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    Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata Arachnids are named after the mythological figure Arachne They are chiefly terrestrial arthropods comprising some 65000 to 73000 named species including spiders scorpions harvestmen ticks and mites


    Arachnids may be easily distinguished from insects by the fact that arachnids have eight legs whereas insects have six The chelicerae serve to macerate food particles The first post-oral pair of appendages — pedipalps (leg-like mouthparts) — of some species have been adapted for sensory prey capture or reproductive functions In Solifugae the palpi are quite leg-like and make Solifugae appear to have ten legs The larvae of mites have only six legs; the fourth pair appears when they moult into nymph Arachnids are further distinguished by the fact they have no antennae and no wings They have a two-segmented body made up of a cephalothorax and an abdomen the cephalothorax being derived from the fusion of the cephalon (head) and the thorax
    Arachnids are mostly carnivorous feeding on the pre-digested bodies of insects and other small animals Many are venomous - they secrete poison from specialised glands to kill prey or enemies Others are parasites some of which are carriers of disease Arachnids usually lay eggs which hatch into immature adults

    Arachnida in human culture

    Arachnids since ancient times have captured the human imagination for example as objects of superstitious fear They make their appearance in the Western zodiac in the constellation of Scorpius They are also present in the Egyptian hieroglyphs and diverse mythological characters such as the Greek Arachné and the west African Anansi There are a number of gods based on arachnids such as the Egyptian Serket Images of scorpion-men are present in Assyrian reliefs


    • Trigonotarbida - extinct
    • Amblypygi - "blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more
    • Araneae - spiders (40000 species)
      • Mesothelae - very rare basal spiders with abdomen segmented and spinnerets median
      • Opisthothelae - spiders with abdomen unsegmented and spinnerets located posteriorly
        • Araneomorphae - most common spiders
        • Mygalomorphae - tarantulas and tarantula-like spiders
    • Phalangiotarbida - extinct
    • Opiliones - phalangids harvestmen or daddy-long-legs (6300 species)
    • Palpigradi - microwhip scorpions
    • Pseudoscorpionida - pseudoscorpions
    • Ricinulei - ricinuleids hooded tickspiders
    • Schizomida - "split middle" whip scorpions with divided exoskeletons
    • Scorpiones - scorpions (2000 species)
    • Solifugae - solpugids windscorpions sun spiders or camel spiders (900 species)
    • Haptopoda - extinct
    • Uropygi - whip scorpions with first legs modified as whip-like sensory organs and with a long thin tail at end of abdomen (100 species)
    • Acarina - mites and ticks (30000 species)
      • Acariformes
        • Sarcoptiformes
        • Trombidiformes
      • Opilioacariformes
      • Parasitiformes - holothyrans ticks and mesostigmatic mites
    It is estimated that a total of 98000 arachnid species have been described and that there may be up to 600000 in total including undescribed species 


    a tick
    Acarina or Acari are a taxon of arachnids that contains mites and ticks Its fossil history goes back to the Devonian era In most modern treatments the Acari is considered a subclass of Arachnida and is composed of 2-3 orders or superorders: Acariformes Parasitiformes and Opilioacariformes Most acarines are minute to small (eg 0080-100mm) but the giants of the Acari (some ticks and red velvet mites) may reach lengths of 10-20mm It is estimated that over 50000 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 (eg 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 


    Amblypygids are also known as
    tailless whip scorpions Approximately 5 families 17 genera and 136 species have been described They are found in tropical and subtropical regions worldwide Some species are subterranean; many are nocturnal During the day they may hide under logs bark stones or leaves They prefer a humid environment Amblypygids may range from 5 to 40 mm Their bodies are broad and highly flattened and the first pair of legs (the first walking legs in most arachnid orders) are modified to act as sensory organs (Compare solifugids uropygids and schizomids) These very thin modified legs can extend several times the length of body They have no silk glands or venomous fangs but can have prominent pincer-like pedipalps Amblypygids often move about sideways on their six walking legs with one "whip" pointed in the direction of travel while the other probes on either side of them Prey are located with these "whips" captured with pedipalps then torn to pieces with chelicerae Fossilised amblypygids have been found dating back to the Carboniferous period
    Amblypygids particularly the species Phrynus marginemaculatus and Damon diadema are thought to be one of the few species of arachnids that show signs of social behavior Research conducted at Cornell University by entomologists suggests that mother amblypygids comfort their young by gently caressing the offspring with her feelers Further when two or more siblings were placed in an unfamiliar environment such a cage they would seek each other out and gather back in a group 


