Arachnid
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{{Taxobox
|\ name = Arachnida
| image = Haeckel Arachnida.jpg
| image_width = 250px
| image_caption = "Arachnida" from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| subphylum = [[Chelicerata]]
| classis = '''Arachnida'''
| classis_authority = [[Georges Cuvier|Cuvier]], 1812
| subdivision_ranks = Extant orders
| subdivision =
[[Acarina]]<br />
[[Amblypygi]]<br />
[[Spider|Araneae]]<br />
[[Opiliones]]<br />
[[Palpigradi]]<br />
[[Pseudoscorpion]]ida<br />
[[Ricinulei]]<br />
[[Schizomida]]<br />
[[Scorpiones]] <br />
[[Solifugae]]<br />
[[Uropygi]]
}}
'''Arachnids''' are a class ('''Arachnida''') of [[Arthropod|joint-legged]] [[invertebrate]] [[animal]]s in the subphylum [[Chelicerata]]. The term arachnid is from the [[Greek language|Greek]] word άράχνη or ''arachne'', meaning spider,<ref>{{cite encyclopedia|encyclopedia=[[Oxford English Dictionary]]|year=1989|edition=2nd edition|title=arachnid}}</ref> and also referring to the [[Greek mythology|mythological]] figure [[Arachne]].
Arachnids are chiefly terrestrial [[arthropod]]s, but are also found in freshwater and, with the exception of the [[pelagic zone]], in all marine environments. They comprise over 100,000{{fact|date=May 2008}} named [[species]], including [[spider]]s, [[scorpion]]s, [[opiliones|harvestmen]], [[tick]]s, and [[mite]]s.
It is commonly understood that arachnids have four pairs of legs, and that arachnids may be easily distinguished from [[insect]]s 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 [[reproduction|reproductive]] functions. In [[Solifugae]], the palpi are quite leg-like and make Solifugae appear to have ten legs. The [[larva]]e of mites (and [[Ricinulei]]) have only six legs; the fourth pair appears when they [[ecdysis|moult]] into [[nymph (biology)|nymph]]s. However, there are also adult mites with six, or even four legs.<ref name=schmidt93>Schmidt 1993: 58</ref>
Arachnids are further distinguished by the fact they have no [[antenna (biology)|antennae]] and no [[insect wing|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.<ref>Ruppert, E., Fox, R., & Barnes, R. (2007) Invertebrate Zoology: A functional evolutionary approach. 7th Edition. ''Thomson Learning'' ISBN 0-03-025982-7</ref>
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 [[Invertebrate trachea|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 [[carnivore|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 <ref name=pinto2007>Pinto-da-Rocha, R., Machado, G. & Giribet, G. (2007) Harvestmen — The Biology of Opiliones. ''Harvard University Press'' ISBN 0-674-02343-9</ref>, althougth it is not unusual for spiders to eat their own silk. Several groups are largely [[venom (poison)|venomous]] — they secrete venom from specialized [[gland]]s to kill prey or enemies. Several mites are [[parasite]]s, some of which are carriers of [[disease]]. Arachnids usually lay [[egg (biology)|eggs]], which hatch into immatures that resemble adults, but scorpions bear live young.
It is true that all arthropods have exoskeletons, 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 [http://www.ncbi.nlm.nih.gov/pubmed/750069].
