Temporal range: Silurian–Recent
|Asian forest scorpion in Khao Yai National Park, Thailand|
C. L. Koch, 1837
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9 mm (Typhlochactas mitchelli) to 20 cm (Hadogenes troglodytes).
Scorpions are found widely distributed over all continents, except Antarctica, in a variety of terrestrial habitats except the high latitude tundra. Scorpions number about 1,752 described species, with 13 extant families recognised to date. The taxonomy has undergone changes and is likely to change further, as a number of genetic studies are bringing forth new information.
Scorpion venom has a fearsome reputation, but only about 25 out of almost 1500 species are known to have venom capable of killing a human being.:1
The word scorpion is thought to have originated in Middle English between 1175 and 1225 AD from Old French scorpion, or from Italian scorpione, both derived from the Latin word scorpius, which is the romanization of the Greek word σκορπίος – skorpíos.
Scorpions are found on all major land masses except Antarctica. Scorpions did not occur naturally in Great Britain, New Zealand and some of the islands in Oceania, but have now been accidentally introduced in some of these places by human trade and commerce.:249 The greatest diversity of scorpions in the Northern Hemisphere is to be found in the subtropical areas lying between latitudes 23° N and 38° N. Above these latitudes, the diversity decreases, with the northernmost occurrence of scorpions being the northern scorpion Paruroctonus boreus at 50° N.:251
Today, scorpions are found in virtually every terrestrial habitat, including high-elevation mountains, caves and intertidal zones, with the exception of boreal ecosystems, such as the tundra, high-altitude taiga and the permanently snow-clad tops of some mountains.:251–252 As regards microhabitats, scorpions may be ground-dwelling, tree-living, lithophilic (rock-loving) or psammophilic (sand-loving); some species, such as Vaejovis janssi, are versatile and found in every type of habitat in Baja California, while others occupy specialised niches such as Euscorpius carpathicus, which occupies the littoral zone of the shore.
Five colonies of scorpions (Euscorpius flavicaudis) have established themselves in Sheerness on the Isle of Sheppey in the United Kingdom. This small population has been resident since the 1860s, having probably arrived with imported fruit from Africa. This scorpion species is small and completely harmless to humans. This marks the northernmost limit where scorpions live in the wild.
There are thirteen families and about 1,750 described species and subspecies of scorpions. In addition, there are 111 described taxa of extinct scorpions.
This classification is based on that of Soleglad & Fet (2003), which replaced the older, unpublished classification of Stockwell. Additional taxonomic changes are from papers by Soleglad et al. (2005).
Scorpions have been found in many fossil records, including marine Silurian deposits, coal deposits from the Carboniferous Period and in amber. The oldest known scorpions lived around 430 million years ago in the Silurian period, on the bottom of shallow tropical seas. These first scorpions had gills instead of the present forms' book lungs. 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 that 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 centimetres (3.9 in) to 2.5 metres (8.2 ft) 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 a shore crab.
The cephalothorax, also called the prosoma, comprises the carapace, eyes, chelicerae (mouth parts), pedipalps (the pedipalps of scorpions have chelae, commonly called claws or pincers) 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 cephalothorax, and usually two to five pairs of eyes along the front corners of the cephalothorax. The position of the eyes on the cephalothorax depends in part on the hardness or softness of the soil upon which they spend their lives.
The pedipalp is a segmented, chelate (clawed) appendage used for prey immobilization, defense, and sensory purposes. The segments of the pedipalp (from closest to the body outwards) are coxa, trochanter, femur (humerus), patella, tibia (including the fixed claw and the manus) and tarsus (moveable claw). A scorpion has darkened or granular raised linear ridges, called "keels" or carinae on the pedipalp segments and on other parts of the body, which are useful taxonomically.:12
The abdomen, also called the opisthosoma, consists of seven segments (somites), each covered dorsally by a sclerotosed plate (tergum) and also ventrally for segments 3 to 7. The first abdominal segment bears a pair of genital opercula covering the gonopore. Segment 2 consists of the basal plate with the pectines. Each of the mesosomal segments 3 to 7 have a pair of spiracles, the openings for the scorpion's respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical, or oval.:13–15
The metasoma, the scorpion's tail, comprises five caudal segments (the first tail segment looks like a last mesosoman segment), and sixth 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.
Scorpions are also known to glow when exposed to certain wavelengths of ultraviolet light such as that produced by a black light, due to the presence of fluorescent chemicals in the cuticle. One 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. Fluorescence occurs as a result of sclerotization and increases in intensity with each successive instar. This fluorescence may have an active role in scorpion light detection.
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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 areas where the temperatures range from 20 to 37 °C (68 to 99 °F), but may survive temperatures ranging from well below freezing to desert heat. Scorpions of the genus Scorpiops living in high Asian mountains, bothriurid scorpions from Patagonia and small Euscorpius scorpions from Central Europe can all survive winter temperatures of about −25 °C (−13 °F). In Repetek (Turkmenistan), there live seven species of scorpions (of which Pectinibuthus birulai is endemic) in temperatures varying from -31–50 °C (-24–122 °F).
They are nocturnal and fossorial, finding shelter during the day in the relative cool of underground holes or undersides of rocks, and emerging at night to hunt and feed. Scorpions exhibit photophobic behavior, primarily to evade detection by predators such as birds, centipedes, lizards, mice, possums, and rats.
Scorpions are opportunistic predators of small arthropods, although the larger kinds have been known to kill small lizards and mice. The large pincers are studded with highly sensitive tactile hairs, and the moment an insect touches these, they use their chelae (pincers) to catch the prey. 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 an unusual 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 into a pre-oral cavity below the chelicerae and carapace. Scorpions can ingest food only in a liquid form; they have external digestion. The digestive juices from the gut are egested onto the food and the digested food sucked in liquid form. Any solid indigestible matter (fur, exoskeleton, etc.) is trapped by setae in the pre-oral cavity, which is ejected by the scorpion.:296–297
Scorpions can consume huge amounts of food at one sitting. They have a very efficient food storage organ and a very low metabolic rate combined with a relatively inactive lifestyle. This enables scorpions to survive long periods when deprived of food; some are able to survive 6 to 12 months of starvation.:297–298 Scorpions excrete very little; their waste consists mostly of insoluble nitrogenous compounds such as xanthine, guanine and uric acid.
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 unfertilised 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 the 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 this "dance," the male leads 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 – pincers – 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 continues too long, the female may lose interest, ending the process.
Once the mating is complete, the male and female will separate. The male will generally retreat quickly, most likely to avoid being cannibalised by the female, although sexual cannibalism is infrequent with scorpions.
Unlike the majority of species in the class arachnid, which are oviparous, scorpions seem to be universally ovoviviparous. The young are born one by one after hatching and expelling the embryonic membrane, if any, 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 that 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 commences with a split in the old exoskeleton 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.
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 1000+ known species of scorpion, only 25 have venom that is deadly to humans; most of those belong to the family Buthidae. However, all scorpions are able to penetrate human skin and deliver sharp, unpleasant stings, most of which usually leave redness around the stung area.
First aid for scorpion stings is generally symptomatic. It includes strong analgesia, either systemic (opiates or paracetamol) or locally applied (such as a cold compress). Hypertensive crises are treated with anxiolytics and vasodilators.
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).
Toxins being investigated include:
Iranian researchers have also reported that the venom of M. eupeus has anti-inflammatory properties and is effective as an anti-arthritis treatment in an experimental species model, but the mechanism of action is unknown.
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