Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. Arachnids are named after the mythological figure Arachne. They are chiefly terrestrial arthropods, comprising some 65,000 to 73,000 named species including spiders, scorpions, harvestmen, ticks, and mites.
Characteristics
Arachnids may be easily distinguished from insects by the fact that arachnids have eight legs whereas insects have six. The chelicerae serve to macerate food particles. The first post-oral pair of appendages — pedipalps (leg-like mouthparts) — of some species have been adapted for sensory, prey capture or reproductive functions. In Solifugae, the palpi are quite leg-like and make Solifugae appear to have ten legs. The larvae of mites have only six legs; the fourth pair appears when they moult into nymphs. Arachnids are further distinguished by the fact they have no antennae and no wings. They have a two-segmented body, made up of a cephalothorax and an abdomen, the cephalothorax being derived from the fusion of the cephalon (head) and the thorax.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals.[citation needed] Many are venomous - they secrete poison from specialised glands to kill prey or enemies.[citation needed] Others are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immature adults.
[edit] Classification
Palpigradi
Araneae
Amblypygi
Thelyphonida
Schizomida
Ricinulei
Acari
Opiliones
Scorpionida
Pseudoscorpiones
Solifugae
Phylogeny of the Arachnida.[1][2]
Trigonotarbida - extinct
Amblypygi - "blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more
Araneae - spiders (40,000 species)
Mesothelae - very rare, basal spiders, with abdomen segmented and spinnerets median
Opisthothelae - spiders with abdomen unsegmented and spinnerets located posteriorly
Araneomorphae - most common spiders
Mygalomorphae - tarantulas and tarantula-like spiders
Phalangiotarbida - extinct
Opiliones - phalangids, harvestmen or daddy-long-legs (6,300 species)
Palpigradi - microwhip scorpions
Pseudoscorpionida - pseudoscorpions
Ricinulei - ricinuleids, hooded tickspiders
Schizomida - "split middle" whip scorpions with divided exoskeletons
Scorpiones - scorpions (2,000 species)
Solifugae - solpugids, windscorpions, sun spiders or camel spiders (900 species)
Haptopoda - extinct
Uropygi - whip scorpions, with first legs modified as whip-like sensory organs and with a long thin tail at end of abdomen (100 species)
Acarina - mites and ticks (30,000 species)
Acariformes
Sarcoptiformes
Trombidiformes
Opilioacariformes
Parasitiformes - holothyrans, ticks and mesostigmatic mites
It is estimated that a total of 98,000 arachnid species have been described, and that there may be up to 600,000 in total, including undescribed species [3].
[edit] Acarina
Ixodes ricinus, a tickMain article: Acarina
Acarina or Acari are a taxon of arachnids that contains mites and ticks. Its fossil history goes back to the Devonian era. In most modern treatments, the Acari is considered a subclass of Arachnida and is composed of 2-3 orders or superorders: Acariformes, Parasitiformes, and Opilioacariformes. Most acarines are minute to small (e.g. 0.080-1.00mm), but the giants of the Acari (some ticks and red velvet mites) may reach lengths of 10-20mm. It is estimated that over 50,000 species have been described (as of 1999) and that a million or more species are currently living. The study of mites and ticks is called acarology[4].
Only the faintest traces of primary segmentation remain in mites, the prosoma and opisthosoma being insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the capitulum or gnathosoma and is also found in the Ricinulei. The remainder of the body is called the idiosoma and is unique to mites. Most adult mites have four pairs of legs, like other arachnids, but some have fewer. For example, gall mites like Phyllocoptes variabilis (superfamily Eriophyioidea) have a wormlike body with only two pairs of legs; some parasitic mites have only one or three pairs of legs in the adult stage. Larval and prelarval stages have a maximum of three pairs of legs; adult mites with only three pairs of legs may be called 'larviform'.
Acarine ontogeny consists of an egg, a prelarval stage (often absent), a larval stage (hexapod except in Eriophyoidea which have only 2 pairs of legs), and a series of nymphal stages. Larvae (and prelarvae) have a maximum of 3 pairs of legs (legs are often reduced to stubs or absent in prelarvae); legs IV are added at the first nymphal stage.
Acarines live in practically every habitat, and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other arthropods in the soil organic matter and detritus. Many are parasitic, and they affect both vertebrates and invertebrates. Most parasitic forms are external parasites, while the free living forms are generally predaceous and may even be used to control undesirable arthropods. Others are detritivores that help to break down forest litter and dead organic matter such as skin cells. Others still are plant feeders and may damage crops. Damage to crops is perhaps the most costly economic effect of mites, especially by the spider mites and their relatives (Tetranychoidea), earth mites (Penthaleidae), thread-footed mites (Tarsonemidae) and the gall and rust mites (Eriophyoidea). Some parasitic forms affect humans and other mammals, causing damage by their feeding, and can even be vectors of diseases such as scrub typhus and rickettsial pox. A well known effect of mites on humans is their role as an allergen and the stimulation of asthma in people affected by the repiratory disease. The use of predatory mites (e.g. Phytoseiidae) in pest control and herbivorous mites that attack weeds are also of importance. An unquantified, but major positive contribution of the Acari is their normal functioning in ecosystems, especially their roles in the decomposer subsystem [4].