Arachnids have two kinds of eyes, the lateral and median [[ocelli]]. The lateral ocelli evolved from [[compound eye]]s and may have a [[tapetum lucidum|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.<ref name=pinto2007/>
==Systematics==
* † [[Trigonotarbida]] — extinct
*[[Amblypygid|Amblypygi]] — "blunt rump" tailless whip scorpions with front legs modified into [[whip]]-like sensory structures as long as 25 cm or more (140 species)
{{userboxtop|toptext= |extra-css=width:40%}}
{{clade|style=font-size:75%;line-height:75%
|1=†[[Trilobita]]
|2=[[Xiphosura]]
|3={{clade
|1=[[Eurypterida]]
|label2='''Arachnida'''
|2={{clade
|1={{clade
|1=[[Scorpiones]]
|2={{clade
|1=[[Opiliones]]
|2={{clade
|1=[[Pseudoscorpiones]]
|2=[[Solifugae]]
}}
}}
}}
|2={{clade
|1={{clade
|1=[[Acari]]
|2={{clade
|1=[[Palpigradi]]
|2=[[Pycnogonida]]
}}
}}
|2={{clade
|1={{clade
|1={{clade
|1=†[[Trigonotarbida]]
|2=[[Ricinulei]]
}}
|2={{clade
|1=[[Araneae]]
|2={{clade
|1=[[Amblypygi]]
|2={{clade
|1=[[Uropygi]]
|2=[[Schizomida]]
}}
}}
}}
}}
}}
}}
}}
}}
}}
<center><small>
Phylogeny of the Chelicerata<br>
(after Giribet ''et al.'' 2002)
</small></center>
{{userboxbottom}}
*[[Araneae]] — spiders (40,000 species)
**[[Mesothelae]] — very rare, [[basal (phylogenetics)|basal]] spiders, with abdomen segmented and [[spinneret]]s median
**[[Opisthothelae]] — spiders with abdomen unsegmented and spinnerets located posteriorly
***[[Araneomorphae]] — most common [[spiders]]
***[[Mygalomorphae]] — [[tarantula]]s and tarantula-like spiders
*† [[Phalangiotarbida]] — extinct
*[[Opiliones]] — phalangids, harvestmen or daddy-long-legs (6,300 species)
*[[Palpigradi]] — microwhip scorpions (80 species)
*[[Pseudoscorpionida]] — pseudoscorpions (3,000 species)
*[[Ricinulei]] — ricinuleids, hooded tickspiders (60 species)
*[[Schizomid]]a — "split middle" whip scorpions with divided exoskeletons (220 species)
*[[Scorpiones]] — scorpions (2,000 species)
*[[Solifugae]] — solpugids, windscorpions, sun spiders or camel spiders (900 species)
*† [[Haptopoda]] — extinct
*[[Uropygid|Uropygi]] — whip scorpion, forelegs modified into sensory appendages and a long tail on abdomen tip (100 species)
*[[Acarina]] — [[mite]]s and [[tick]]s (30,000 species)
**[[Acariformes]]
***[[Sarcoptiformes]]
***[[Trombidiformes]]
**[[Opilioacariformes]]
**[[Parasitiformes]] — [[holothyran]]s, ticks and [[mesostigmatic mite]]s
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 <ref>{{Cite book |author=Arthur D. Chapman |title=Numbers of living species in Australia and the world |publisher=[[Department of the Environment and Heritage]] |year=2005 |id=ISBN 0-642-56850-2 |url=http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/pubs/number-living-species-report.pdf}}</ref>.
===Acarina===
[[Image:Tick 2 (aka).jpg|thumb|right|''[[Ixodes ricinus]]'', a tick]]
{{Main|Acarina}}
'''Acarina''' or '''Acari''' are a [[taxon]] of arachnids that contains [[mite]]s and [[tick]]s. 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]].<ref name="WalProc">{{cite book |author=D. E. Walter & H. C. Proctor |year=1999 |title=Mites: Ecology, Evolution and Behaviour |publisher=[[University of New South Wales Press]], Sydney and [[CABI]], Wallingford |id=ISBN 0-86840-529-9}}</ref>
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 mite]]s 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 [[Ecoregion|habitat]], and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other [[arthropod]]s in the soil [[organic material|organic matter]] and [[detritus (biology)|detritus]]. Many are [[parasitism|parasitic]], and they affect both [[vertebrate]]s and [[invertebrate]]s. Most parasitic forms are external parasites, while the free living forms are generally [[predation|predaceous]] and may even be used to control undesirable arthropods. Others are [[detritivore]]s that help to break down forest [[plant litter|litter]] and dead organic matter such as [[skin]] cells. Others still are [[herbivory|plant feeders]] and may damage [[Crop (agriculture)|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 [[mammal]]s, causing damage by their feeding, and can even be [[vector (biology)|vectors]] of diseases such as [[scrub typhus]] and [[rickettsia]]l 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 [[weed]]s are also of importance. An unquantified, but major positive contribution of the Acari is their normal functioning in [[ecosystem]]s, especially their roles in the decomposer subsystem <ref name="WalProc"/>.
