Arachnid

For other uses, see Arachnid (disambiguation).“Arachnida” redirects here. For the arachnida curve, seeSectrix of Maclaurin.

Arachnids is a class (Arachnida) of joint-leggedinvertebrate animals (arthropods), in the subphylumChelicerata. All arachnids have eight legs, althoughthe front pair of legs in some species has convertedto a sensory function, while in other species, differentappendages can grow large enough to take on the appear-ance of extra pairs of legs. The term is derived from theGreek word ἀράχνη (aráchnē), meaning “spider”.[2]

Almost all extant arachnids are terrestrial. However,some inhabit freshwater environments and, with the ex-ception of the pelagic zone, marine environments aswell. They comprise over 100,000 named species, in-cluding spiders, scorpions, harvestmen, ticks, mites, andsolifuges.[3]

1 Anatomy

Basic characteristics of arachnids include four pairs of legs (1)and a body divided into two tagmata: the cephalothorax (2) andthe abdomen (3)

Almost all adult arachnids have eight legs, and arachnidsmay be easily distinguished from insects by this fact, sinceinsects have six legs. However, arachnids also have twofurther pairs of appendages that have become adapted forfeeding, defense, and sensory perception. The first pair,the chelicerae, serve in feeding and defense. The nextpair of appendages, the pedipalps, have been adapted forfeeding, locomotion, and/or reproductive functions. In

Solifugae, the palps are quite leg-like, so that these an-imals appear to have ten legs. The larvae of mites andRicinulei have only six legs; a fourth pair usually appearswhen they moult into nymphs. However, mites are vari-able: as well as eight, there are adult mites with six oreven four legs.[4]

Arachnids are further distinguished from insects by thefact they do not have antennae or wings. Their bodyis organized into two tagmata, called the prosoma, orcephalothorax, and the opisthosoma, or abdomen. Thecephalothorax is derived from the fusion of the cephalon(head) and the thorax, and is usually covered by a sin-gle, unsegmented carapace. The abdomen is segmentedin the more primitive forms, but varying degrees of fu-sion between the segments occur in many groups. It istypically divided into a preabdomen and postabdomen,although this is only clearly visible in scorpions, and insome orders, such as the Acari, the abdominal sectionsare completely fused.[5]

Like all arthropods, arachnids have an exoskeleton, andthey also have an internal structure of cartilage-like tissue,called the endosternite, to which certain muscle groupsare attached. The endosternite is even calcified in someOpiliones.[6]

1.1 Locomotion

Most arachnids lack extensor muscles in the distal jointsof their appendages. Spiders and whipscorpions ex-tend their limbs hydraulically using the pressure of theirhemolymph.[7] Solifuges and some harvestmen extendtheir knees by the use of highly elastic thickeningsin the joint cuticle.[7] Scorpions, pseudoscorpions andsome harvestmen have evolved muscles that extend twoleg joints (the femur-patella and patella-tibia joints) atonce.[8][9] The equivalent joints of the pedipalps of scor-pions though, are extended by elastic recoil.[10]

2 Physiology

There are characteristics that are particularly importantfor the terrestrial lifestyle of arachnids, such as internalrespiratory surfaces in the form of tracheae, or modifica-tion of the book gill into a book lung, an internal seriesof vascular lamellae used for gas exchange with the air.While the tracheae are often individual systems of tubes,similar to those in insects, ricnuleids, pseudoscorpions,

1

2 6 SYSTEMATICS

and some spiders possess sieve tracheae, in which severaltubes arise in a bundle from a small chamber connectedto the spiracle. This type of tracheal system has almostcertainly evolved from the book lungs, and indicates thatthe tracheae of arachnids are not homologous with thoseof insects.[11]

Further adaptations to terrestrial life are appendagesmodified for more efficient locomotion on land, internalfertilisation, special sensory organs, and water conserva-tion enhanced by efficient excretory structures as well asa waxy layer covering the cuticle.The excretory glands of arachnids include up to four pairsof coxal glands along the side of the prosoma, and one ortwo pairs of Malpighian tubules, emptying into the gut.Many arachnids have only one or the other type of excre-tory gland, although several do have both. The primarynitrogenous waste product in arachnids is guanine.[11]

Arachnid blood is variable in composition, dependingon the mode of respiration. Arachnids with an efficienttracheal system do not need to transport oxygen in theblood, and may have a reduced circulatory system. Inscorpions and some spiders, however, the blood con-tains haemocyanin, a copper-based pigment with a sim-ilar function to haemoglobin in vertebrates. The heartis located in the forward part of the abdomen, and mayor may not be segmented. Some mites have no heart atall.[11]