[edit] Amblypygi
An amblypygidMain article: Amblypygi
Amblypygids are also known as tailless whip scorpions. Approximately 5 families, 17 genera and 136 species have been described. They are found in tropical and subtropical regions worldwide. Some species are subterranean; many are nocturnal. During the day, they may hide under logs, bark, stones, or leaves. They prefer a humid environment. Amblypygids may range from 5 to 40 mm. Their bodies are broad and highly flattened and the first pair of legs (the first walking legs in most arachnid orders) are modified to act as sensory organs. (Compare solifugids, uropygids, and schizomids.) These very thin modified legs can extend several times the length of body. They have no silk glands or venomous fangs, but can have prominent pincer-like pedipalps. Amblypygids often move about sideways on their six walking legs, with one "whip" pointed in the direction of travel while the other probes on either side of them. Prey are located with these "whips", captured with pedipalps, then torn to pieces with chelicerae. Fossilised amblypygids have been found dating back to the Carboniferous period.
Amblypygids, particularly the species Phrynus marginemaculatus and Damon diadema, are thought to be one of the few species of arachnids that show signs of social behavior. Research conducted at Cornell University by entomologists suggests that mother amblypygids comfort their young by gently caressing the offspring with her feelers. Further, when two or more siblings were placed in an unfamiliar environment, such a cage, they would seek each other out and gather back in a group [5].
[edit] Araneae
Araneus diadematusMain article: spider
Spiders are the most numerous and familiar of the arachnids. All spiders produce silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in webs, although there are many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, and temporarily hold sperm, among other applications.
All spiders except those in the families Uloboridae and Holarchaeidae, and in the suborder Mesothelae (together about 350 species) can inject venom to protect themselves or to kill and liquefy prey. Only about 200 species, however, have bites that can pose health problems to humans [6]. Many larger species' bites may be painful, but will not produce lasting health concerns.
Spiders are found all over the world, from the tropics to the Arctic, living underwater in silken domes they supply with air, and on the tops of mountains.
[edit] Haptopoda
Main article: Haptopoda
Haptopoda is an extinct order known exclusively from a few specimens from the Upper Carboniferous of the United Kingdom. It is monotypic, i. e., has only one species, Plesiosiro madeleyi Pocock 191. Relationships with other arachnids are obscure, but closest relatives may be the Amblypygi, Thelyphonida and Schizomida of the tetrapulmonate clade [7].
[edit] Opiliones
Paroligolophus agrestisMain article: Opiliones
Opiliones (better known as "harvestmen") are arachnids which are harmless to people and are known for their exceptionally long walking legs, compared to their body size. As of 2005, over 6,300 species of Phalangids have been discovered worldwide. The order Opiliones can be divided in four suborders: Cyphophthalmi, Eupnoi, Dyspnoi and Laniatores. Well-preserved fossils have been found in the 400-million year old Rhynie cherts of Scotland, which looks surpringly modern, indicating that the basic structure of the harvestmen hasn't changed much since then. Their closest relatives are probably the mites (Acari).
The difference between harvestmen and spiders is that in harvestmen the two main body sections (the abdomen with ten segments and cephalothorax, or prosoma and opisthosoma) are nearly joined, so that they appear to be one oval structure. In more advanced species, the first five abdominal segments are often fused into a dorsal shield called the scutum, which is normally fused with the carapace. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates laying next to each other. The second pair of legs are longer than the others and works as antennae. They have a single pair of eyes in the middle of their heads, orientated sideways. They have a pair of prosomatic scent glands that secrete a peculiar smelling fluid when disturbed. Harvestmen do not have silk glands and do not possess poison glands, posing absolutely no danger to humans. They breathe through tracheae. Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located, one opening on each side. In more active species, spiracles are also found upon the tibia of the legs. They have a gonopore on the ventral cephalothorax, and the copulation is direct as the male has a penis (while the female has an ovipositor).
Typical body length does not exceed 7 mm (about ¼ in) even in the largest species. However, leg span is much larger and can exceed 160 mm (over 6 in). Most species live for a year. Many species are omnivorous, eating primarily small insects and all kinds of plant material and fungi; some are scavengers of the decays of any dead animal, bird dung and other fecal material. Mating involves direct copulation, rather than the deposition of a spermatophore. They are mostly nocturnal and coloured in hues of brown, although there are a number of diurnal species which have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
[edit] Palpigradi
Main article: Palpigradi
Palpigradi, commonly known as "microwhip scorpions", are tiny cousins of the uropygid, or whip scorpion, no more than 3 mm in length. They have a thin, pale, segmented carapace which terminates in a whip-like flagellum, made up of 15 segments. The carapace is divided into two plates between the third and fourth leg set. They have no eyes. Some species have three pairs of book lungs, while others have no lungs at all. Approximately 80 species of Palpigradi have been described worldwide, all in the family Eukoeneniidae, which contains four genera.
They are believed to be predators like their larger relatives, feeding on minuscule insects in their habitat. Their mating habits are unknown, except that they lay only a few relatively large eggs at a time. Microwhip scorpions need a damp environment to survive, and they always hide from light, so they are commonly found in the moist earth under buried stones and rocks. They can be found on every continent, except in arctic and antarctic regions.
[edit] Phalangiotarbida
Main article: Phalangiotarbida
Phalangiotarbi (Haase, 1890) is an extinct arachnid order known exclusively from the Upper Carboniferous of Europe and North America.
The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida has been recently proposed to be sister group to (Palpigradi+Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). (Pollitt et al., 2004).