===Amblypygi===
[[Image:Amblypigid.jpg|thumb|right|An amblypygid]]
{{Main|Amblypygi}}
Amblypygids are also known as '''tailless whip scorpions''' or '''cave spiders'''. Approximately 5 families, 17 genera and 136 species have been described. They are found in [[tropics|tropical]] and [[subtropics|subtropical]] regions worldwide. Some species are subterranean; many are [[nocturnal animal|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 [[millimetre|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 [[solifugid]]s, [[uropygid]]s, and [[schizomid]]s.) These very thin modified legs can extend several times the length of body. They have no [[silk]] glands or [[venom]]ous fangs, but can have prominent pincer-like [[pedipalp]]s. 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]]. [[Fossil]]ised amblypygids have been found dating back to the [[Carboniferous|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 as a cage, they would seek each other out and gather back in a group <ref>{{cite web |author=Jeanna Bryner |title= Creepy: Spiders Love to Snuggle | format= | work= LiveScience| url= http://www.livescience.com/animalworld/070319_sweet_spiders.html | date=[[2007-03-19]]}}</ref>
===Araneae===
[[Image:Araneus diadematus (aka).jpg|thumb|right|''[[Araneus diadematus]]'']]
{{Main|spider}}
Spiders are the most familiar of the arachnids, and the most numerous, if only described species are counted. All spiders produce [[spider silk|silk]], a thin, strong [[protein]] strand extruded by the spider from [[spinneret]]s most commonly found on the end of the abdomen. Many species use it to trap insects in [[spider web|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 [[ballooning (spider)|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 [[Venomous Spiders|bites]] that can pose health problems to humans.<ref name="global">{{cite journal |url=http://www.ajtmh.org/cgi/content/full/71/2/239 |author=James H. Diaz |year=2004 |title=The global epidemiology, syndromic classification, management, and prevention of spider bites |journal=[[American Journal of Tropical Medicine and Hygiene]] |volume=71 |issue=2 |pages=239–250 |quotes=no |pmid=15306718 |doi=10.1126/science.153.3744.1647}}</ref> 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===
{{Main|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 1911. Relationships with other arachnids are obscure, but closest relatives may be the [[Amblypygi]], [[Thelyphonida]] and [[Schizomida]] of the tetrapulmonate clade.<ref>{{cite book |author=Dunlop |year=2000 |chapter=The Carboniferous arachnid ''Plesiosiro'' |title=19th European Colloquium of Arachnology, 17–22 July 2000 |publisher=[[University of Aarhus]], Denmark}}</ref>
===Opiliones===
[[Image:Paroligolophus.agrestis.jpg|thumb|right|''[[Paroligolophus agrestis]]'']]
{{Main|Opiliones}}
'''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 chert]]s 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 gland]]s that secrete a peculiar smelling fluid when disturbed. Harvestmen do not have [[silk gland]]s and do not possess poison glands, posing absolutely no danger to humans. They breathe through [[invertebrate trachea|tracheae]]. Between the base of the fourth pair of legs and the abdomen a pair of [[spiracle]]s 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 [[millimetre|mm]] (about ¼ [[inch|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 [[fungus|fungi]]; some are [[scavenger]]s 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 animal|nocturnal]] and coloured in hues of brown, although there are a number of [[diurnal animal|diurnal]] species that have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
===Palpigradi===
{{Main|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 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 lung]]s, 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.
