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Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. There are about 2,000 species of scorpions, found widely distributed south of about 49° N, except New Zealand and Antarctica. The northernmost part of the world where scorpions live in the wild is Sheerness on the Isle of Sheppey in the UK, where a small colony of Euscorpius flavicaudis has been resident since the 1860s. The word scorpion derives from Greek σκορπιός – skorpios.

Anatomy
The body of a scorpion is divided into two parts: the cephalothorax (also called the prosoma) and the abdomen (opisthosoma). The abdomen consists of the mesosoma and the metasoma.

Cephalothorax
The cephalothorax, also called the prosoma, is the scorpion's “head”, comprising the carapace, eyes, chelicerae (mouth parts), pedipalps (claws) and four pairs of walking legs. The scorpion's exoskeleton is thick and durable, providing good protection from predators. Scorpions have two eyes on the top of the head, and usually two to five pairs of eyes along the front corners of the head. The position of the eyes on the head depends in part on the hardness or softness of the soil upon which they spend their lives.

Metasoma


The metasoma, the scorpion's tail, comprises six segments (the first tail segment looks like a last mesosoman segment), 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.

On rare occasions, scorpions can be born with two metasomata (tails). Two-tailed scorpions are not a different species, merely a genetic abnormality.

Reproduction
Most scorpions reproduce sexually, and most species have male and female individuals. However, some species, such as Hottentotta hottentotta, Hottentotta caboverdensis, Liocheles australasiae, Tityus columbianus, Tityus metuendus, Tityus serrulatus, Tityus stigmurus, Tityus trivittatus, and Tityus urugayensis, reproduce through parthenogenesis, a process in which unfertilized eggs develop into living embryos. Parthenogenic reproduction starts following the scorpion's final moult to maturity and continues thereafter.

Sexual reproduction is accomplished by the transfer of a spermatophore from the male to the female; scorpions possess a complex courtship and mating ritual to effect this transfer. Mating starts with the male and female locating and identifying each other using a mixture of pheromones and vibrational communication. Once they have satisfied each other that they are of opposite sex and of the correct species, mating can commence.

The courtship starts with the male grasping the female’s pedipalps with his own; the pair then perform a "dance" called the "promenade à deux". In reality this is the male leading the female around searching for a suitable place to deposit his spermatophore. The courtship ritual can involve several other behaviours such as juddering and a cheliceral kiss, in which the male's chelicerae—clawlike mouthparts—grasp the female's in a smaller more intimate version of the male's grasping the female's pedipalps and in some cases injecting a small amount of his venom into her pedipalp or on the edge of her cephalothorax, probably as a means of pacifying the female.

When the male has identified a suitable location, he deposits the spermatophore and then guides the female over it. This allows the spermatophore to enter her genital opercula, which triggers release of the sperm, thus fertilizing the female. The mating process can take from 1 to 25+ hours and depends on the ability of the male to find a suitable place to deposit his spermatophore. If mating goes on for too long, the female may eventually lose interest, breaking off the process.

Once the mating is complete, the male and female will separate. The male will generally retreat quickly, most likely to avoid being cannibalized by the female, although sexual cannibalism is infrequent with scorpions.

Birth and development


Unlike the majority of arachnid 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 molt. Before the first molt, scorplings cannot survive naturally without the mother, since they depend on her for protection and to regulate their moisture levels. Especially in species which display more advanced sociability (e.g. Pandinus spp.), the young/mother association can continue for an extended period of time. The size of the litter depends on the species and environmental factors, and can range from two to over a hundred scorplings. The average litter however, consists of around 8 scorplings.

The young generally resemble their parents. Growth is accomplished by periodic shedding of the exoskeleton (ecdysis). A scorpion's developmental progress is measured in instars (how many moults it has undergone). Scorpions typically require between five and seven moults to reach maturity. Moulting is effected by means of a split in the old exoskeleton which takes place just below the edge of the carapace (at the front of the prosoma). The scorpion then emerges from this split; the pedipalps and legs are first removed from the old exoskeleton, followed eventually by the metasoma. When it emerges, the scorpion’s new exoskeleton is soft, making the scorpion highly vulnerable to attack. The scorpion must constantly stretch while the new exoskeleton hardens to ensure that it can move when the hardening is complete. The process of hardening is called sclerotization. The new exoskeleton does not fluoresce; as sclerotization occurs, the fluorescence gradually returns.

Life and habits
Scorpions have quite variable lifespans and the actual 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 Hadrurus arizonensis). Lifespan of Hadogenes species in the wild is estimated at 25–30 years.