[edit] Pseudoscorpiones
A pseudoscorpion on a printed pageMain article: pseudoscorpion
Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of scorpions. They range from 2 to 8 mm (1⁄12 to ⅓ inch) in length [8]. The opisthosoma is made up of twelve segments, each guarded by plate-like tergites above and sternites below. The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions. The colour of the body can be yellowish-tan to dark-brown, with the paired claws often a contrasting colour. They may have two, four or no eyes. They have two very long palpal chelae (pedipalps or pincers) which strongly resemble the pincers found on a scorpion. The pedipalps generally consist of an immobile "hand" and "finger", with a separate movable finger controlled by an adductor muscle. A venom gland and duct are usually located in the mobile finger; the poison is used to capture and immobilise the pseudoscorpion's prey. During digestion, pseudoscorpions pour a mildly corrosive fluid over the prey, then ingest the liquefied remains. Pseudoscorpions spin silk from a gland in their jaws to make disk-shaped cocoons for mating, molting, or waiting out cold weather. Another trait they share with their closest relatives, the spiders, is breathing through spiracles. Most spiders have one pair of spiracles, and one of book lungs, but pseudoscorpions do not have book lungs.
There are more than 2,000 species of pseudoscorpions recorded. They range worldwide, even in temperate to cold regions, but have their most dense and diverse populations in the tropics and subtropics. The fossil record of pseudoscorpions dates back over 380 million years, to the Devonian period, near the time when the first land-animal fossils appear.
During the elaborate mating dance, the male of some pseudoscorpion species pulls a female over a spermatophore previously laid upon a surface [9]. In other species, the male also pushes the sperm into the female genitals using the forelegs [10].The female carries the fertilised eggs in a brood pouch attached to her abdomen, and the young ride on the mother for a short time after they hatch [8]. Up to two dozen young are hatched in a single brood; there may be more than one brood per year. The young go through three molts over the course of several years before reaching adulthood. Adult pseudoscorpions live 2 to 3 years. They are active in the warm months of the year, overwintering in silken coccoons when the weather grows cold.
Pseudoscorpions are generally beneficial to humans since they prey on clothes moth larvae, carpet beetle larvae, booklice, ants, mites, and small flies. They are small and inoffensive, and are rarely seen due to their size. They usually enter the home by "riding along" with larger insects (known as phoresy), or are brought in with firewood. They are often observed in bathrooms or laundry rooms, since they seek humidity. They may sometimes be found feeding on mites under the wing covers of certain beetles.
[edit] Ricinulei
Main article: Ricinulei
Riniculei (hooded tickspiders) are 5–10 mm long. Their most notable feature is a "hood" which can be raised and lowered over the head; when lowered, it covers the mouth and the chelicerae. Ricinulei have no eyes. The pedipalps end in pincers that are small relative to their bodies, when compared to those of the related orders of scorpions and pseudoscorpions. The heavy-bodied abdomen forms a narrow pedicel, or waist, where it attaches to the prosoma. In males, the third pair of legs are modified to form copulatory organs. Malpighian tubules and a pair of coxal glands make up the excretory system. They have no lungs, as gas exchange takes place through the trachea.
Ricinulei are predators, feeding on other small arthropods. Little is known about their mating habits; the males have been observed using their modified third leg to transfer a spermatophore to the female. The eggs are carried under the mother's hood, until the young hatch into six-legged "larva", which later molt into their adult forms. Ricinulei require moisture to survive. Approximately 57 species of ricinuleids have been described worldwide, all in a single family which contains 3 genera.
[edit] Schizomida
Main article: Schizomida
Schizomida is an order of arachnids which tend to live in the top layer of soils. Schizomids present the prosoma covered by a large protopeltidium and smaller, paired, mesopeltidia and metapeltidia. There are no eyes. The opisthosoma is a smooth oval of 12 recognisable somites. The first is reduced and forms the pedicel. The last three are much constricted, forming the pygidium. The last somite bears the flagellum, which in this order is short and consists of not more than four segments.
The name means "split or cleaved middle", referring to the way the cephalothorax is divided into two separate plates. Like the related orders Uropygi, Amblypygi, and Solpugida, the schizomids use only six legs for walking, having modified their first two legs to serve as sensory organs. They also have large well-developed pedipalps (pincers) just behind the sensory legs.
[edit] Scorpiones
Scorpio maurus palmatusMain article: scorpion
Scorpions are characterised by a metasoma (tail) comprising six segments, the last containing the scorpion's anus and bearing the telson (the sting). The telson, in turn, consists of the vesicle, which holds a pair of venom glands and the hypodermic aculeus, the venom-injecting barb. The abdomen's front half, the mseosoma, is made up of six segments. The first segment contains the sexual organs as well as a pair of vestigial and modified appendages forming a structure called the genital operculum. The second segment bears a pair of featherlike sensory organs known as the pectines; the final four segments each contain a pair of book lungs. The mesosoma is armored with chitinous plates, known as tergites on the upper surface and sternites on the lower surface.
The cuticle of scorpions is covered with hairs in some places that act like balance organs. An outer layer that makes them fluorescent green under ultraviolet light is called the hyaline layer. Newly molted scorpions do not glow until after their cuticle has hardened. The fluoresent hyaline layer can be intact in fossil rocks that are hundreds of millions of years old.
