight in insects.

Give examples of insect behavior.

Present facts and a logical argument to support the following statement: “Insects dominate life on Earth.”

Explain why distinctive life stages and metamorphosis are adaptive.

Diagram an insect life cycle.

This page titled 11.11: Insects is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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11.10: Arthropods

11.12: Echinoderms

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HomepageDiscover & LearnAnimal factsheetsInsectsWhat are insects?

What are insects?

Author(s)

David Britton

Updated

16/10/20

Read time

8 minutes

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What are insects?

Insects are arthropods

The insect body

Insect evolution

Types of insects

Why most animals are insects

In order to answer this question we must look at where insects fit in the animal kingdom. The animal kingdom is divided into several groups called phyla. An example of a phylum is the Chordata, which holds all the backboned animals. Insects belong to the phylum Arthropoda.

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Insect - Hairy grub SEM image

Image: Sue Lindsay

Insects are arthropods

Arthropods are characterised by having the following features:a hard external skeleton (called a exoskeleton)a segmented bodyat least three pairs of jointed legsThe Arthropoda is divided into a number of classes. These include the:Crustacea (crabs, crayfish, prawns)Arachnida (spiders, mites, scorpions)Myriapoda (millipedes & centipedes)Insecta (insects)

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Robber Fly

Image: Bruce Hulbert

Insect are successful and importantThe insects have proved to be the most successful arthropods. There are far more species in the class Insecta than in any other group of animals. These amazingly diverse animals have conquered all the environments on earth except for the frozen polar environments at the highest altitudes and in the immediate vicinity of active volcanoes.Insects are the only invertebrates (animals without backbones) with wings. Much of their success results from their ability to fly and colonise new habitats. The study of insects is called entomology and entomologists are scientists who study insects.Insects play a very important role in the web of life, in every environment. Some of their jobs include pollinating flowering plants, being a source of food for insectivorous animals and assisting in the decomposition of plants and animals.

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Insect classificationModern insect classification divides the Insecta into 29 orders, many of which have common names. Some of the more common orders are:Mantodea - praying mantidsBlattodea - cockroachesIsoptera - termitesSiphonaptera - fleasOdonata - dragonflies and damselfliesDermaptera - earwigsDiptera - fliesLepidoptera - butterflies and mothsOrthoptera - grasshoppers, katydids, cricketsColeoptera - beetlesHymenoptera - wasps, bees, ants, sawflies

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Insect Diagram

Image: Design Unit

The insect body

It is very difficult to provide a simple answer to the question: What external features characterise an insect? This is because the class Insecta is full of exceptions. It is not easy to produce a typical body plan for what most insects look like, but there are some very general features that most insects possess.Insect featuresThe insect body is divided into three main parts, the head, thorax and abdomen.Insects have no internal skeleton, instead they are covered in an external shell (exoskeleton) that protects their soft internal organs.No insect has more than three pairs of legs, except for some immature forms such as caterpillars that have prolegs. These are appendages that serve the purpose of legs.The typical insect mouth has a pair of lower jaws (maxillae) and upper jaws (mandibles) which are designed to bite. There are many variations to this structure, as many moths and butterflies have tubular sucking mouthparts, many bugs and other blood-sucking insects have sucking stabbing mouthparts and some adult insects simply don't have functional mouthparts.Insects have one pair of antennae located on the headMost insects have one or two pairs of wings although some insects such as lice, fleas, bristletails and silverfish are completely wingless.Together these features can help us distinguish insects from other arthropods.

Insect evolution

Insects are an ancient group of animals. The first insects probably appeared before the Devonian period (400 - 360 million years ago) and by the Carboniferous period (360 - 285 million years ago) had taken to the air.Adaptation to flight proved a highly successful strategy and during the Permian period (285 - 245 million years ago) insects achieved their greatest diversity. No other arthropod group has achieved flight. By the Permian, the basic physical structure of many of the modern orders of insects had evolved.The more recently evolved Hymenoptera (ants, bees, wasps and sawflies) and Lepidoptera (butterflies and moths) appear as fossils in the Jurassic period (210 - 145 million years ago). The Mantodea (praying mantids) appeared in Eocene period in fossilised amber (60 - 35 million years ago).

Types of insects

Moths and butterflies

The scale-winged insects, Order Lepidoptera.Moths, butterflies and skippers: Order LepidopteraWhat are the differences between butterflies and moths?Butterfly and moth sketches by the Scott family

Ants, wasps, bees and sawflies

The heavy-winged insects, Order Hymenoptera.Ants, Wasps, Bees and Sawflies: Order HymenopteraAnts: Family FormicidaeAnts as pestsWhat are the differences between ants and termites?Wasps: Suborder ApocritaWhat are the differences between flies and wasps?Bees: Suborder Apocrita

Grasshoppers, crickets, locusts and earwigs

The straight-winged insects, Order Orthoptera, and the skin-winged insects, Order Dermaptera.Grasshoppers, crickets, katydids and locusts: Order OrthopteraWhat do grasshoppers, crickets, katydids and locusts look like?

Flies, dragonflies and lacewings

The two-winged insects, Order Diptera; the toothed insects, Order Odonata; and the net-winged insects, Order Neuroptera.Flies and mosquitoes: Order DipteraWombat FliesTrue FliesLacewingsWhat do Lacewings look like?

Bugs, cicadas & beetles

The true bugs, Order Hemiptera & the sheath-winged insects, Order Coleoptera.What are the differences between bugs and beetles?Cicadas: Superfamily CicadoideaWhat do Cicadas look like?Beetles: Order Coleoptera

Cockroaches, termites, mantids and stick insects

The nett-winged insects, Order Dictyoptera; and the phantom insects, Order Phasmatodea.Cockroaches: Order BlattodeaWhat do Cockroaches look like?What are the differences between ants and termites?What do termites look like?Praying mantises: Order MantodeaWhat do praying mantids look like?Leaf and Stick Insects: Order PhasmatodeaCare of Stick Insects

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