3 Diet and digestive system

Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Onlyin the harvestmen and among mites, such as the housedust mite, is there ingestion of solid food particles, andthus exposure to internal parasites,[12] although it is notunusual for spiders to eat their own silk. Several groupssecrete venom from specialized glands to kill prey or en-emies. Several mites are parasites, some of which arecarriers of disease.Arachnids produce digestive juices in their stomachs, anduse their pedipalps and chelicerae to pour them over theirdead prey. The digestive juices rapidly turn the prey intoa broth of nutrients, which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth.Behind the mouth is a muscular, sclerotised pharynx,which acts as a pump, sucking the food through themouthand on into the oesophagus and stomach. In some arach-nids, the oesophagus also acts as an additional pump.The stomach is tubular in shape, with multiple diverticulaextending throughout the body. The stomach and its di-verticula both produce digestive enzymes and absorb nu-trients from the food. It extends through most of thebody, and connects to a short sclerotised intestine andanus in the hind part of the abdomen.[11]

4 Senses

Arachnids have two kinds of eyes, the lateral and medianocelli. The lateral ocelli evolved from compound eyesand may have a tapetum, which enhances the ability tocollect light. With the exception of scorpions, which canhave up to five pairs of lateral ocelli, there are never morethan three pairs present. The median ocelli develop froma transverse fold of the ectoderm. The ancestors of mod-ern arachnids probably had both types, but modern onesoften lack one type or the other.[12] The cornea of theeye also acts as a lens, and is continuous with the cuticleof the body. Beneath this is a transparent vitreous body,and then the retina and, if present, the tapetum. In mostarachnids, the retina probably does not have enough lightsensitive cells to allow the eyes to form a proper image.[11]

In addition to the eyes, almost all arachnids have twoother types of sensory organs. The most important tomost arachnids are the fine sensory hairs that cover thebody and give the animal its sense of touch. These can berelatively simple, but many arachnids also possess morecomplex structures, called trichobothria.Finally, slit sense organs are slit-like pits covered witha thin membrane. Inside the pit, a small hair touchesthe underside of the membrane, and detects its mo-tion. Slit sense organs are believed to be involved inproprioception, and possibly also hearing.[11]

5 Reproduction

See also: Spider § Reproduction and life cycle andScorpion § Reproduction

Arachnids may have one or two gonads, which are locatedin the abdomen. The genital opening is usually locatedon the underside of the second abdominal segment. Inmost species, the male transfers sperm to the female ina package, or spermatophore. Complex courtship ritualshave evolved inmany arachnids to ensure the safe deliveryof the sperm to the female.[11]

Arachnids usually lay yolky eggs, which hatch into im-matures that resemble adults. Scorpions, however, areeither ovoviviparous or viviparous, depending on species,and bear live young.

6 Systematics

• † Trigonotarbida—extinct (late Silurian earlyPermian)

• Amblypygi—"blunt rump” tailless whip scorpionswith front legs modified into whip-like sensorystructures as long as 25 cm or more (140 species)

6.1 Acari 3

• Araneae—true spiders (40,000 species)

• Mesothelae—very rare, basal spiders, with ab-domen segmented and spinnerets median

• Opisthothelae—spiders with abdomen unseg-mented and spinnerets located posteriorly• Araneomorphae—most common spiders• Mygalomorphae—tarantulas andtarantula-like spiders

• † Phalangiotarbida—extinct

• Opiliones—phalangids, harvestmen or daddy-long-legs (6,300 species)

• Palpigradi—microwhip scorpions (80 species)

• Pseudoscorpionida—pseudoscorpions (3,000species)

• Ricinulei—ricinuleids, hooded tickspiders (60species)

• Schizomida—"split middle” whip scorpions with di-vided exoskeletons (220 species)

• Scorpiones—scorpions (2,000 species)

• Solifugae—solpugids, windscorpions, sun spiders orcamel spiders (900 species)

• † Haptopoda—extinct

• Thelyphonida—vinegarroons or whip scorpions(formerly uropygida) forelegs modified into sensoryappendages and a long tail on abdomen tip (100species)

• Acari—mites and ticks (30,000 species)

• Acariformes• Sarcoptiformes• Trombidiformes

• Opilioacariformes• Parasitiformes—holothyrans, ticks andmesostigmatic mites

It is estimated that 98,000 arachnid species have been de-scribed, and that there may be up to 600,000 in total.[13]

6.1 Acari

Main article: Acari

Acari or Acarina is a taxon of arachnids that containsmites and ticks. Its fossil history goes back to theDevonian period, although there is also a questionableOrdovician record. The Devonian period was the timeframe in which certain species of animals developed legs.In most modern treatments, the Acari is considered a

Ixodes hexagonus, a tick

subclass of Arachnida and is composed of two or threeorders or superorders: Acariformes, Parasitiformes, andOpilioacariformes. Most acarines are minute to small(e.g. 0.080–1.00 mm), but the giants of the Acari (someticks and red velvet mites) may reach lengths of 10–20mm. It is estimated that over 50,000 species have beendescribed (as of 1999) and that a million or more speciesare currently living. The study of mites and ticks is calledacarology.[14]