    Spiders are the most numerous and familiar of the arachnids All spiders produce silk a thin strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen Many species use it to trap insects in webs although there are many species that hunt freely Silk can be used to aid in climbing form smooth walls for burrows build egg sacs wrap prey and temporarily hold sperm among other applications
    All spiders except those in the families Uloboridae and Holarchaeidae and in the suborder Mesothelae (together about 350 species) can inject venom to protect themselves or to kill and liquefy prey Only about 200 species however have bites that can pose health problems to humans  Many larger species' bites may be painful but will not produce lasting health concerns
    Spiders are found all over the world from the tropics to the Arctic living underwater in silken domes they supply with air and on the tops of mountains


    Haptopoda' is an extinct order known exclusively from a few specimens from the Upper Carboniferous of the United Kingdom It is monotypic i. e., has only one species Plesiosiro madeleyi Pocock 191 Relationships with other arachnids are obscure but closest relatives may be the Amblypygi Thelyphonida and Schizomida of the tetrapulmonate clade 


    Opiliones (better known as "harvestmen") are arachnids which are harmless to people and are known for their exceptionally long walking legs compared to their body size As of 2005 over 6300 species of Phalangids have been discovered worldwide The order Opiliones can be divided in four suborders: Cyphophthalmi Eupnoi Dyspnoi and Laniatores Well-preserved fossils have been found in the 400-million year old Rhynie cherts of Scotland which looks surpringly modern indicating that the basic structure of the harvestmen hasn't changed much since then Their closest relatives are probably the mites (Acari)
    The difference between harvestmen and spiders is that in harvestmen the two main body sections (the abdomen with ten segments and cephalothorax or prosoma and opisthosoma) are nearly joined so that they appear to be one oval structure In more advanced species the first five abdominal segments are often fused into a dorsal shield called the scutum which is normally fused with the carapace Sometimes this shield is only present in males The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates laying next to each other The second pair of legs are longer than the others and works as antennae They have a single pair of eyes in the middle of their heads orientated sideways They have a pair of prosomatic scent glands that secrete a peculiar smelling fluid when disturbed Harvestmen do not have silk glands and do not possess poison glands posing absolutely no danger to humans They breathe through tracheae Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located one opening on each side In more active species spiracles are also found upon the tibia of the legs They have a gonopore on the ventral cephalothorax and the copulation is direct as the male has a penis (while the female has an ovipositor)
    Typical body length does not exceed 7 mm (about ¼ in) even in the largest species However leg span is much larger and can exceed 160 mm (over 6 in) Most species live for a year Many species are omnivorous eating primarily small insects and all kinds of plant material and fungi; some are scavengers of the decays of any dead animal bird dung and other fecal material Mating involves direct copulation rather than the deposition of a spermatophore They are mostly nocturnal and coloured in hues of brown although there are a number of diurnal species which have vivid patterns in yellow green and black with varied reddish and blackish mottling and reticulation


    Palpigradi commonly known as "microwhip scorpions" are tiny cousins of the uropygid or whip scorpion no more than 3 mm in length They have a thin pale segmented carapace which terminates in a whip-like flagellum made up of 15 segments The carapace is divided into two plates between the third and fourth leg set They have no eyes Some species have three pairs of book lungs while others have no lungs at all Approximately 80 species of Palpigradi have been described worldwide all in the family
    Eukoeneniidae which contains four genera
    They are believed to be predators like their larger relatives feeding on minuscule insects in their habitat Their mating habits are unknown except that they lay only a few relatively large eggs at a time Microwhip scorpions need a damp environment to survive and they always hide from light so they are commonly found in the moist earth under buried stones and rocks They can be found on every continent except in arctic and antarctic regions