Scorpions are opportunistic predators of small arthropods and insects. They use their chela (pincers) to catch the prey initially. Depending on the toxicity of their venom and size of their claws, they will then either crush the prey or inject it with neurotoxic venom. The neurotoxins consist of a variety of small proteins as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim. Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting, allowing for effective prey capture. Scorpion venoms are optimised for action upon other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). A few scorpion species, however, mostly in the family Buthidae, can be dangerous to humans. The scorpion which is responsible for the most human deaths is the Androctonus australis, or fat-tailed scorpion of North Africa. The toxicity of A. australis's venom is roughly half that of L. quinquestriatus, but since A. australis injects quite a bit more venom into its prey, it is the most deadly to humans. Human deaths normally occur in the young, elderly, or infirm; scorpions are generally unable to deliver enough venom to kill healthy adults. Some people, however may be allergic to the venom of some species, in which case the scorpion's sting can more likely kill. A primary symptom of a scorpion sting is numbing at the injection site, sometimes lasting for several days. It has been found that scorpions have two types of venom: a translucent, weaker venom designed to stun only, and an opaque, more potent venom designed to kill heavier threats [11][12].
Unlike the majority of Arachnida species, scorpions are viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have undergone at least one moult [13]. The young generally resemble their parents, requiring between five and seven moults to reach maturity. Scorpions have quite variable lifespans and the lifespan of most species is not known. The age range appears to be approximately 4-25 years (25 years being the maximum reported life span in the species H. arizonensis). They are nocturnal and fossorial, finding shelter during the day in the relative cool of underground holes or undersides of rocks and coming out at night to hunt and feed. Scorpions prefer to live in areas where the temperatures range from 20°C to 37°C (68°F to 99°F), but may survive in the temperature range of 14°C to 45°C (57°F to 113°F) [14][15].
Scorpions have been found in many fossil records, including coal deposits from the Carboniferous Period and in marine Silurian deposits. They are thought to have existed in some form since about 425–450 million years ago. They are believed to have an oceanic origin, with gills and a claw like appendage that enabled them to hold onto rocky shores or seaweed.
[edit] Solifugae
Galeodes sp.Main article: Solifugae
Solifugae is a group of 900 species of arachnid, commonly known as camel spiders, wind scorpions, and sun spiders. The name derives from Latin, and means those that flee from the sun. Most Solifugae live in tropical or semitropical regions where they inhabit warm and arid habitats, but some species have been known to live in grassland or forest habitats. The most distinctive feature of Solifugae is their large chelicerae. Each of the two chelicerae are composed of two articles forming a powerful pincer; each article bears a variable number of teeth. Males in all families but Eremobatidae possess a flagellum on the basal article of the chelicera. Solifugae also have long pedipalps, which function as sense organs similar to insects' antennae and give the appearance of the two extra legs. Pedipalps terminate in eversible adhesive organs.
Solifugae are carnivorous or omnivorous, with most species feeding on termites, darkling beetles, and other small arthropods; however, solifugae have been videotaped consuming larger prey such as lizards. Prey is located with the pedipalps and killed and cut into pieces by the chelicerae. The prey is then liquefied and the liquid ingested through the pharynx. Reproduction can involve direct or indirect sperm transfer; when indirect, the male emits a spermatophore on the ground and then inserts it with his chelicerae in the female's genital pore.
[edit] Trigonotarbida
Main article: Trigonotarbida
The Order Trigonotarbida is an extinct group of arachnids whose fossil record extends from the Silurian to the Lower Permian and are known from several localities in Europe and North America. They superficially resemble spiders, to which they were clearly related. It was once thought that trigonotarbids lacked the silk-producing spinnerets that have apparently been crucial to the spider's evolutionary success, though in recent years at least one fossil find seems to show distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk.
These early arachnids seem to have been adapted to stalking prey on the ground. They have been found within the very structure of ground-dwellings plants, possibly where they hid to await their prey. Trigonotarbids are currently the oldest known land arthropods. They lack silk glands on the opisthosoma and cheliceral poison glands, and most likely represented independent offshoots of the Arachnida.
[edit] Uropygi
A uropygidMain article: Uropygi
The Uropygi, commonly known as whip scorpions, range from 25 to 85 mm in length; the largest species, of the genus Mastigoproctus, reaches 85 mm. Like the related orders Schizomida, Amblypygi, and Solifugae, the uropygids use only six legs for walking, having modified their first two legs to serve as antennae-like sensory organs. Many species also have very large scorpion-like pedipalps (pincers). They have one pair of eyes at the front of the cephalothorax and three on each side of the head. Whip scorpions have no poison glands, but they do have glands near the rear of their abdomen that can spray a combination of acetic acid and octanoic acid when they are bothered. Other species spray formic acid or chlorine. As of 2006, over 100 species of uropygids have been described worldwide.
Whip scorpions are carnivorous, nocturnal hunters feeding mostly on insects but sometimes on worms and slugs. The prey is crushed between special teeth on the inside of the trochanters (the second segment of the leg) of the front legs. They are valuable in controlling the population of roaches and crickets.
Males secrete a sperm sac, which is transferred to the female. Up to 35 eggs are laid in a burrow, within a mucous membrane that preserves moisture. Mothers stay with the eggs and do not eat. The white young that hatch from the eggs climb onto their mother's back and attach themselves there with special suckers. After the first molt they look like miniature whip scorpions, and leave the burrow; the mother dies soon after. The young grow slowly, going through three molts in about three years before reaching adulthood.
Uropygids are found in tropical and subtropical areas worldwide, usually in underground burrows which they dig with their pedipalps. They may also burrow under logs, rotting wood, rocks, and other natural debris. They enjoy humid, dark places and avoid the light.
Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the Earth, with over a million described species—more than all other animal groups combined.[1] Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans where crustaceans tend to predominate instead. There are approximately 5,000 dragonfly species, 2,000 praying mantis, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 360,000 beetle, and 110,000 bee, wasp and ant species described to date. Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower figure of about six to ten million.[1][2][3] Adult modern insects range in size from a 0.139 mm Mymarid wasp (Dicopomorpha echmepterygis) to a 55.5 cm long Stick insect (Phobaeticus serratipes).[4] The heaviest documented insect is a Giant Weta (at 70 grams), but other possible candidates include the Goliath beetles Goliathus goliatus, Goliathus regius and Cerambycid beetles such as Titanus giganteus, though no one is certain which is truly the heaviest.[4]
The study of insects (from Latin insectus, meaning "cut into sections") is called entomology, from the Greek εντομος, also meaning "cut into sections".[5]
Roles in the environment and human society
Aedes aegypti, a parasite, and vector of dengue fever and yellow feverMany insects are considered pests by humans. Insects commonly regarded as pests include those that are parasitic (mosquitoes, lice, bedbugs), transmit diseases (mosquitoes, flies), damage structures (termites), or destroy agricultural goods (locusts, weevils). Many entomologists are involved in various forms of pest control, often using insecticides, but more and more relying on methods of biocontrol.
Although pest insects attract the most attention, many insects are beneficial to the environment and to humans. Some pollinate flowering plants (for example wasps, bees, butterflies, ants). Pollination is a trade between plants that need to reproduce, and pollinators that receive rewards of nectar and pollen. A serious environmental problem today is the decline of populations of pollinator insects, and a number of species of insects are now cultured primarily for pollination management in order to have sufficient pollinators in the field, orchard or greenhouse at bloom time.
Insects also produce useful substances such as honey, wax, lacquer and silk. Honey bees have been cultured by humans for thousands of years for honey, although contracting for crop pollination is becoming more significant for beekeepers. The silkworm has greatly affected human history, as silk-driven trade established relationships between China and the rest of the world. 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. Adult insects such as crickets, and insect larvae of various kinds are also commonly used as fishing bait.
Chorthippus biguttatus, a grasshopperIn some parts of the world, insects are used for human food ("Entomophagy"), while being a taboo in other places. There are proponents of developing this use to provide a major source of protein in human nutrition. Since it is impossible to entirely eliminate pest insects from the human food chain, insects already are present in many foods, especially grains. Most people do not realize that food laws in many countries do not prohibit insect parts in food, but rather limit the quantity. According to cultural materialist anthropologist Marvin Harris, the eating of insects is taboo in cultures that have protein sources that require less work, like farm birds or cattle.
Many insects, especially beetles, are scavengers, feeding on dead animals and fallen trees, recycling the biological materials into forms found useful by other organisms, and insects are responsible for much of the process by which topsoil is created. The ancient Egyptian religion adored beetles and represented them as scarabeums.
Although mostly unnoticed by most humans, the most useful of all insects are insectivores, those that feed on other insects. Many insects, such as grasshoppers, can potentially reproduce so quickly that they could literally bury the earth in a single season. However, there are hundreds of other insect species that feed on grasshopper eggs, and some that feed on grasshopper adults. This role in ecology is usually assumed to be primarily one of birds, but insects, though less glamorous, are much more significant. For any pest insect one can name, there is a species of wasp that is either a parasitoid or predator upon that pest, and plays a significant role in controlling it.
Human attempts to control pests by insecticides can backfire, because important but unrecognized insects already helping to control pest populations are also killed by the poison, leading eventually to population explosions of the pest species.
[edit] Morphology
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 artery
8. tracheal tubes (trunk with spiracle)
9. mesothorax
10. metathorax
11. forewing
12. hindwing
13. mid-gut (stomach)
14. heart
15. ovary
16. hind-gut (intestine, rectum & anus)
17. anus
18. vagina
19. nerve chord (abdominal ganglia)
20. Malpighian tubes
21. pillow
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 possess segmented bodies supported by an exoskeleton, a hard outer covering made mostly of chitin. The body is divided into a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, and mouth parts. The thorax has six legs (one pair per segment) and wings (if present in the species). The abdomen (made up of eleven segments some of which may be reduced or fused) has respiratory, excretory and reproductive structures.
[edit] Nervous system
Their nervous system can be divided into a brain and a ventral nerve cord. The head capsule (made up of six fused segments) has six pairs of ganglia. The first three pairs are fused into the brain, while the three following pairs are fused into a structure called the subesophageal ganglion.
The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction. Some cockroaches have just six ganglia in the abdomen, whereas the wasp Vespa crabro has only two in the thorax and three in the abdomen. And some like the house fly Musca domestica have fused all the body ganglia into a single large thoracic ganglion.
[edit] Exoskeleton
Most insects have two pairs of wings located on the second and third thoracic segments. Insects are the only invertebrates to have developed flight, and this has played an important part in their success. The winged insects, and their wingless relatives, make up the subclass Pterygota. Insect flight is not very well understood, relying heavily on turbulent aerodynamic effects. The primitive insect groups use muscles that act directly on the wing structure. The more advanced groups making up the Neoptera have foldable wings and their muscles act on the thorax wall and power the wings indirectly. These muscles are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would ordinarily be possible (see insect flight).
Their outer skeleton, the cuticle, is made up of two layers; the epicuticle which is a thin and waxy water resistant outer layer and contains no chitin, and another layer under it called the procuticle. This is chitinous and much thicker than the epicuticle and has two layers. The outer being the exocuticle while the inner is the endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each others in a sandwich pattern, while the exocuticle is rigid and sclerotized.
