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 sting. Scorpions range in size from 9–12 mm (0.35–0.47 in) in Microtityus minimus to 23 cm (9.1 in) in Heterometrus swammerdami. The evolutionary history of scorpions goes back to the Silurian period 435 million years ago. They have adapted to a wide range of environmental conditions, and they can now be found on all continents except Antarctica. There are about 1,750 described species, with 13 extant (living) families recognised to date. Their taxonomy is being revised in the light of genomic studies.
|Hottentotta tamulus from Mangaon, Maharashtra, India|
C. L. Koch, 1837
|Native range of Scorpiones|
Scorpions primarily prey on insects and other invertebrates, but some species take vertebrates including frogs, lizards and mammals. They use their pincers to restrain and kill prey, but may also use their venomous sting. Scorpions themselves are also preyed on by large animals. During courtship, the male and female scorpion grasp each other's pincers and move around in a "dance" were the male tries to maneuver the female on his deposited spermatophore. Most species give live birth and the female cares for the young as their exoskeletons harden, transporting them on her back. The exoskeleton of a scorpion contains fluorescent chemicals and glows under ultraviolet light.
The vast majority of species do not represent a serious threat to humans, and healthy adults usually do not need medical treatment after being stung. Only about 25 species have venom capable of killing a human. In some parts of the world with highly venomous species, human fatalities regularly occur, primarily in areas with limited access to medical treatment. Scorpions with their powerful sting appear in art, folklore, mythology and numerous brands. Scorpion motifs are woven into kilim carpets for protection. Scorpio is the name of a constellation and the corresponding astrological sign; a classical myth tells how the giant scorpion and its enemy, Orion, became constellations on opposite sides of the sky.
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 scorpius, which is the romanization of the Greek σκορπίος – skorpíos. The Proto-Indo-European root word *(s)ker- has been translated as "to cut".
Scorpion fossils have been found in many strata, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber. Whether the early scorpions were marine or terrestrial has been debated, though they had book lungs like modern terrestrial species. Over 100 fossil species of scorpion have been described. The oldest found to date is Parioscorpio venator, which lived 437 million years ago, during the Silurian, in present-day Wisconsin. Unlike present day scorpions, but like its marine ancestors, it had compound eyes. Gondwanascorpio from the Devonian is among the earliest known terrestrial animals on the Gondwana supercontinent.
The Scorpiones are a clade of pulmonate Arachnida within the Chelicerata, a subphylum of Arthropoda that contains sea spiders and horseshoe crabs, and terrestrial animals without book-lungs such as ticks and harvestmen. The extinct Eurypterida, sometimes called sea scorpions, though they were not all marine, are not scorpions; their grasping pincers were chelicerae, not homologous with the pincers (second appendages) of scorpions. Scorpiones is sister to the Tetrapulmonata, a terrestrial group with book-lungs that contains the spiders and whip scorpions, as in this 2019 cladogram:
The internal phylogeny of the scorpions has been debated, but genomic analysis consistently places the Bothriuridae as sister to a clade consisting of Scorpionoidea and "Chactoidea". The scorpions diversified between the Devonian and the early Carboniferous. The main division is into the clades Buthida and Iurida. The Bothriuridae diverged starting before temperate Gondwana broke up into separate land masses, completed by the Jurassic. The Iuroidea and Chactoidea are both seen not to be single clades, and are shown as "paraphyletic" (with quotation marks) in this 2018 cladogram.
Carl Linnaeus described six species of scorpion in his genus Scorpio in 1758 and 1767; three of these are now considered valid and are called Scorpio maurus, Androctonus australis, and Euscorpius carpathicus; the other three are dubious names. He placed the scorpions among his "Insecta aptera" (wingless insects), a group that included Crustacea, Arachnida and Myriapoda. In 1801, Jean-Baptiste Lamarck divided up the "Insecta aptera", creating the taxon Arachnides for spiders, scorpions, and acari (mites and ticks), though it also contained the Thysanura (thrips), Myriapoda and parasites such as lice.