Only the faintest traces of primary segmentation remainin mites, the prosoma and opisthosoma being insensi-bly fused, and a region of flexible cuticle (the cirum-capitular furrow) separates the chelicerae and pedipalpsfrom the rest of the body. This anterior body region iscalled the gnathosoma (or capitulum) and is also foundin the Ricinulei. The remainder of the body is called theidiosoma and is unique to mites. Most adult mites havefour pairs of legs, like other arachnids, but some havefewer. For example, gall mites like Phyllocoptes variabilis(superfamily Eriophyioidea) have a wormlike body withonly two pairs of legs; some parasitic mites have only oneor three pairs of legs in the adult stage. Larval and prelar-val stages have a maximum of three pairs of legs; adultmites with only three pairs of legs may be called 'larvi-form'.Acarine ontogeny consists of an egg, a prelarval stage (of-ten absent), a larval stage (hexapod except in Eriophy-oidea, which have only two pairs of legs), and a series ofnymphal stages. Larvae (and prelarvae) have a maximumof three pairs of legs (legs are often reduced to stubs orabsent in prelarvae); the fourth pair of legs is added at thefirst nymphal stage.

4 6 SYSTEMATICS

Acarines live in practically every habitat, and includeaquatic (freshwater and sea water) and terrestrial species.They outnumber other arthropods in the soil organic mat-ter and detritus. Many are parasitic, and they affect bothvertebrates and invertebrates. Most parasitic forms areexternal parasites, while the free living forms are gener-ally predaceous and may even be used to control unde-sirable arthropods. Others are detritivores that help tobreak down forest litter and dead organic matter suchas skin cells. Others still are plant feeders and maydamage crops. Damage to crops is perhaps the mostcostly economic effect of mites, especially by the spi-der mites and their relatives (Tetranychoidea), earth mites(Penthaleidae), thread-footed mites (Tarsonemidae) andthe gall and rust mites (Eriophyoidea). Some parasiticforms affect humans and other mammals, causing dam-age by their feeding, and can even be vectors of diseasessuch as scrub typhus and rickettsial pox. A well-knowneffect of mites on humans is their role as an allergenand the stimulation of asthma in people affected by therespiratory disease. The use of predatory mites (e.g.Phytoseiidae) in pest control and herbivorous mites thatattack weeds is also important. An unquantified, but ma-jor positive contribution of the Acari is their normal func-tioning in ecosystems, especially their roles in the decom-poser subsystem.[14]

6.2 Amblypygi

An amblypygid

Main article: Amblypygi

Amblypygids are also known as tailless whip scorpionsor cave spiders. Approximately 5 families, 17 generaand 136 species have been described. They are foundin tropical and subtropical regions worldwide. Somespecies are subterranean; many are nocturnal. During theday, they may hide under logs, bark, stones, or leaves.They prefer a humid environment. Amblypygids mayrange from 5 to 40 mm. Their bodies are broad andhighly flattened and the first pair of legs (the first walk-ing legs in most arachnid orders) are modified to actas sensory organs. (Compare solifugids, uropygids, andschizomids.) These very thin modified legs can extend

several times the length of body. They have no silk glandsor venomous fangs, but can have prominent pincer-likepedipalps. Amblypygids often move about sideways ontheir six walking legs, with one “whip” pointed in thedirection of travel while the other probes on either sideof them. Prey are located with these “whips”, cap-tured with pedipalps, then torn to pieces with chelicerae.Fossilised amblypygids have been found dating back tothe Carboniferous period.Amblypygids, particularly the species Phrynusmarginemaculatus and Damon diadema, are thought tobe one of the few species of arachnids that show signs ofsocial behavior. Research conducted at Cornell Univer-sity by entomologists suggests that mother amblypygidscomfort their young by gently caressing the offspringwith her feelers. Further, when two or more siblingswere placed in an unfamiliar environment, such as acage, they would seek each other out and gather back ina group.[15]

6.3 Araneae

Araneus diadematus

Main article: Spider

Araneae, or spiders, are the most familiar of the arach-nids, and the most species-rich with around 40,000 de-scribed species.[16] All spiders produce silk, a thin, strongprotein strand extruded by the spider from spinneretsmost commonly found on the end of the abdomen. Manyspecies use it to trap insects in webs, although there are

6.6 Palpigradi 5

many species that hunt freely. Silk can be used to aid inclimbing, form smooth walls for burrows, build egg sacs,wrap prey, temporarily hold sperm, and even fly, amongother applications.All spiders except those in the families Uloboridae andHolarchaeidae, and in the suborder Mesothelae (togetherabout 350 species) can inject venom to protect them-selves or to kill and liquefy prey. Only about 200 species,however, have bites that can pose health problems tohumans.[17]Many larger species’ bites may be painful, butwill not produce lasting health concerns.Spiders are found all over the world, from the tropics tothe Arctic, with some extreme species even living under-water in silken domes that they supply with air,[18] and onthe tops of the highest mountains.