[edit] Development
Hoverflies mating in flightMost insects hatch from eggs, but others are ovoviviparous or viviparous, and all undergo a series of moults as they develop and grow in size. This manner of growth is necessitated by the inelastic exoskeleton. Moulting is a process by which the individual escapes the confines of the exoskeleton in order to increase in size, then grows a new and larger outer covering. In some insects, the young are called nymphs and are similar in form to the adults except that the wings are not developed until the adult stage. This is called incomplete metamorphosis and insects showing this are termed hemimetabolous. Holometabolous insects show complete metamorphosis, which distinguishes the Endopterygota and includes many of the most successful insect groups. In these species, an egg hatches to produce a larva, which is generally worm-like in form, and can be divided into five different forms; eruciform (caterpillar-like), scarabaeiform (grublike), campodeiform (elongated, flattened, and active), elateriform (wireworm-like) and vermiform (maggot-like). The larva grows and eventually becomes a pupa, a stage sealed within a cocoon in some species. There are three types of pupae; obtect (the pupa is compact with the legs and other appendages enclosed), exarate (where the pupa has the legs and other appendages free and extended) and coarctate (where the pupa develops inside the larval skin). In the pupal stage, the insect undergoes considerable change in form to emerge as an adult, or imago. Butterflies are an example of an insect that undergoes complete metamorphosis. Some insects have even evolved hypermetamorphosis.
Some insects (parastic wasps) show polyembryony where a single fertilized egg can divide into many and in some cases thousands of separate embryos. Other developmental and reproductive variations include haplodiploidy, polymorphism, paedomorphosis (metathetely and prothetely), sexual dimorphism, parthenogenesis and more rarely hermaphroditism.
[edit] Behavior
Flies attracted to an incandescent light bulbMany insects possess very sensitive and/or specialized organs of perception. 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 many kilometers. There is a pronounced tendency for there to be 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. There is a variety of different mechanisms by which insects perceive sound, and it is by no means universal; the general pattern, however, is that if an insect can produce sound, then it can also hear sound, though the range of frequencies they can hear is often quite narrow (and may in fact be limited to only the frequency that they themselves produce). Some nocturnal moths can perceive the ultrasonic emissions of bats, a mechanism which helps them avoid predation. Certain predatory and parasitic insects can detect the characteristic sounds made by their prey/hosts. Bloodsucking insects have special sensory structures that can detect infrared emissions, and use them to home in on their hosts.
Sensillae: sensory structures on insectsMost insects lead short lives as adults, and rarely interact with one another except to mate, or compete for mates. A small number exhibit some form of parental care, where they will at least guard their eggs, and sometimes continue guarding their offspring until adulthood, and possible even actively feeding them. Another simple form of parental care is to construct a nest (a burrow or an actual construction, either of which may be simple or complex), store provisions in it, and lay an egg upon those provisions. The adult does not contact the growing offspring, but it nonetheless does provide food. This sort of care is typical of bees and various types of wasps.
A few such insects also have a well-developed number sense, among the solitary wasps that provision with a single species of prey. The mother wasp lays her eggs in individual cells and provides each egg with a number of live caterpillars on which the young feed when hatched. Some species of wasp always provide five, others twelve, and others as high as twenty-four caterpillars per cell. The number of caterpillars is different among species, but it is always the same for each sex of larvae. The male solitary wasp in the genus Eumenes is smaller than the female, so the mother of one species supplies him with only five caterpillars; the larger female receives ten caterpillars in her cell. She can in other words distinguish between both the numbers five and ten in the caterpillars she is providing and which cell contains a male or a female.
[edit] Sociality
A termite mound made by the cathedral termiteSocial insects, such as the termites, ants and many bees and wasps, are the most familiar species of eusocial animal. They live together in large well-organized colonies that may be so tightly integrated and genetically similar that the colonies of some species are sometimes considered superorganisms. It is sometimes argued that the various species of honey bee are the only invertebrates (and indeed one of the few non-human groups) to have evolved a system of abstract symbolic communication (i.e., where a behavior is used to represent and convey specific information about something in the environment), called the "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.
Only those insects which live in nests or colonies demonstrate any true capacity for fine-scale spatial orientation or "homing" - this can be quite sophisticated, however, and allow an insect to return unerringly to a single hole a few millimeters in diameter among a mass of thousands of apparently identical holes all clustered together, after a trip of up to several kilometers' distance, and (in cases where an insect hibernates) as long as a year after last viewing the area (a phenomenon known as philopatry). A few insects migrate, but this is a larger-scale form of navigation, and often involves only large, general regions (e.g., the overwintering areas of the Monarch butterfly).
[edit] Light production and vision
A few insects, notably the beetles of the family Lampyridae have evolved light generating organs. They are also able to control this light generation to produce flashes and some species use the light to attract mates.
Most insects except some species of cave dwelling crickets are able to perceive light and dark. Many species have acute vision capable of detecting minute movements. The eyes include simple eyes or ocelli as well as compound eyes of varying sizes. Many species are able to detect light in the infrared, ultraviolet as well as the visible light wavelengths. Colour vision has been demonstrated in many species.