More recently, thirteen families and about 1,750 species and subspecies of scorpions have been described. In addition, there are 111 described taxa of fossil scorpions. This classification is based on Soleglad and Fet (2003), which replaced Stockwell's older, unpublished classification. Additional taxonomic changes are from papers by Soleglad et al. (2005).
- Order Scorpiones
- Parvorder Pseudochactida Soleglad et Fet, 2003
- Parvorder Buthida Soleglad et Fet, 2003
- Parvorder Chaerilida Soleglad et Fet, 2003
- Parvorder Iurida Soleglad et Fet, 2003
- Superfamily Chactoidea Pocock, 1893
- Superfamily Iuroidea Thorell, 1876
- Superfamily Scorpionoidea Latreille, 1802
Scorpions are found on all major land masses except Antarctica. The diversity of scorpions is greatest in subtropical areas; it decreases towards both the poles and the equator, though scorpions are found in the tropics. Scorpions did not occur naturally in Great Britain, New Zealand and some of the islands in Oceania, but have now been accidentally introduced into these places by humans. Five colonies of Euscorpius flavicaudis have established themselves in Sheerness in England at 51°N, while Paruroctonus boreus lives as far north as Red Deer, Alberta, at 52°N.
Scorpions are xerocoles, meaning they primarily live in deserts, but they can be found in virtually every terrestrial habitat including high-elevation mountains, caves, and intertidal zones. However, they are largely absent from boreal ecosystems such as the tundra, high-altitude taiga, and mountain tops. As regards microhabitats, scorpions may be ground-dwelling, tree-living, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and are found in every type of habitat in Baja California, while others such as Euscorpius carpathicus, endemic to the littoral zone of rivers in Romania, occupy specialized niches.
The body of a scorpion is divided into two parts or tagmata: the cephalothorax or prosoma, and the abdomen or opisthosoma.[a] The opisthosoma is subdivided into a broad anterior portion, the mesosoma or pre-abdomen, and a narrow tail-like posterior, the metasoma or post-abdomen.
The cephalothorax comprises the carapace, eyes, chelicerae (mouth parts), pedipalps (which have chelae, commonly called claws or pincers) and four pairs of walking legs. 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 starlight to navigate at night. Some species also have light receptors in their tail. The chelicerae are at the front and underneath the carapace. They are pincer-like and have three segments and sharp "teeth". The brain of a scorpion is in the back of the cephalothorax, just above the esophagus. As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but it also has a long ventral nerve cord with segmented ganglia. This may be a "primitive" trait.
The pedipalp is a segmented, 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, 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; these are useful taxonomically. Unlike those of some other arachnids, the legs have not been modified for other purposes, though they may occasionally be used for digging, and females may use them to catch emerging young. The legs are covered in proprioceptors, bristles and sensory setae. Depending on the species, the legs may also have spines and spurs.
The mesosoma or preabdomen is the broad part of the opisthosoma. It consists of the anterior seven somites (segments) of the opisthosoma, each covered dorsally by a sclerotised plate, its tergite. Ventrally, somites 3 to 7 are armoured with matching plates called sternites. The ventral side of somites 1 has 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, all bear pairs 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. There are thus four pairs of book lungs; each consists of some 140 to 150 thin lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles hold the lamellae apart. A muscle opens the spiracle and widens the atrial chamber; dorsoventral muscles contract to compress the pulmonary chamber, forcing air out, and relax to allow the chamber to refill. The 7th and last somite do not bear appendages or any other significant external structures.