Scorpions prefer to live in areas where the temperatures range from 20 °C to 37 °C (68 °F to 99 °F), but may survive from freezing temperatures to the desert heat. Scorpions of the genus Scorpiops living in high Asian mountains, bothriurid scorpions from Patagonia and small Euscorpius scorpions from middle Europe can all survive winter temperatures of about −25 °C. In Repetek (Turkmenistan) there live seven species of scorpions (of which Pectinibuthus birulai is endemic) in temperatures which vary from 49,9 °C to -31 °C.

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 exhibit photophobic behavior, primarily to evade detection by their predators such as birds, centipedes, lizards, mice, possums, and rats.

Scorpions are opportunistic predators of small arthropods and insects. They use their chelae (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. This will kill or paralyze the prey so the scorpion can eat it. Scorpions have a relatively unique style of eating using chelicerae, small claw-like structures that protrude from the mouth that are unique to the Chelicerata among arthropods. The chelicerae, which are very sharp, are used to pull small amounts of food off the prey item for digestion. Scorpions can only digest food in a liquid form; any solid matter (fur, exoskeleton, etc) is disposed of by the scorpion.

Venom
All known scorpion species possess venom and use it primarily to kill or paralyze their prey so that it can be eaten; in general it is fast-acting, allowing for effective prey capture. It is also used as a defense against predators. The venom is a mixture of compounds (neurotoxins, enzyme inhibitors, etc.) each not only causing a different effect, but possibly also targeting a specific animal. Each compound is made and stored in a pair of glandular sacs, and is released in a quantity regulated by the scorpion itself. Of the over thousand known species of scorpion, only a few have venom that is dangerous to humans.

Medical use
The key ingredient of the venom is a scorpion toxin protein.

Short chain scorpion toxins constitute the largest group of potassium (K+) channel blocking peptides; an important physiological role of the KCNA3 channel, also known as KV1.3, is to help maintain large electrical gradients for the sustained transport of ions such as Ca2+ that controls T lymphocyte (T cell) proliferation. Thus KV1.3 blockers could be potential immunosuppressants for the treatment of autoimmune disorders (such as rheumatoid arthritis, inflammatory bowel disease and multiple sclerosis).

The venom of Uroplectes lineatus is clinically important in dermatology.

Toxins being investigated include:


 * Chlorotoxin is a 36-amino acid peptide found in the venom of the deathstalker scorpion (Leiurus quinquestriatus) which blocks small‐conductance chloride channels. The fact that Chlorotoxin binds preferentially to glioma cells has allowed the development of new methods, that still are under investigation, for the treatment and diagnosis of several types of cancer.
 * Maurotoxin from the venom of the Tunisian Scorpio maurus palmatus

Fossil record
Scorpions have been found in many fossil records, including marine Silurian deposits, coal deposits from the Carboniferous Period and in amber. They are thought to have existed in some form since about 430 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, although the assumption that the oldest scorpions were aquatic has been questioned. Currently, 111 fossil species of scorpion are known. Unusually for arachnids, there are more species of Palaeozoic scorpion than Mesozoic or Cenozoic ones.

The eurypterids, marine creatures which lived during the Paleozoic era, share several physical traits with scorpions and may be closely related to them. Various species of Eurypterida could grow to be anywhere from 10 to 250 cm in length. However, they exhibit anatomical differences marking them off as a group distinct from their Carboniferous and Recent relatives. Despite this, they are commonly referred to as "sea scorpions." Their legs are thought to have been short, thick, tapering and to have ended in a single strong claw; it appears that they were well-adapted for maintaining a secure hold upon rocks or seaweed against the wash of waves, like the legs of shore-crab.

Geographical distribution
Scorpions are almost universally distributed south of 49° N, and their geographical distribution shows in many particulars a close and interesting correspondence with that of the mammals, including their entire absence from New Zealand. The facts of their distribution are in keeping with the hypothesis that the order originated in the northern hemisphere and migrated southwards into the southern continent at various epochs, their absence from the countries to the north of the above-mentioned latitudes being due, no doubt, to the comparatively recent glaciation of those areas. When they reached Africa, Madagascar was part of that continent; but their arrival in Australia was subsequent to the separation of New Zealand from the Austro-Malayan area to the north of it.

In the United States, scorpions are most common in southern Arizona and in a swath of land extending through central Texas and central Oklahoma. The common striped scorpion, Centruroides vittatus, reaches from northern and northeastern Mexico to southern Colorado, Kansas, southern Missouri, and Louisiana. A small population is native to Monroe County, Illinois. Species of the genus Vaejovis are found from Georgia north to Kentucky, the Carolinas, and Tennessee, and as far west as Washington and California. Paruroctonus boreus is found through the Northwest U.S. and into Canada (Southern Saskatchewan, Southern Alberta and the Okanagan Valley of British Columbia). Scorpions can be found in 31 different states in the U.S., including Hawaii (Isometrus maculatus). They are absent from areas that were affected by Pleistocene glaciation in the eastern U.S. California and Arizona boast the greatest scorpion species diversity, although areas in the Trans-Pecos region of Texas have 9 species within 100 meters.