    Phalangiotarbi (Haase 1890) is an extinct arachnid order known exclusively from the Upper Carboniferous of Europe and North America
    The affinities of phalangiotarbids are obscure with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks) Phalangiotarbida has been recently proposed to be sister group to (Palpigradi+Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990) (Pollitt et al 2004)


    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 ⅓ 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) which strongly resemble the pincers found on a scorpion The pedipalps generally consist of an immobile "hand" and "finger" with a separate movable finger controlled by an adductor muscle A venom gland and duct are usually located in the mobile finger; the poison is used to capture and immobilise the pseudoscorpion's prey During digestion pseudoscorpions pour a mildly corrosive fluid over the prey then ingest the liquefied remains Pseudoscorpions spin silk from a gland in their jaws to make disk-shaped cocoon 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 2000 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 dozen young are hatched in a single brood; there may be more than one brood per year The young go through three molts over the course of several years before reaching adulthood Adult pseudoscorpions live 2 to 3 years They are active in the warm months of the year overwintering in silken coccoons when the weather grows cold
    Pseudoscorpions are generally beneficial to humans since they prey on clothes moth larvae carpet beetle larvae booklice ants mites and small flies They are small and inoffensive and are rarely seen due to their size They usually enter the home by "riding along" with larger insects (known as phoresy) or are brought in with firewood They are often observed in bathrooms or laundry rooms since they seek humidity They may sometimes be found feeding on mites under the wing covers of certain beetles


    Riniculei (hooded tickspiders) are 5–10 mm long Their most notable feature is a "hood" which can be raised and lowered over the head; when lowered it covers the mouth and the chelicerae Ricinulei have no eyes The pedipalps end in pincers that are small relative to their bodies when compared to those of the related orders of scorpions and pseudoscorpions The heavy-bodied abdomen forms a narrow pedicel or waist where it attaches to the prosoma In males the third pair of legs are modified to form copulatory organs Malpighian tubules and a pair of coxal glands make up the excretory system They have no lungs as gas exchange takes place through the trachea
    Ricinulei are predators feeding on other small arthropods Little is known about their mating habits; the males have been observed using their modified third leg to transfer a spermatophore to the female The eggs are carried under the mother's hood until the young hatch into six-legged "larva" which later molt into their adult forms Ricinulei require moisture to survive Approximately 57 species of ricinuleids have been described worldwide all in a single family which contains 3 genera


    Schizomida is an order of arachnids which tend to live in the top layer of soils Schizomids present the prosoma covered by a large protopeltidium and smaller paired mesopeltidia and metapeltidia There are no eyes The opisthosoma is a smooth oval of 12 recognisable somites The first is reduced and forms the pedicel The last three are much constricted forming the pygidium The last somite bears the flagellum which in this order is short and consists of not more than four segments
    The name means "split or cleaved middle" referring to the way the cephalothorax is divided into two separate plates Like the related orders Uropygi Amblypygi and Solpugida the schizomids use only six legs for walking having modified their first two legs to serve as sensory organs They also have large well-developed pedipalps (pincers) just behind the sensory legs