[edit] Sound production and hearing
Insects were the earliest organisms to produce sounds and to sense them. Soundmaking in insects is achieved mostly by mechanical action of appendages. In the grasshoppers and crickets this is achieved by stridulation. The cicadas have the loudest sounds among the insects and have special modifications to their body and musculature to produce and amplify sounds. Some species such as the African cicada, Brevisana brevis have been measured at 106.7 decibels at a distance of 50 cm.[4] Some insects can hear ultrasound and take evasive action when they sense detection by bats. Some moths produce clicks and these were earlier thought to have a role in jamming the bat echolocation. However studies suggest that these are produced mostly by unpalatable moths to warn the bats, just as warning colouration is used visually.[6]
Very low sounds are also produced in various species of Lepidoptera, Coleoptera and Hymenoptera, mostly through the use of wing movement or friction at the joints of appendages.
Most soundmaking insects also have tympanal organs that can perceive airborne sounds. Most insects are also able to sense vibrations transmitted by the substrate.
[edit] Chemical communication
In addition to the use of sound for communication, a wide range of insects have evolved chemical means for communication. These chemicals, termed semiochemicals, are often derived from plant metabolites include those meant to attract, repel and provide other kinds of information. 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 known developed in the social insects.
[edit] Locomotion
[edit] Flight
Main article: insect flight
Insects are the only group of invertebrates to have developed flight. The evolution of insect wings has been the subject of debate. Some proponents suggest that the wings are para-notal in origin while others have suggested that these are modified gills. In the Carboniferous age, some of the Meganeura dragonflies had as much as a 50cms wide wingspan. The largest flying insects today are much smaller and include several moth species such as the Atlas moth and the White Witch (Thysania agrippina).
Insect flight has been a topic of great interest in aerodynamics due partly to the inability of steady-state theories to explain the lift generated by the tiny wings of insects.
[edit] Walking
Many adult insects use six legs for walking and have adopted a tripedal gait. The tripedal gait allows for rapid walking whilst always having a stable stance and has been studied extensively in cockroaches. The legs are used in alternate triangles touching the ground. 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, whilst 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 and so on.
The purest form of the tripedal gait is seen in insects moving at speed and is illustrated in the gif animation of a 7-spot ladybird (Coccinellidae, Coccinella septempunctata). However, this type of locomotion is not rigid and insects can adapt a variety of gaits; for example, when moving slowly, turning, or avoiding obstacles, four or more feet may be touching the ground. Insects can also adapt their gait to cope with the loss of one or more limbs.
Cockroaches are amongst the fastest insect runners and at full speed actually adopt a bipedal run to reach a high velocity in proportion to their body size. As Cockroaches move extremely rapidly, they need recording at several hundred frames per second to reveal their gait. More sedate locomotion is also studied by scientists in stick insects Phasmatodea.
A few insects have evolved to walk on the surface of the water, especially the bugs of the family, Gerridae, also known as water striders. A few species in the genus Halobates even live on the surface of open oceans, a habitat that has few insect species.
Insect walking is of particular interest as an alternative form of locomotion to the use of wheels for robots (Robot locomotion).
[edit] Swimming
The backswimmer Notonecta glauca underwater, showing the paddle like hindleg adaptationA large number of insects live either a part or their whole lives underwater. In many orders the immature stages are spent in water while the adults are either aerial or terrestrial in habit. A few species spend a part of their adult life either under or over water. Many of these species have adaptations to help in locomotion under water. The water beetles and water bugs have legs adapted into paddle like structures. Some Odonate larvae, such as dragonfly naiads, propel themselves rapidly by expelling water forcibly out of the rectal chamber.
[edit] Evolution
Evolution has produced astonishing variety in insects. Pictured are some of the possible shapes of antennae.Main article: Insect evolution
The relationships of insects to other animal groups remain unclear. Although more traditionally grouped with millipedes and centipedes, evidence has emerged favoring closer evolutionary ties with the crustaceans. In the Pancrustacea theory insects, together with Remipedia and Malacostraca, make up a natural clade.
Apart from some tantalizing Devonian fragments, insects first appear suddenly in the fossil record at the very beginning of the Late Carboniferous period, Early Bashkirian age, about 350 million years ago. Insect species were already diverse and highly specialized by this time, with fossil evidence reflecting the presence of more than half a dozen different orders. Thus, the first insects probably emerged earlier in the Carboniferous period, or even in the preceding Devonian. The oldest insect traces are found in amber from Lebanon representing the Lower Cretaceous (120 mya). Research to discover these earliest insect ancestors in the fossil record continues.
The origins of insect flight remain obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects had an additional pair of winglets attaching to the first segment of the thorax, for a total of three pairs. So far, there is nothing that suggests that the insects were a particularly successful group of animals before they got their wings.
Late Carboniferous and Early Permian insect orders include both several current very long-lived groups and a number of Paleozoic forms. During this era, some giant dragonfly-like forms reached wingspans of 55 to 70 cm, making them far larger than any living insect. Also their nymphs must have had a very impressive size. This gigantism may have been due to higher atmospheric oxygen levels that allowed increased respiratory efficiency relative to today. The lack of flying vertebrates could have been another factor.
Most extant orders of insects developed during the Permian era that began around 270 million years ago. 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.
The remarkably successful Hymenopterans appeared in the Cretaceous but achieved their diversity more recently, in the Cenozoic. A number of highly-successful insect groups evolved in conjunction with flowering plants, a powerful illustration of co-evolution.
Many modern insect genera developed during the Cenozoic; insects from this period on are often found preserved in amber, often in perfect condition. Such specimens are easily compared with modern species. The study of fossilized insects is called paleoentomology.