6.4 Haptopoda

Main article: Haptopoda

Haptopoda is an extinct order known exclusively froma few specimens from the Upper Carboniferous of theUnited Kingdom. It is monotypic, i.e. has only onespecies: Plesiosiro madeleyi Pocock 1911. Relationshipswith other arachnids are obscure, but closest relativesmaybe the Amblypygi, Thelyphonida and Schizomida of thetetrapulmonate clade.[19]

6.5 Opiliones

Male Opilio canestrinii cleaning its legs

Main article: Opiliones

Opiliones (formerly Phalangida, and better known as"harvestmen" or "daddy longlegs") are arachnids thatare harmless to people and are known for their excep-tionally long walking legs, compared to their body size.As of December 2011, over 6,500 species of harvest-men have been discovered worldwide.[20] The order Opil-iones is divided into four suborders: Cyphophthalmi,Eupnoi, Dyspnoi and Laniatores. Well-preserved fossilshave been found in the 410-million year old Rhynie chertsof Scotland; they look surprisingly modern, suggesting

that the basic structure of the harvestmen has not changedmuch since then.[21]

The difference between harvestmen and spiders is thatin harvestmen the two main body sections (the abdomenor opisthosoma with ten segments and the cephalothoraxor prosoma) are nearly joined, so that they appear to beone oval structure. In more advanced species, the firstfive abdominal segments are often fused into a dorsalshield called the scutum, which is normally fused with thecarapace. Sometimes this shield is only present in males.The two hindmost abdominal segments may be reducedor separated in the middle on the surface to form twoplates lying next to each other. The second pair of legs islonger than the others and works as antennae. They havea single pair of eyes in the middle of their heads, orientedsideways. They have a pair of prosomatic scent glandsthat secrete a peculiar smelling fluidwhen disturbed. Har-vestmen do not have spinnerets and do not possess poi-son glands, posing absolutely no danger to humans. Theybreathe through tracheae. Between the base of the fourthpair of legs and the abdomen is a pair of spiracles, oneopening on each side. In more active species, spiraclesare also found upon the tibia of the legs. They have agonopore on the ventral cephalothorax, and copulation isdirect, as the male has a penis (while the female has anovipositor).Typical body length does not exceed 7 millimetres (0.28in) even in the largest species. However, leg span is muchlarger and can exceed 160 mm (6.3 in). Most specieslive for a year. Many species are omnivorous, eating pri-marily small insects and all kinds of plant material andfungi; some are scavengers of the decays of any dead an-imal, bird dung and other fecal material. They are mostlynocturnal and coloured in hues of brown, although thereare a number of diurnal species that have vivid patterns inyellow, green and black with varied reddish and blackishmottling and reticulation.

6.6 Palpigradi

Main article: Palpigradi

Palpigradi, commonly known as “microwhip scorpions”,are tiny cousins of the uropygid, or whip scorpion, nomore than 3 mm in length. They have a thin, pale, seg-mented carapace that terminates in a whip-like flagellum,made up of 15 segments. The carapace is divided intotwo plates between the third and fourth leg set. Theyhave no eyes. Some species have three pairs of booklungs, while others have no respiratory organs at all.[22]Approximately 80 species of Palpigradi have been de-scribed worldwide, in the families Eukoeneniidae andProkoeneniidae, with a total of seven genera.They are believed to be predators like their larger rela-tives, feeding on minuscule insects in their habitat. Theirmating habits are unknown, except that they lay only a

6 6 SYSTEMATICS

few relatively large eggs at a time. Microwhip scorpionsneed a damp environment to survive, and they always hidefrom light, so they are commonly found in the moist earthunder buried stones and rocks. They can be found on ev-ery continent, except in Arctic and Antarctic regions.

6.7 Phalangiotarbida

Main article: Phalangiotarbida

Phalangiotarbi is an extinct arachnid order known ex-clusively from the Palaeozoic (Devonian to Permian) ofEurope and North America.The affinities of phalangiotarbids are obscure, with mostauthors favouring affinities with Opiliones (harvestmen)and/or Acari (mites and ticks). Phalangiotarbida hasbeen recently proposed to be sister group to (Palpi-gradi+Tetrapulmonata): the taxon Megoperculata sensuShultz (1990).[23]