    Scorpions' are characterised by a metasoma (tail) comprising six segments the last containing the scorpion's anus and bearing the telson (the sting) The telson in turn consists of the vesicle which holds a pair of venom glands and the hypodermic aculeus the venom-injecting barb The abdomen's front half the mseosoma is made up of six segments The first segment contains the sexual organs as well as a pair of vestigial and modified appendages forming a structure called the genital operculum The second segment bears a pair of featherlike sensory organs known as the pectines; the final four segments each contain a pair of book lungs The mesosoma is armored with chitinous plates known as tergites on the upper surface and sternites on the lower surface
    The cuticle of scorpions is covered with hairs in some places that act like balance organs An outer layer that makes them fluorescent green under ultraviolet light is called the hyaline layer Newly molted scorpions do not glow until after their cuticle has hardened The fluoresent hyaline layer can be intact in fossil rocks that are hundreds of millions of years old
    Scorpions are opportunistic predators of small arthropods and insects They use their chela (pincers) to catch the prey initially Depending on the toxicity of their venom and size of their claws they will then either crush the prey or inject it with neurotoxic venom The neurotoxins consist of a variety of small proteins as well as sodium and potassium cations which serve to interfere with neurotransmission in the victim Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting allowing for effective prey capture Scorpion venoms are optimised for action upon other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain numbness or swelling) A few scorpion species however mostly in the family Buthidae can be dangerous to humans The scorpion which is responsible for the most human deaths is the Androctonus australis or fat-tailed scorpion of North Africa The toxicity of A australis's venom is roughly half that of L quinquestriatus but since A australis injects quite a bit more venom into its prey it is the most deadly to humans Human deaths normally occur in the young elderly or infirm; scorpions are generally unable to deliver enough venom to kill healthy adults Some people however may be allergic to the venom of some species in which case the scorpion's sting can more likely kill A primary symptom of a scorpion sting is numbing at the injection site sometimes lasting for several days It has been found that scorpions have two types of venom: a translucent weaker venom designed to stun only and an opaque more potent venom designed to kill heavier threats 
    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 arachnid commonly known as camel spiders wind scorpions and sun spiders The name derives from Latin and means those that flee from the sun Most Solifugae live in tropical or semitropical regions where they inhabit warm and arid habitats but some species have been known to live in grassland or forest habitats The most distinctive feature of Solifugae is their large chelicerae Each of the two chelicerae are composed of two articles forming a powerful pincer; each article bears a variable number of teeth Males in all families but Eremobatidae possess a flagellum on the basal article of the chelicera Solifugae also have long pedipalps which function as sense organs similar to insects' antennae and give the appearance of the two extra legs Pedipalps terminate in eversible adhesive organs
    Solifugae are carnivorous or omnivorous with most species feeding on termites darkling beetles and other small arthropods; however solifugae have been videotaped consuming larger prey such as lizards Prey is located with the pedipalps and killed and cut into pieces by the chelicerae The prey is then liquefied and the liquid ingested through the pharynx Reproduction can involve direct or indirect sperm transfer; when indirect the male emits a spermatophore on the ground and then inserts it with his chelicerae in the female's genital pore


    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


    Uropygi commonly known as whip scorpions range from 25 to 85 mm in length; the largest species of the genus Mastigoproctus reaches 85 mm. Like the related orders Schizomida Amblypygi and Solifugae the uropygids use only six legs for walking having modified their first two legs to serve as antennae-like sensory organs Many species also have very large scorpion-like pedipalps (pincers) They have one pair of eyes at the front of the cephalothorax and three on each side of the head Whip scorpions have no poison glands but they do have glands near the rear of their abdomen that can spray a combination of acetic acid and octanoic acid when they are bothered Other species spray formic acid or chlorine As of 2006 over 100 species of uropygids have been described worldwide
    Whip scorpions are carnivorous nocturnal hunters feeding mostly on insects but sometimes on worms and slugs The prey is crushed between special teeth on the inside of the trochanters (the second segment of the leg) of the front legs They are valuable in controlling the population of roaches and crickets
    Males secrete a sperm sac which is transferred to the female Up to 35 eggs are laid in a burrow within a mucous membrane that preserves moisture Mothers stay with the eggs and do not eat The white young that hatch from the eggs climb onto their mother's back and attach themselves there with special suckers After the first molt they look like miniature whip scorpions and leave the burrow; the mother dies soon after The young grow slowly going through three molts in about three years before reaching adulthood
    Uropygids are found in tropical and subtropical areas worldwide usually in underground burrows which they dig with their pedipalps They may also burrow under logs rotting wood rocks and other natural debris They enjoy humid dark places and avoid the light