The mesosoma contains the heart or "dorsal vessel" which is the center of the scorpion's open circulatory system. The heart is continuous with a deep arterial system which spreads throughout the body. Sinuses return deoxygenated hemolymph to the heart; the hemolymph is re-oxygenated by cardiac pores. The mesosoma also contains the reproductive system. The female gonads are made of three or four tubes that run parallel to each other and are connected by two to four transverse anastomoses. These tubes are the sites for both oocyte formation and embryonic development. They connect to two oviducts which connect to a single atrium leading to the genital orifice. Males have two gonads made of two cylindrical tubes with a ladder-like configuration; they contain cysts which produce spermatozoa. Both tubes end in a spermiduct, one on each side of the mesosoma. They connect to glandular symmetrical structures called paraxial organs, which end at the genital orifice. These secrete chitin-based structures which come together to form the spermatophore.
The "tail" or metasoma consists of five segments and the telson, not strictly a segment. The five segments are merely body rings; they lack apparent sterna or terga, and become larger distally. These segments have keels, setae and bristles which may be used for taxonomic classification. The anus is at the distal and ventral end of the last segment, and is encircled by four anal papillae and the anal arch.
The telson includes the vesicle, which contains a symmetrical pair of venom glands. Externally it bears the curved sting, the hypodermic aculeus, equipped with sensory hairs. Each of the venom glands has its own duct to convey its secretion 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. An extrinsic muscle system in the tail moves it forward and propels and penetrates with the aculeus, while an intrinsic muscle system attached to the glands pumps venom.
Scorpions range in size from the 9–12 mm (0.35–0.47 in) Microtityus minimus in the Buthidae, to the 23 cm (9.1 in) Heterometrus swammerdami in the Scorpionidae. Most scorpions species are nocturnal or crepuscular, finding shelter during the day in burrows and other shelters such as cracks in rocks and tree bark. They prefer areas where the temperature remains between 11 to 40 °C (52 to 104 °F), but may survive temperatures from well below freezing to desert heat. Scorpions can withstand intense heat: Leiurus quinquestriatus, Scorpio maurus and Hadrurus arizonensis can live in temperatures of 45–50 °C (113–122 °F) if they are sufficiently hydrated. Desert species must deal with the extreme changes in temperature from day to night or between seasons; Pectinibuthus birulai lives in a temperature range of −30–50 °C (−22–122 °F). Scorpions that live outside deserts prefer lower temperatures. The ability to resist cold may be related to the increase in the sugar trehalose when the temperature drops. Scorpions may also hibernate.
Desert scorpions have several adaptations for water conservation. They excrete insoluble compounds such as xanthine, guanine, and uric acid, not requiring water for their removal from the body. Guanine is the main component and maximizes the amount of nitrogen excreted. A scorpion's cuticle holds in moisture via lipids and waxes from epidermal glands, and protects against ultraviolet radiation. Even when dehydrated, a scorpion can tolerate high osmotic ion concentrations in its hemolymph.
Scorpions may be attacked by other arthropods like ants, spiders, solifugids and centipedes. Major predators include frogs, lizards, snakes, birds, and mammals. Meerkats are somewhat specialized in preying on scorpions, biting off their stingers and being immune to their venom. When threatened, a scorpion raises its claws and tail in a defensive posture. Some species stridulate to warn off predators by rubbing certain hairs, the sting or the claws. Scorpions host parasites including mites, scuttle flies, nematodes and bacteria.
Diet and feeding
Scorpions generally prey on insects, particularly grasshoppers, crickets, termites, beetles and wasps. They also take spiders, solifugids, woodlice and even small vertebrates including lizards, snakes and mammals. Species with large claws may prey on earthworms and mollusks. The majority of species are opportunistic and consume a variety of prey though some may be highly specialized. Prey size depends on the size of the species. Several scorpion species are sit-and-wait predators, which involves them waiting for prey at or near the entrance to their burrow. Others actively seek them out. Scorpions detect their prey with mechanoreceptive and chemoreceptive hairs on their bodies. Scorpions capture prey with their claws. Small animals are merely killed with the claws, particularly by large-clawed species. Larger and more aggressive prey is given a sting, which can happen very quickly at 0.75 seconds.