[edit] Coevolution
Main article: Coevolution
Insects were among the earliest terrestrial herbivores and they acted as major selection agents on plants. 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. And such insects advertise their toxicity using warning colours. This successful evolutionary pattern has also been utilized by mimics. Over time, this has led to complex groups of co-evolved species. Conversely, some interactions between plants and insects are beneficial (see pollination), and coevolution has led to the development of very specific mutualisms in such systems.
[edit] Taxonomy
A Syrphid fly on a Grape hyacinth
Orthetrum caledonicum, the Blue Skimmer dragonflyThis is a list of the orders and higher taxa of insects.
Within the subphylum Hexapoda, a few groups such as springtails (Collembola), are often treated as insects; however some authors treat them as distinct from the insects in having a different evolutionary origin. This may also be the case for the rest of the members of the Entognatha; Protura and Diplura.
The true insects, those of the Class Insecta, are distinguished from all other arthropods in part by having ectognathous, or exposed, mouthparts and eleven abdominal segments. The true insects are therefore sometimes also referred to as the Ectognatha. Many insect groups are winged as adults. The exopterygote part of the Neoptera are sometimes divided into Orthopteroida (cerci present) and Hemipteroida (cerci absent), also called lower and higher Exopterygota; a full classification is given below.
Subclass Apterygota
Archaeognatha (bristletails)
Thysanura (silverfish)
Monura - extinct
Subclass Pterygota
Infraclass Paleoptera (may be paraphyletic)
Ephemeroptera (mayflies)
Palaeodictyoptera - extinct
Megasecoptera - extinct
Archodonata - extinct
Diaphanopterodea - extinct
Protodonata - extinct
Odonata (dragonflies and damselflies)
Infraclass Neoptera
Superorder Exopterygota
Caloneurodea - extinct
Titanoptera - extinct
Protorthoptera - extinct
Grylloblattodea (ice-crawlers)
Mantophasmatodea (gladiators)
Plecoptera (stoneflies)
Embioptera (webspinners)
Zoraptera (angel insects)
Dermaptera (earwigs)
Orthoptera (grasshoppers, etc)
Phasmatodea (stick insects)
Blattodea (cockroaches)
Isoptera (termites)
Mantodea (mantids)
Psocoptera (booklice, barklice)
Thysanoptera (thrips)
Phthiraptera (lice)
Hemiptera (true bugs)
Superorder Endopterygota
Hymenoptera (ants, bees, etc.)
Coleoptera (beetles)
Strepsiptera (twisted-winged parasites)
Raphidioptera (snakeflies)
Megaloptera (alderflies, etc.)
Neuroptera (net-veined insects)
Mecoptera (scorpionflies, etc.)
Siphonaptera (fleas)
Diptera (true flies)
Protodiptera extinct
Superorder Amphiesmenoptera
Trichoptera (caddisflies)
Lepidoptera (butterflies, moths)
Incertae sedis
Glosselytrodea extinct
Miomoptera extinct
As seen above, insects are divided into two subclasses; Apterygota and Pterygota (flying insects), but this could relatively soon change. Apterygota is made up of two orders; Archaeognatha (bristletails) and Thysanura (silverfish). In the suggested classification, the Archaeognatha makes up the Monocondylia while Thysanura and Pterygota are grouped together as Dicondylia. It is even possible that the Thysanura itself are not monophyletic, making the family Lepidotrichidae a sister group to the Dicondylia (Pterygota + the rest of the Thysanura).
Also within the infraclass Neoptera we will probably see some re-organization in not too long. Today Neoptera is divided into the superorders Exopterygota and Endopterygota. But even if the Endopterygota are monophyletic, the Exopterygota seems to be paraphyletic, and can be separated into smaller groups; Paraneoptera, Dictyoptera, Orthopteroidea and to other groups (Grylloblattodea + Mantophasmatodea and Plecoptera + Zoraptera + Dermaptera). Phasmatodea and Embioptera has been suggested to form Eukinolabia, while Strepsiptera and Diptera are sometimes grouped together in Halteria. Paraneoptera has turned out to be more closeley related to Endopterygota than to the rest of the Exopterygota. It is not still clear how closely related the remaining Exopterygote groups are and if they belongs together in a larger unit. Only more research will give the answer.
[edit] Relationship to other arthropods
Hexapoda (Insecta, collembola, diplura, protura)
Crustacea (crabs, shrimp, isopods)
Myriapoda
Pauropoda
Diplopoda (Millipedes)
Chilopoda (Centipedes)
Symphyla
Chelicerata
Arachnida (Spiders, scorpions and allies)
Eurypterida (Sea scorpions: Extinct)
Xiphosura (King crabs)
Pycnogonida (Sea spiders)
Trilobites (Extinct)
A phylogenetic tree of the arthropods and related groups.[7]
Other terrestrial arthropods, such as centipedes, millipedes, scorpions and spiders, are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointed exoskeleton. However upon closer examination their features differ significantly; most noticeably they do not have the six legs characteristic of adult insects.
The higher level phylogeny of the arthropods continues to be a matter of debate and research.
[edit] Quotations
"Something in the insect seems to be alien to the habits, morals, and psychology of this world, as if it had come from some other planet: more monstrous, more energetic, more insensate, more atrocious, more infernal than our own."
—Maurice Maeterlinck (1862–1949)
When asked what can be learned about the Creator by examining His work, J.B.S. Haldane said "an inordinate fondness for beetles."
"To understand the success of insects is to appreciate our own shortcomings" —Thomas Eisner