6.8 Pseudoscorpions

A pseudoscorpion on a printed page

Main 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 (0.079 to 0.315 in) long.[24]The opisthosoma is made up of twelve segments, eachguarded by plate-like tergites above and sternites below.The abdomen is short and rounded at the rear, ratherthan extending into a segmented tail and stinger like truescorpions. The colour of the body can be yellowish-tanto dark-brown, with the paired claws often a contrastingcolour. They may have two, four or no eyes. They havetwo very long pedipalps with palpal chelae (pincers) thatstrongly resemble the pincers found on a scorpion. Thepedipalps generally consist of an immobile “hand” and“finger”, with a separate movable finger controlled by anadductor muscle. A venom gland and duct are usually

located in the mobile finger; the poison is used to cap-ture and immobilise the pseudoscorpion’s prey. Duringdigestion, pseudoscorpions pour a mildly corrosive fluidover the prey, then ingest the liquefied remains. Pseu-doscorpions spin silk from a gland in their jaws to makedisk-shaped cocoons for mating, molting, or waiting outcold weather. Another trait they share with their closestrelatives, the spiders, is breathing through spiracles. Mostspiders 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 pseudoscorpionsrecorded. They range worldwide, even in temperate tocold regions, but have their most dense and diverse popu-lations in the tropics and subtropics. The fossil record ofpseudoscorpions dates back over 380 million years, to theDevonian period, near the time when the first land-animalfossils appear.During the elaborate mating dance, the male ofsome pseudoscorpion species pulls a female over aspermatophore previously laid upon a surface.[25] In otherspecies, the male also pushes the sperm into the femalegenitals using the forelegs.[26] The female carries the fer-tilised eggs in a brood pouch attached to her abdomen,and the young ride on the mother for a short time af-ter they hatch.[24] Up to two dozen young are hatched ina single brood; there may be more than one brood peryear. The young go through three molts over the courseof several years before reaching adulthood. Adult pseu-doscorpions live 2 to 3 years. They are active in the warmmonths of the year, overwintering in silken cocoons whenthe weather grows cold.Pseudoscorpions are generally beneficial to humans sincethey prey on clothes moth larvae, carpet beetle larvae,booklice, ants, mites, and small flies. They are small andinoffensive, and are rarely seen due to their size. Theyusually enter the home by “riding along” with larger in-sects (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 foundfeeding on mites under the wing covers of certain beetles.

6.9 Ricinulei

Main article: Ricinulei

Ricinulei (hooded tickspiders) are 5–10 mm long. Theirmost notable feature is a “hood” that can be raised andlowered over the head; when lowered, it covers the mouthand the chelicerae. Ricinulei have no eyes. The pedipalpsend in pincers that are small relative to their bodies, whencompared to those of the related orders of scorpions andpseudoscorpions. The heavy-bodied abdomen forms anarrow pedicel, or waist, where it attaches to the prosoma.In males, the third pair of legs are modified to form cop-ulatory organs. Malpighian tubules and a pair of coxalglandsmake up the excretory system. They have no lungs,

6.11 Scorpions 7

as gas exchange takes place through the trachea.Ricinulei are predators, feeding on other small arthro-pods. Little is known about their mating habits; the maleshave been observed using their modified third leg to trans-fer a spermatophore to the female. The eggs are carriedunder 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 57species of ricinuleids have been described worldwide, allin a single family that contains 3 genera.

6.10 Schizomida

Main article: Schizomida

Schizomida is an order of arachnids that tend to live inthe top layer of soils. Schizomids present the prosomacovered by a large protopeltidium and smaller, paired,mesopeltidia and metapeltidia. There are no eyes. Theopisthosoma is a smooth oval of 12 recognisable somites.The first is reduced and forms the pedicel. The last threeare much constricted, forming the pygidium. The lastsomite bears the flagellum, which in this order is shortand consists of not more than four segments.The name means “split or cleaved middle”, referring tothe way the cephalothorax is divided into two separateplates. Like the related orders Uropygi, Amblypygi, andSolpugida, the schizomids use only six legs for walking,having modified their first two legs to serve as sensoryorgans. They also have large well-developed pedipalps(pincers) just behind the sensory legs.

6.11 Scorpions

Scorpio maurus palmatus

Main article: Scorpion

Scorpions are characterised by a metasoma (tail) com-prising six segments, the last containing the scorpion’sanus and bearing the telson (the sting). The telson, in

