Temporal range: 430-0 Ma Early Silurian - present
|Hottentotta tamulus from Mangaon, Maharashtra, India|
C. L. Koch, 1837
See classification for families.
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognized by the pair of grasping pedipalps 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 / 0.3 in. (Typhlochactas mitchelli) to 23 cm / 9 in. (Heterometrus swammerdami).
The evolutionary history of scorpions goes back to the Silurian era 430 million years ago. They have adapted to a wide range of environmental conditions and can now be found on all continents except Antarctica. Scorpions number about 1750 described species, with 13 extant families recognised to date. Only about 25 of these species are known to have venom capable of killing a human being.:1 The taxonomy has undergone changes and is likely to change further, as genetic studies are bringing forth new information.
Scorpion stings are painful but are usually harmless to humans. For stings from species found in North America, no treatment is normally needed for healthy adults, although medical care should be sought for children and for the elderly. More harmful stings from species found in South America, Africa, and western Asia may require medical attention.
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 - skorpíos.
Scorpions are found on all major land masses except Antarctica. Scorpions did not occur naturally in Great Britain, Japan, South Korea, New Zealand and some of the islands in Oceania, but now have 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 regions between the latitudes 23° N and 38° N. Above these latitudes, the diversity decreases, with the northernmost natural occurrence of scorpions being the northern scorpion Paruroctonus boreus at Medicine Hat, Alberta, Canada 50° N.:251 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. At just over 51° N, this marks the northernmost limit where scorpions live in the wild.
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, rock-loving, or sand-loving. Some species, such as Vaejovis janssi, are versatile and found in every type of habitat in Baja California, while others occupy specialized niches such as Euscorpius carpathicus, which occupies the littoral zone of the shore.
There are thirteen known 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 and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber. The oldest known scorpions lived around 430 million years ago in the Silurian period. Though once believed to have lived on the bottom of shallow tropical seas, early scorpions are now believed to have been terrestrial and to have washed into marine settings together with plant matter. These first scorpions were believed to have had gills instead of the present forms' book lungs though this has subsequently been refuted. The oldest Gondwanan scorpiones (Gondwanascorpio) comprise the earliest known terrestrial animals from Gondwana. 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, commonly called "sea scorpions", were marine creatures that lived during the Palaeozoic era that 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. Cladistic analyses have supported the idea that the eurypterids are a distinct group from the scorpions.
The body of a scorpion is divided into two parts (tagmata): the head (cephalothorax) and the abdomen (opisthosoma), which is subdivided into a broad anterior (mesosoma), or preabdomen, and a narrow tail-like posterior (metasoma), or postabdomen.:10
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. While unable to form sharp images, their central eyes are amongst the most light sensitive in the animal kingdom, especially in dim light, and makes it possible for nocturnal species to use star light to navigate at night. Some species also have light receptions in their tail. 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 mesosoma is the broad part of the opisthosoma. Sometimes it is loosely called the abdomen. It consists of the anterior seven somites (segments) of the opisthosoma, each covered dorsally by a sclerotosed plate, its tergite. Ventrally somites 3 to 7 are armoured with matching plates called sternites.
Ventrally somites 1 and 2 are more complex; the first abdominal sternite is modified into a pair of genital opercula covering the gonopore. Sternite 2 forms the basal plate bearing the pectines. Morphologically the pectines are a pair of limbs that function as sensory organs.
The next four somites, 3 to 6, each bears a pair of spiracles; they serve as openings for the scorpion's respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical, or oval, according to the species of scorpion.:13-15
The 7th and last somite do not bear appendages or any other significant external structures.
The metasoma is commonly known as the scorpion's "tail", though this is in some ways misleading because unlike most so-called tails it is not an appendage or limb; it is in fact part of the opisthosoma. It comprises five segments, of which the fifth segment bears the telson. In many species it superficially seems as though the metasoma has four segments only, because their first (anterior) metasomal segment gives the impression of being the posterior segment of the mesosoma. The fifth segment of the metasoma is the caudal segment of the opisthosoma and accordingly bears the anus. The scorpion's telson is the part commonly called the stinger; it is attached to the end of the fifth segment just dorsad from the anus, but as the distal end of the tail at rest normally is carried upside down with the sting pointing forward, the anus usually is above the base of the telson and facing upwards.
The scorpion's telson is the part commonly called the stinger; it includes the vesicle, containing a symmetrical pair of venom glands. Externally it bears the curved sting, the hypodermic aculeus or venom-injecting barb. It is equipped with various sensory hairs, as the sting cannot be guided visually. Each of the venom glands has its own duct to convey its secretion internally along the aculeus from the bulb of the gland to immediately subterminal of the point of the aculeus, where each of the paired ducts has its own venom pore. 
On rare occasions, scorpions are born with two metasomata. Two-tailed scorpions are no more than examples of adventitious ontogenic abnormality. Whether there ever is a genetic component to the condition is uncertain, but such evidence as is available from offspring is negative so far as no two-tailed scorpions have been observed among the rarely-observed progeny of multiple-tailed scorpion specimens.
Scorpions are also known to glow a vibrant blue-green 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 sclerotisation and increases in intensity with each successive instar. This fluorescence may have an active role in scorpion light detection.
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), seven species of scorpion (of which Pectinibuthus birulai is endemic) live in temperatures varying from -31 to 50 °C (-24 to 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, opossums, 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 hair, 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 fertilising 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 cannibalized by the female, although sexual cannibalism is infrequent with scorpions.
Unlike the majority of species in the class Arachnida, 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 moult. Before the first moult, 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 sclerotisation. The new exoskeleton does not fluoresce; as sclerotisation 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. However, as a general rule, they will kill their prey with brute force if they can, as opposed to using venom. 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 1,000+ known species of scorpion, only 25 have venom that is deadly to humans; most of those belong to the family Buthidae (including Leiurus quinquestriatus, Hottentotta, Centruroides and Androctonus).
According to the United States National Institute for Occupational Safety and Health (NIOSH), the following steps should be taken to prevent scorpion stings:
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. Scorpion envenomation with high morbidity and mortality is usually due to either excessive autonomic activity and cardiovascular toxic effects or neuromuscular toxic effects. Antivenom is the specific treatment for scorpion envenomation combined with supportive measures including vasodilators in patients with cardiovascular toxic effects and benzodiazepines when there is neuromuscular involvement. Although rare, severe hypersensitivity reactions including anaphylaxis to scorpion antivenin (SAV) are possible.
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 the following:
Scorpions for use in the pharmaceutical industry are collected from the wild in Pakistan. Farmers in the Thatta District are paid about US$100 for each 40 gram scorpion and 60 gram specimens are reported to fetch at least US$50,000. The trade is reported to be illegal but thriving.
The Guinness Book of Records claims [...] Heterometrus swammerdami, to be the largest scorpion in the world [9 inches (23 cm)