Five colonies of scorpions (Euscorpius flavicaudis) have established themselves in southern England having probably arrived with imported fruit from Africa, but the number of colonies could be lower now because of the destruction of their habitats. This scorpion species is small and completely harmless to humans.

Ultraviolet light


Scorpions are also known to glow when exposed to certain wavelengths of ultraviolet light such as that produced by a blacklight, due to the presence of fluorescent chemicals in the cuticle. The principal fluorescent component is now known to be beta-Carboline. A hand-held UV lamp has long been a standard tool for nocturnal field surveys of these animals. However, a glow will only be produced in adult specimens as the substances in the skin required to produce the glow are not found in adolescents.

Classification
This classification is based on that of Soleglad & Fet (2003), which replaced the older, unpublished classification of Stockwell. Additional taxonomic changes are from Soleglad et al. (2005).


 * ORDER SCORPIONES
 * Infraorder Orthosterni Pocock, 1911
 * Parvorder Pseudochactida Soleglad et Fet, 2003
 * Superfamily Pseudochactoidea Gromov, 1998
 * Family Pseudochactidae Gromov, 1998
 * Parvorder Buthida Soleglad et Fet, 2003
 * Superfamily Buthoidea C. L. Koch, 1837
 * Family Buthidae C. L. Koch, 1837 (thick-tailed scorpions)
 * Family Microcharmidae Lourenço, 1996
 * Parvorder Chaerilida Soleglad et Fet, 2003
 * Superfamily Chaeriloidea Pocock, 1893
 * Family Chaerilidae Pocock, 1893
 * Parvorder Iurida Soleglad et Fet, 2003
 * Superfamily Chactoidea Pocock, 1893
 * Family Chactidae Pocock, 1893
 * Subfamily Chactinae Pocock, 1893
 * Tribe Chactini Pocock, 1893
 * Tribe Nullibrotheini Soleglad et Fet, 2003
 * Subfamily Brotheinae Simon, 1879
 * Tribe Belisariini Lourenço, 1998
 * Tribe Brotheini Simon, 1879
 * Subtribe Brotheina Simon, 1879
 * Subtribe Neochactina Soleglad et Fet, 2003
 * Subfamily Uroctoninae
 * Family Euscorpiidae Laurie, 1896
 * Subfamily Euscorpiinae Laurie, 1896
 * Subfamily Megacorminae Kraepelin, 1905
 * Tribe Chactopsini Soleglad et Sissom, 2001
 * Tribe Megacormini Kraepelin, 1905
 * Subfamily Scorpiopinae Kraepelin, 1905
 * Tribe Scorpiopini Kraepelin, 1905
 * Tribe Troglocormini Soleglad et Sissom, 2001
 * Family Superstitioniidae Stahnke, 1940
 * Subfamily Superstitioniinae Stahnke, 1940
 * Subfamily Typlochactinae Mitchell, 1971
 * Family Vaejovidae Thorell, 1876
 * Superfamily Iuroidea Thorell, 1876
 * Family Iuridae Thorell, 1876
 * Family Caraboctonidae Kraepelin, 1905 (hairy scorpions)
 * Subfamily Caraboctoninae Kraepelin, 1905
 * Subfamily Hadrurinae Stahnke, 1974
 * Superfamily Scorpionoidea Latreille, 1802
 * Family Bothriuridae Simon, 1880
 * Subfamily Bothriurinae Simon, 1880
 * Subfamily Lisposominae Lawrence, 1928
 * Family Diplocentridae Karsch, 1880
 * Family Scorpionidae Latreille, 1802 (burrowing scorpions or pale-legged scorpions)
 * Subfamily Diplocentrinae Karsch, 1880
 * Tribe Diplocentrini Karsch, 1880
 * Tribe Nebini Kraepelin, 1905
 * Subfamily Scorpioninae Latreille, 1802
 * Subfamily Urodacinae Pocock, 1893
 * Family Hemiscorpiidae Pocock, 1893 (= Ischnuridae, =Liochelidae) (rock scorpions, creeping scorpions, or tree scorpions)
 * Subfamily Hemiscorpiinae Pocock, 1893
 * Subfamily Heteroscorpioninae Kraepelin, 1905
 * Subfamily Hormurinae Laurie, 1896