turn, consists of the vesicle, which holds a pair of venomglands and the hypodermic aculeus, the venom-injectingbarb. The abdomen’s front half, the mesosoma, is madeup of six segments. The first segment contains the sexualorgans as well as a pair of vestigial and modified ap-pendages forming a structure called the genital opercu-lum. The second segment bears a pair of featherlike sen-sory organs known as the pectines; the final four segmentseach contain a pair of book lungs. The mesosoma isarmored with chitinous plates, known as tergites on theupper surface and sternites on the lower surface.The cuticle of scorpions is covered with hairs in someplaces that act like balance organs. An outer layer thatmakes them fluorescent green under ultraviolet light iscalled the hyaline layer. Newly molted scorpions do notglow until after their cuticle has hardened. The fluores-cent hyaline layer can be intact in fossil rocks that arehundreds of millions of years old.Scorpions are opportunistic predators of small arthropodsand insects. They use their chela (pincers) to catch theprey initially. Depending on the toxicity of their venomand size of their claws, they will then either crush theprey or inject it with neurotoxic venom. The neuro-toxins consist of a variety of small proteins as well assodium and potassium cations, which serve to interferewith neurotransmission in the victim. Scorpions use theirvenom to kill or paralyze their prey so that it can be eaten;in general, it is fast acting, allowing for effective preycapture. Scorpion venoms are optimised for action onother arthropods and therefore most scorpions are rela-tively harmless to humans; stings produce only local ef-fects (such as pain, numbness or swelling). A few scor-pion species, however, mostly in the family Buthidae, canbe dangerous to humans. The scorpion that is responsi-ble for the most human deaths is the Androctonus aus-tralis, or fat-tailed scorpion of North Africa. The tox-icity of A. australis's venom is roughly half that of thedeathstalker (Leiurus quinquestriatus), but since A. aus-tralis injects quite a bit more venom into its prey, it is themost deadly to humans. Human deaths normally occur inthe young, elderly, or infirm; scorpions are generally un-able to deliver enough venom to kill healthy adults. Somepeople, however, may be allergic to the venom of somespecies, in which case the scorpion’s sting can more likelykill. A primary symptom of a scorpion sting is numbingat the injection site, sometimes lasting for several days. Ithas been found that scorpions have two types of venom:a translucent, weaker venom designed to stun only, andan opaque, more potent venom designed to kill heavierthreats.[27][28]

Unlike the majority of Arachnida species, scorpions areviviparous. The young are born one by one, and the broodis carried about on its mother’s back until the young haveundergone at least one moult.[29] The young generally re-semble their parents, requiring between five and sevenmoults to reach maturity. Scorpions have quite variablelifespans and the lifespan of most species is not known.

8 6 SYSTEMATICS

The age range appears to be approximately 4–25 years(25 years being the maximum reported life span in thegiant desert hairy scorpion, Hadrurus arizonensis). Theyare nocturnal and fossorial, finding shelter during the dayin the relative cool of underground holes or undersides ofrocks and coming out at night to hunt and feed. Scorpionsprefer to live in areas where the temperature is 20–37 °C(68–99 °F), but may survive in the temperature range of14–45 °C (57–113 °F).[30][31]

Scorpions have been found in many fossil records, includ-ing coal deposits from the Carboniferous Period and inmarine Silurian deposits. They are thought to have ex-isted in some form since about 450 to 425 million yearsago. They are believed to have an oceanic origin, withgills and a claw-like appendage that enabled them to holdonto rocky shores or seaweed.

6.12 Solifugae

Galeodes sp.

Main article: Solifugae

Solifugae is a group of 900 species of arachnids, com-monly known as camel spiders, wind scorpions, and sunspiders. The name derives from Latin, and means thosethat flee from the sun. Most Solifugae live in tropical orsemitropical regions where they inhabit warm and aridhabitats, but some species have been known to live ingrassland or forest habitats. The most distinctive fea-ture of Solifugae is their large chelicerae. Each of thetwo chelicerae are composed of two articles forming apowerful pincer; each article bears a variable number ofteeth. Males in all families but Eremobatidae possess aflagellum on the basal article of the chelicera. Solifugaealso have long pedipalps, which function as sense organssimilar to insects’ antennae and give the appearance of thetwo extra legs. Pedipalps terminate in reversible adhesiveorgans.Solifugae are carnivorous or omnivorous, with mostspecies feeding on termites, darkling beetles, and othersmall arthropods; however, solifugae have been video-taped consuming larger prey, such as lizards. Prey is lo-

cated with the pedipalps and killed and cut into pieces bythe chelicerae. The prey is then liquefied and the liquidingested through the pharynx. Reproduction can involvedirect or indirect sperm transfer; when indirect, the maleemits a spermatophore on the ground and then inserts itwith his chelicerae in the female’s genital pore.

6.13 Trigonotarbida

Main article: Trigonotarbida

The Order Trigonotarbida is an extinct group of arach-nids whose fossil record extends from the Silurian to theLower Permian. They are known from several localitiesin North Asia, North America and Argentina. They su-perficially resemble spiders, to which they were clearlyrelated.These early arachnids seem to have been adapted to stalk-ing prey on the ground. They have been found within thevery structure of ground-dwelling plants, possibly wherethey hid to await their prey. Trigonotarbids are currentlyamong the oldest known land arthropods. They lack silkglands on the opisthosoma and cheliceral poison glands,and most likely represented independent offshoots of theArachnida.