===Phalangiotarbida===
{{Main|Phalangiotarbida}}
'''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===
[[Image:Ar 1.jpg|thumb|right|A pseudoscorpion on a printed page]]
{{Main|pseudoscorpion}}
Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of [[scorpion]]s. They range from 2 to 8 [[millimetre|mm]] ({{frac|1|12}} to 1/3 [[inch]]) in length <ref name=Penn>{{cite web |url=http://www.ento.psu.edu/extension/factsheets/pseudoscorpion.htm |title=Entomological Notes: Pseudoscorpion Fact Sheet |publisher=[[Pennsylvania State University]], Department of Entomology |author=Steve Jacobs |monthyear=August 2006}}</ref>. The opisthosoma is made up of twelve segments, each guarded by plate-like [[tergite]]s above and [[sternite]]s 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'' ([[pedipalp]]s 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 (poison)|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 [[Cocoon (silk)|cocoon]]s for mating, molting, or waiting out cold weather. Another trait they share with their closest relatives, the [[spider]]s, is breathing through [[spiracle]]s. 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 <ref>{{cite journal |quotes=no |author=Peter Weygoldt |url=http://www.sciencemag.org/cgi/content/abstract/153/3744/1647 |title=Spermatophore Web Formation in a Pseudoscorpion |journal=[[Science (journal)|Science]] |volume=153 |issue=3744 |pages=1647–1649 |year=1966 | doi = 10.1126/science.153.3744.1647 <!--Retrieved from URL by DOI bot--> |pmid=17802636}}</ref>. In other species, the male also pushes the sperm into the female genitals using the forelegs <ref>{{cite journal |quotes=no |url=http://www.americanarachnology.org/JoA_free/JoA_v21_n2/JoA_v21_p156.pdf |title=Mating biology resolves trichotomy for cheliferoid pseudoscorpions (Pseudoscorpionida, Cheliferoidea) |journal=[[Journal of Arachnology]] |volume=21 |issue=2 |year=1993 |doi=10.1126/science.153.3744.1647 |pmid=17802636 |author=Weygoldt, P. |pages=1647 |unused_data=|Heather C. Proctor}}</ref>.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 <ref name=Penn />. 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 [[ecdysis|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 [[moth|clothes moth]] larvae, [[carpet beetle]] larvae, [[Psocoptera|booklice]], [[ant]]s, [[mite]]s, and small [[Diptera|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.
===Ricinulei===
{{Main|Ricinulei}}
'''Riniculei''' (''hooded tickspiders'') are 5–10 [[millimetre|mm]] long. Their most notable feature is a "hood" that 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 [[scorpion]]s and [[pseudoscorpion]]s. 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 tubule]]s 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 [[ecdysis|molt]] into their adult forms. Ricinulei require moisture to survive. Approximately 57 species of ricinuleids have been described worldwide, all in a single family that contains 3 genera.
===Schizomida===
{{Main|Schizomida}}
'''Schizomida''' is an [[Order (biology)|order]] of arachnids that tend to live in the top layer of soils. Schizomids present the prosoma covered by a large [[protopeltidium]] and smaller, paired, [[mesopeltidium|mesopeltidia]] and [[metapeltidium|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 [[Uropygid|Uropygi]], [[Amblypygid|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.