Scorpions, like other arachnids, digest their food externally. The chelicerae, which are very sharp, are used to pull small amounts of food off the prey item into a pre-oral cavity below the chelicerae and carapace. The digestive juices from the gut are egested onto the food, and the digested food is then sucked into the gut in liquid form. Any solid indigestible matter (such as exoskeleton fragments) is trapped by setae in the pre-oral cavity and ejected. The sucked-in food is pumped into the midgut by the pharynx, where it is further digested. The waste passes through the hindgut and out of the anus. Scorpions can consume large amounts of food at one sitting. They have an efficient food storage organ and a very low metabolic rate, and a relatively inactive lifestyle. This enables them to survive long periods without food. Some are able to survive 6 to 12 months of starvation.
Most scorpions reproduce sexually, with male and female individuals; however, species in some genera, such as Hottentotta and Tityus, and the species Centruroides gracilis, Liocheles australasiae, and Ananteris coineaui have been reported, not necessarily reliably, to reproduce through parthenogenesis, in which unfertilized eggs develop into living embryos. Receptive females produce pheromones which are picked up by wandering males using their pectines to comb the substrate. Males begin courtship by moving their bodies back and forth, without moving the legs, a behavior known as juddering. This appears to produce ground vibrations that are picked up by the female.
The pair then make contact using their pedipalps, and perform a "dance" called the promenade à deux (from the French for "a walk for two"). In this dance, the male and female move backwards and forwards while facing each other, searching for a suitable place for the male to deposit his spermatophore. The courtship ritual can involve several other behaviors such as a cheliceral kiss, in which the male and female grasp each other's mouth-parts, and sexual stinging, in which the male stings the female in the chelae or mesosoma to subdue her. When the male has located a suitably stable substrate, such as hard ground, agglomerated sand, rock, or tree bark, he deposits the spermatophore and guides the female over it. This allows the spermatophore to enter her genital opercula, which triggers release of the sperm, thus fertilizing the female. A mating plug then forms in the female to prevent her from mating again before the young are born. The male and female then abruptly separate. Sexual cannibalism after mating has only been reported anecdotally in scorpions.
Birth and development
Unlike the majority of arachnids, which are oviparous, hatching from eggs, scorpions seem to be universally viviparous, with live births. They are also unusual among terrestrial arthropods in the amount of care a female gives to her offspring. Gestation can last for over a year in some species. The size of a brood varies by species, from three to over 100. Before giving birth, the female elevates the front of her body and positions her pedipalps and front legs under her to catch the young. The young emerge one by one from the genital opercula, expel the embryonic membrane, if any, and are placed on the mother's back where they remain until they have gone though at least one molt.
The period before the first molt is called the pro-juvenile stage; the young are unable to feed or sting, but have suckers on their tarsi, used to hold on to their mother. This period lasts 5 to 25 days, depending on the species. The brood molt for the first time simultaneously in a process that lasts 6 to 8 hours, marking the beginning of the juvenile stage. Juvenile stages or instars generally resemble smaller versions of adults, with fully-developed pincers, trichobothria and stings. They are still soft and lack pigments, and thus continue to ride on their mother's back for protection. They became harder and more pigmented over the next couple of days. They may leave their mother temporarily, returning when they sense potential danger. Once the tegument is fully hardened, the young can hunt prey on their own and may soon leave their mother. A scorpion may molt six times on average before reaching maturity, which may not occur until it is 6 to 83 months old, depending on the species. They may live up to 25 years.
Scorpions 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 beta-carboline. Accordingly, a hand-held ultraviolet 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 the scorpion's ability to detect light.
Relationship with humans
Effect of sting
Scorpion venom serves to kill or paralyze prey rapidly. Only 25 species have venom that is deadly to humans; most of those belong to the family Buthidae (including Leiurus quinquestriatus, Hottentotta spp., Centruroides spp., and Androctonus spp.). People with allergies are especially at risk; otherwise, first aid is symptomatic, with analgesia. Cases of very high blood pressure are treated with medications that relieve anxiety and relax the blood vessels. 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 are possible.