6.14 Thelyphonida

A whip scorpion

Main article: Thelyphonida

The Thelyphonida (formerly Uropygida), commonlyknown as vinegarroons or whip scorpions, range from 25to 85 mm in length; the largest species, of the genusMastigoproctus, reaches 85 mm (3.3 in). Like the relatedorders Schizomida, Amblypygi, and Solifugae, the vine-garroons use only six legs for walking, having modifiedtheir first two legs to serve as antennae-like sensory or-gans. Many species also have very large scorpion-likepedipalps (pincers). They have one pair of eyes at thefront of the cephalothorax and three on each side of the

9

head. Whip scorpions have no poison glands, but they dohave glands near the rear of their abdomen that can spraya combination of acetic acid and octanoic acid when theyare bothered. Other species spray formic acid or chlorine.As of 2006, over 100 species have been described world-wide.Whip scorpions are carnivorous, nocturnal hunters feed-ing mostly on insects but sometimes on worms and slugs.The prey is crushed between special teeth on the insideof the trochanters (the second segment of the leg) of thefront legs. They are valuable in controlling cockroach andcricket populations.Males secrete a sperm sac, which is transferred to the fe-male. Up to 35 eggs are laid in a burrow, within a mucousmembrane that preserves moisture. Mothers stay with theeggs and do not eat. The white young that hatch from theeggs climb onto their mother’s back and attach themselvesthere with special suckers. After the first molt, they looklike miniature whip scorpions, and leave the burrow; themother dies soon after. The young grow slowly, goingthrough three molts in about three years before reachingadulthood.Vinegarroons are found in tropical and subtropical areasworldwide, usually in underground burrows that they digwith their pedipalps. They may also burrow under logs,rotting wood, rocks, and other natural debris. They enjoyhumid, dark places and avoid the light.

7 See also

• Arachnophobia

• Endangered spiders

• Glossary of arachnology terms

• List of extinct arachnids

8 References[1] J. A. Dunlop (September 1996). “A trigonotarbid arach-

nid from the Upper Silurian of Shropshire” (PDF).Palaeontology 39 (3): 605–614. Retrieved October 12,2008.

[2] “Arachnid”. Oxford English Dictionary (2nd ed.). 1989.

[3] Joel Cracraft & Michael Donoghue, ed. (2004). Assem-bling the Tree of Life. Oxford University Press. p. 297.

[4] Günther Schmidt (1993). Giftige und gefährliche Spinnen-tiere [Poisonous and dangerous arachnids] (in German).Westarp Wissenschaften. p. 75. ISBN 3-89432-405-8.

[5] E. Ruppert, R. Fox & R. Barnes (2007). InvertebrateZoology: A Functional Evolutionary Approach (7th ed.).Thomson Learning. ISBN 0-03-025982-7.

[6] J. Kovoor (1978). “Natural calcification of the pro-somatic endosternite in the Phalangiidae (Arachnida:Opiliones)". Calcified Tissue Research 26 (3): 267–269.doi:10.1007/BF02013269. PMID 750069.

[7] Sensenig, Andrew T; Jeffrey W Shultz (2003-02-15).“Mechanics of Cuticular Elastic Energy Storage inLeg Joints Lacking Extensor Muscles in Arachnids”.Journal of Experimental Biology 206 (4): 771–784.doi:10.1242/jeb.00182. ISSN 1477-9145. Retrieved2012-05-18.

[8] Shultz, Jeffrey W (2005-02-06). “Evolution of lo-comotion in arachnida: The hydraulic pressure pumpof the giant whipscorpion, Mastigoproctus Giganteus(Uropygi)". Journal of Morphology 210 (1): 13–31.doi:10.1002/jmor.1052100103. ISSN 1097-4687.

[9] Shultz, Jeffrey W (1992-01-01). “Muscle Firing Patternsin Two Arachnids Using Different Methods of PropulsiveLeg Extension”. Journal of Experimental Biology 162 (1):313–329. ISSN 1477-9145. Retrieved 2012-05-19.

[10] Sensenig, Andrew T.; Jeffrey W. Shultz (2004). “ELAS-TIC ENERGY STORAGE IN THE PEDIPALPALJOINTS OF SCORPIONS AND SUN-SPIDERS(ARACHNIDA, SCORPIONES, SOLIFUGAE)".Journal of Arachnology 32 (1): 1–10. doi:10.1636/S02-73. ISSN 0161-8202.

[11] Robert D. Barnes (1982). Invertebrate Zoology. Philadel-phia, PA: Holt-Saunders International. pp. 596–604.ISBN 0-03-056747-5.

[12] Glauco Machado, Ricardo Pinto-da-Rocha & Gon-zalo Giribet (2007). Ricardo Pinto-da-Rocha, GlaucoMachado & Gonzalo Giribet, ed. Harvestmen: the Biol-ogy of Opiliones. Harvard University Press. ISBN 0-674-02343-9.

