AskDefine | Define arachnid

Dictionary Definition

arachnid n : air-breathing arthropods characterized by simple eyes and four pairs of legs [syn: arachnoid]

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From (arachne) αραχνη, spider, attributed to mythological woman named Arachne.



  1. Any of the eight-legged creatures, including spiders and scorpions, of the class Arachnida


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Extensive Definition

Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. The term arachnid is from the Greek word άράχνη or arachne, meaning spider, and also referring to the mythological figure Arachne.
Arachnids are chiefly terrestrial arthropods, but are also found in freshwater and, with the exception of the pelagic zone, in all marine environments. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, and mites.
It is commonly understood that arachnids have four pairs of legs, and that arachnids may be easily distinguished from insects by this fact (insects have six legs or three pairs). Interestingly, arachnids generally have a total of 6 pairs of appendages — two pairs of which have become adapted for feeding, defense, and sensory perception. The first pair of appendages, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps have been adapted for feeding, locomotion, and/or reproductive functions. In Solifugae, the palpi are quite leg-like and make Solifugae appear to have ten legs. The larvae of mites (and Ricinulei) have only six legs; the fourth pair appears when they moult into nymphs. However, there are also adult mites with six, or even four legs.
Arachnids are further distinguished by the fact they have no antennae and no wings. Their body is organized into two tagma called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. The cephalothorax (prosoma) is derived from the fusion of the cephalon (head) and the thorax. The abdomen (opisthosoma) can be further divided into the preabdomen and postabdomen in many taxa, although in some orders such as the Acari the abdominal sections are fused.
There are some important modifications that are particularly important for the terrestrial lifestyle of an arachnid, such as internal respiratory surfaces in the form of trachea, or modification of the book gill into a book lung, an internal series of vascular lamellae used for gas exchange with the air. Further adaptations are appendages modified for more efficient locomotion on land, internal fertilisation, special sensory organs, and water conservation enhanced by more efficient excretory structures (coxal glands and Malpighian tubules) as well as a waxy layer covering the cuticle.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only in the harvestmen and among mites, such as the house dust mite, is there ingestion of solid food particles and thus exposure to internal parasites , althougth it is not unusual for spiders to eat their own silk. Several groups are largely venomous — they secrete venom from specialized glands to kill prey or enemies. Several mites are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immatures that resemble adults, but scorpions bear live young.
As all arthopods, they have an external skeleton, and they also have an internal structure of cartilage like tissue called the endosternite, where certain muscle groups are attached. Calcification in the endosternite has been found in some Opiliones
Arachnids have two kinds of eyes, the lateral and median ocelli. The lateral ocelli evolved from compound eyes and may have a tapetum, which enhances the efficiency of photon capture. The median ocelli develop from a transverse fold of the ectoderm. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other..


Acarina or Acari are a taxon of arachnids that contains mites and ticks. Its fossil history goes back to the Devonian era, although there is also a questionable Ordovician record. The Devonian era was the time frame in which certain species of animals developed legs. 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. 0.080–1.00 mm), but the giants of the Acari (some ticks and red velvet mites) may reach lengths of 10–20 mm. 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.
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 


Spiders are the most familiar of the arachnids, and the most numerous, if only described species are counted. 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, even flying, 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. 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, with some extreme species even living underwater in silken domes they supply with air, and on the tops of the highest mountains.


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 1911. Relationships with other arachnids are obscure, but closest relatives may be the Amblypygi, Thelyphonida and Schizomida of the tetrapulmonate clade 


Opiliones (better known as "harvestmen" or ""daddy longlegs"") are arachnids that 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 look 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 lying 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 that have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.


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 that 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.


Phalangiotarbi (Haase, 1890) is an extinct arachnid order known exclusively from the Palaeozoic (Devonian to Permian) 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).


Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of scorpions. They range from 2 to 8 mm ( to 1/3 inch) in length . 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) that 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 . In other species, the male also pushes the sperm into the female genitals using the forelegs .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 .
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. 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).
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.


Solifugae is a group of 900 species of arachnids, 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.


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.


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 that 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.


arachnid in Arabic: عنكبيات
arachnid in Bulgarian: Паякообразни
arachnid in Catalan: Aràcnid
arachnid in Czech: Pavoukovci
arachnid in Danish: Spindlere
arachnid in German: Spinnentiere
arachnid in Estonian: Ämblikulaadsed
arachnid in Spanish: Arachnida
arachnid in Esperanto: Araneoidoj
arachnid in Persian: عنکبوتیان
arachnid in French: Arachnida
arachnid in Croatian: Paučnjaci
arachnid in Indonesian: Arachnida
arachnid in Icelandic: Áttfætlur
arachnid in Italian: Arachnida
arachnid in Hebrew: עכבישניים
arachnid in Georgian: ობობასნაირნი
arachnid in Latin: Arachnida
arachnid in Luxembourgish: Spannendéieren
arachnid in Lithuanian: Voragyviai
arachnid in Hungarian: Pókszabásúak
arachnid in Dutch: Spinachtigen
arachnid in Japanese: クモ綱
arachnid in Norwegian: Edderkoppdyr
arachnid in Polish: Pajęczaki
arachnid in Portuguese: Aracnídeos
arachnid in Quechua: Pakcha
arachnid in Russian: Паукообразные
arachnid in Sicilian: Arachnida
arachnid in Simple English: Arachnid
arachnid in Slovak: Pavúkovce
arachnid in Slovenian: Pajkovci
arachnid in Serbian: Арахнида
arachnid in Finnish: Hämähäkkieläimet
arachnid in Swedish: Spindeldjur
arachnid in Thai: แมง
arachnid in Turkish: Örümceğimsiler
arachnid in Ukrainian: Павукоподібні
arachnid in Contenese: 蜘蛛綱
arachnid in Chinese: 蛛形纲
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