Possible use of toxins
Scorpion venom is a mixture of neurotoxins; most of these are peptides, chains of amino acids. Many of them interfere with membrane channels that transport sodium, potassium, calcium, or chloride ions. These channels are essential for nerve conduction, muscle contraction and many other biological processes. Some of these molecules may be useful in medical research and might lead to the development of new disease treatments. Among their potential therapeutic uses are as analgesic, anti-cancer, antibacterial, antifungal, antiviral, antiparasitic, bradykinin-potentiating, and immunosuppressive drugs. As of 2020, no scorpion toxin-based drug is on sale, though chlorotoxin is being trialled for use against glioma, a brain cancer.
Fried scorpion is traditionally eaten in Shandong, China. There, scorpions can be cooked and eaten in a variety of ways, such as roasting, frying, grilling, raw, or alive. The stings are typically not removed, since direct and sustained heat negates the harmful effects of the venom. The scorpion Heterometrus laoticus is farmed for consumption as a novelty food in Vietnam. They are also used to make snake wine (scorpion wine).
Mythology, religion, and folklore
The scorpion is a culturally significant animal, appearing as a motif in art, especially in Islamic art in the Middle East. A scorpion motif is often woven into Turkish kilim flat-weave carpets, for protection from their sting. The scorpion is perceived both as an embodiment of evil and a protective force such as a dervish's powers to combat evil. In Muslim folklore, the scorpion portrays human sexuality. Scorpions are used in folk medicine in South Asia, especially in antidotes for scorpion stings.
One of the earliest occurrences of the scorpion in culture is its inclusion, as Scorpio, in the 12 signs of the Zodiac by Babylonian astronomers during the Chaldean period. This was then taken up by western astrology. In ancient Egypt, the goddess Serket was often depicted as a scorpion, one of several goddesses who protected the Pharaoh. In ancient Greece, a warrior's shield sometimes carried a scorpion device, as seen in red-figure pottery from the 5th century BC. A myth reported by Hesiod and Ovid tells that Gaia sent a giant scorpion to kill the hunter Orion, who had said he would kill all the world's animals; in different versions, either he kills the scorpion or it kills him. Orion and the scorpion both became constellations; as enemies they were placed on opposite sides of the world, so when one rises in the sky, the other sets.
Alongside serpents, scorpions are used to symbolize evil in the New Testament. In Luke 10:19 it is written, "Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you." Here, scorpions and serpents symbolize evil. Revelation 9:3 speaks of "the power of the scorpions of the earth."
"Scorpion and snake fighting", Anglo-Saxon Herbal, c. 1050
Still life with scorpion and frog by Hermenegildo Bustos, 1874
The scorpion with its powerful sting has been used as the name or symbol of various products and brands, including Italy's Abarth racing cars, and since classical times for weapons. In the Roman army, the scorpio was a torsion siege engine used to shoot a projectile. The British Army's FV101 Scorpion was an armoured reconnaissance vehicle or light tank in service from 1972 to 1994. It holds the Guinness world record for the fastest production tank. A version of the Matilda II tank, fitted with a flail to clear mines, was named the Matilda Scorpion. Several ships of the Royal Navy have been named HMS Scorpion, including an 18-gun sloop in 1803, a turret ship in 1863, and a destroyer in 1910. A Montesa scrambler motorcycle was named Scorpion.
A hand- or forearm-balancing asana in modern yoga as exercise with the back arched and one or both legs pointing forwards over the head is called Scorpion pose. A variety of martial arts films and video games have been entitled Scorpion King. Scorpions have equally appeared in western artforms including film and poetry: the surrealist filmmaker Luis Buñuel made symbolic use of scorpions in his 1930 classic L'Age d'or (The Golden Age), while Stevie Smith's last collection of poems was entitled Scorpion and other Poems.
- As there is currently neither paleontological nor embryological evidence that arachnids ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax. Similarly, arguments can be formed against use of the term abdomen, as the opisthosoma of all scorpions contains a heart and book lungs, organs atypical of an abdomen.
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