[13] Arthur D. Chapman (2005). Numbers of living species inAustralia and the world. Department of the Environmentand Heritage. ISBN 0-642-56850-2.

[14] D. E. Walter & H. C. Proctor (1999). Mites: Ecology,Evolution and Behaviour. University of New South WalesPress, Sydney and CAB International, Wallingford. ISBN0-86840-529-9.

[15] Jeanna Bryner (March 19, 2007). “Creepy: Spiders Loveto Snuggle”. LiveScience.

[16] Norman I. Platnick (2009). “The World Spider Catalog,version 9.5”. American Museum of Natural History. Re-trieved April 25, 2009.

[17] James H. Diaz (2004). “The global epidemiology, syn-dromic classification, management, and prevention of spi-der bites”. American Journal of Tropical Medicine andHygiene 71 (2): 239–250. PMID 15306718.

[18] Dolores Schütz & Michael Taborsky (2003).“Adaptations to an aquatic life may be responsiblefor the reversed sexual size dimorphism in the water spi-der, Argyroneta aquatica" (PDF). Evolutionary EcologyResearch 5 (1): 105–117.

10 9 EXTERNAL LINKS

[19] J. A. Dunlop (1999). “A redescription of the Car-boniferous arachnid Plesiosiro madeleyi Pocock, 1911(Arachnida: Haptopoda)" (PDF). Transactions of theRoyal Society of Edinburgh: Earth Sciences 90: 29–47.doi:10.1017/S0263593300002492.

[20] Adriano B. Kury (2011). Z.-Q. Zhang, ed. “Animal bio-diversity: an outline of higher-level classification and sur-vey of taxonomic richness” (PDF). Zootaxa 4138: 112–114. |chapter= ignored (help)

[21] Glauco Machado, Ricardo Pinto-da-Rocha & GonzaloGiribet (2007). “What are harvestmen?". In RicardoPinto-da-Rocha, Glauco Machado & Gonzalo Giribet.Harvestmen: the Biology of Opiliones. Harvard UniversityPress. pp. 1–13. ISBN 0-674-02343-9.

[22] Herbert W. Levi (1967). “Adaptations of respira-tory systems of spiders”. Evolution 21 (3): 571–583.doi:10.2307/2406617. JSTOR 2406617.

[23] Jessica R. Pollitt, Simon J. Braddy & Jason A. Dun-lop (2004). “The phylogenetic position of the extinctarachnid order Phalangiotarbida Haase, 1890, with ref-erence to the fauna from the Writhlington Geological Na-ture Reserve (Somerset, UK)". Transactions of the RoyalSociety of Edinburgh: Earth Sciences 94 (3): 243–259.doi:10.1017/S0263593300000651.

[24] Steve Jacobs (August 2006). “Entomological Notes:Pseudoscorpion Fact Sheet”. Pennsylvania State Univer-sity, Department of Entomology.

[25] Peter Weygoldt (1966). “Spermatophore Web Formationin a Pseudoscorpion”. Science 153 (3744): 1647–1649.doi:10.1126/science.153.3744.1647. PMID 17802636.

[26] Proctor, Heather C. (1993). “Mating Biology ResolvesTrichotomy for Cheliferoid Pseudoscorpions (Pseu-doscorpionida, Cheliferoidea)". Journal of Arachnology(American Arachnological Society) 21 (2): 156–158.

[27] David Cheng (June 23, 2005). “Scorpion sting”.eMedicine.

[28] Jan Ove Rein (1993). “Sting use in two species ofParabuthus scorpions (Buthidae)". Journal of Arachnol-ogy 21: 60–63.

[29] W. R. Lourenco (2000). “Reproduction in scorpions, withspecial reference to parthenogenesis” (PDF). In Scharff,N.; Toft, Søren; Henriksen, Per G. European arachnol-ogy 2000: 19th European Colloquium of Arachnology,Aarhus, Denmark, 17–22 July 2000. Aarhus: AarhusUniversity Press. pp. 71–85. ISBN 87-7934-001-6.

[30] Neil F. Hadley (1970). “Water relations of the desert scor-pion Hadrurus arizonensis" (PDF). Journal of Experimen-tal Biology 53 (3): 547–558. PMID 5487163.

[31] K. Hoshino, A. T. V. Moura & H. M. G. de Paula(2006). “Selection of environmental temperature by theyellow scorpion Tityus serrulatus Lutz & Mello, 1922(Scorpiones, Buthidae)". Journal of Venomous Animalsand Toxins including Tropical Diseases 12 (1): 59–66.doi:10.1590/S1678-91992006000100005.

9 External links• A guide to arachnids with care sheets.

11

10 Text and image sources, contributors, and licenses

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