===Scorpions===
[[Image:SCORPIO MAURUS PALMATUS.jpg|thumb|200px|right|''[[Scorpio maurus|Scorpio maurus palmatus]]'']]
{{Main|scorpion}}
'''Scorpions''' are characterised by a metasoma (tail) comprising six segments, the last containing the scorpion's [[anus]] and bearing the [[telson]] (the [[sting (biology)|sting]]). The telson, in turn, consists of the [[vesicle (biology)|vesicle]], which holds a pair of [[venom]] glands and the hypodermic aculeus, the venom-injecting [[barb]]. The abdomen's front half, the mesosoma, is made up of six segments. The first segment contains the [[sex organ|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 lung]]s. The mesosoma is [[armor]]ed with [[chitin]]ous plates, known as [[tergite]]s on the upper surface and [[sternite]]s 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 [[protein]]s as well as sodium and potassium [[cation]]s, 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 [[arthropod]]s 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 that is responsible for the most human deaths is the ''[[Fattail scorpion|Androctonus australis]]'', or fat-tailed scorpion of [[North Africa]]. The toxicity of ''A. australis''<nowiki>'</nowiki>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 <ref>{{cite web |url=http://www.emedicine.com/med/topic2081.htm |title=Scorpion sting |author=David Cheng |date=[[2005-06-23]] |publisher=[[eMedicine]]}}</ref><ref>{{cite journal |quotes=no |url=http://www.americanarachnology.org/JoA_free/JoA_v21_n1/JoA_v21_p60.pdf |title=Sting use in two species of ''Parabuthus'' scorpions (Buthidae) |year=1993 |journal=[[Journal of Arachnology]] |volume=21 |pages=60–63 |author=Jan Ove Rein}}</ref>.
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 [[ecdysis|moult]].<ref>{{cite journal |quotes=no |author=W. R. Lourenco |year=2000 |title=Reproduction in scorpions, with special reference to parthenogenesis |journal=[[European Arachnology]] |pages=71–85 |volume= 153|issue= |doi=10.1126/science.153.3744.1647 |pmid=17802636}}</ref> 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[[Celsius|°C]] to 37 °C (68[[Fahrenheit|°F]] to 99 °F), but may survive in the temperature range of 14 °C to 45 °C (57 °F to 113 °F)<ref>{{cite journal |url=http://jeb.biologists.org/cgi/reprint/53/3/547.pdf |quotes=no |author=Neil F. Hadley |title=Water relations of the desert scorpion ''Hadrurus arizonensis'' |journal=[[Journal of Experimental Biology]] |volume=53 |pages=547–558 |year=1970}}</ref><ref>{{cite journal |quotes=no |author=K. Hoshino, A. T. V. Moura & H. M. G. de Paula |year=2006 |title=Selection of environmental temperature by the yellow scorpion ''Tityus serrulatus'' Lutz & Mello, 1922 (Scorpiones, Buthidae) |journal=J. Venom. Anim. Toxins incl. Trop. Dis. |url=http://www.scielo.br/pdf/jvatitd/v12n1/28301.pdf |volume=12 |issue=1 |pages=59–66 |doi=10.1590/S1678-91992006000100005}}</ref>.
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===
[[Image:Galeodes.jpg|thumb|right|''[[Galeodes|Galeodes sp.]]'']]
{{Main|Solifugae}}
'''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 [[Eremobates|Eremobatidae]] possess a [[flagellum]] on the basal article of the chelicera. Solifugae also have long [[pedipalp]]s, which function as sense organs similar to insects' [[antenna (biology)|antennae]] and give the appearance of the two extra legs. Pedipalps terminate in eversible adhesive organs.
Solifugae are [[carnivore|carnivorous]] or [[omnivore|omnivorous]], with most species feeding on [[termite]]s, [[darkling beetle]]s, and other small [[arthropod]]s; 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.
===Trigonotarbida===
{{Main|Trigonotarbida}}
The [[Order (biology)|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 [[spider]]s, to which they were clearly related. It was once thought that trigonotarbids lacked the [[Spider silk|silk]]-producing [[spinneret]]s that have apparently been crucial to the spider's [[evolution]]ary success, though in recent years at least one fossil find seems to show distinct [[microtubercle]]s 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.
===Uropygi===
[[Image:Whipscorpion.jpg|thumb|right|A uropygid]]
{{Main|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 [[Schizomid]]a, [[Amblypygid|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 [[carnivore|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 [[subtropics|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.
==References==
{{reflist}}
*
spiders are cousins to rats.
==See also==
*[[Endangered spiders]]
{{wikispecies|Arachnida}}
{{commonscat|Arachnida}}
{{Arthropods}}
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