Scorpion
Scorpions are predatory arachnids with eight legs, a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always ending with a stinger. The evolutionary history of scorpions goes back 435 million years. They mainly live in deserts but have adapted to a wide range of environmental conditions, and can be found on all continents except Antarctica. There are over 2,500 described species, with 22 extant families recognized to date. Their taxonomy is being revised to account for 21st-century genomic studies.
Scorpions primarily prey on insects and other invertebrates, but some species hunt vertebrates. They use their pincers to restrain and kill prey, or to prevent their own predation. The venomous sting is used for offense and defense. During courtship, the male and female grasp each other's pincers and dance while the male tries to move the female onto its sperm packet. All known species give live birth and the female cares for the young as their exoskeletons harden, transporting them on its back. The exoskeleton contains fluorescent chemicals and glows under ultraviolet light.
The vast majority of species do not seriously threaten humans, and healthy adults usually do not need medical treatment after a sting. About 25 species have venom capable of killing a human, which happens frequently in the parts of the world where they live, primarily where access to medical treatment is unlikely.
Scorpions appear in art, folklore, mythology, and commercial brands. Scorpion motifs are woven into kilim carpets for protection from their sting. Scorpius is the name of a constellation; the corresponding astrological sign is Scorpio. A classical myth about Scorpius tells how the giant scorpion and its enemy Orion became constellations on opposite sides of the sky.
Etymology
The word scorpion originated in Middle English between 1175 and 1225 AD from Old French scorpion, or from Italian scorpione, both derived from the Latin scorpio, equivalent to scorpius, which is the romanization of the Greek σκορπίος skorpíos. The Greek word is not traceable further within Indo-European, but is a loanword, borrowed from a non-Indo-European language; it is "akin to Hebrew ʿaqrāb".Evolution
Fossil record
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, and while they had book lungs like modern terrestrial species, the most basal such as Eramoscorpius were originally considered as still aquatic, until it was found that Eramoscorpius had book lungs. Over 100 fossil species of scorpion have been described. The oldest found as of 2021 is Dolichophonus loudonensis, which lived during the Silurian, in present-day Scotland. Gondwanascorpio from the Devonian is among the earliest-known terrestrial animals on the Gondwana supercontinent. Some Palaeozoic scorpions possessed compound eyes similar to those of eurypterids. The Triassic fossils Protochactas and Protobuthus belong to the modern clades Chactoidea and Buthoidea respectively, indicating that the crown group of modern scorpions had emerged by this time. In 2025, a 140 million year old scorpion was discovered in Jordanian amber by Abbas Haddadin.Phylogeny
External
The Scorpiones are a clade within the pulmonate Arachnida. Arachnida is placed within the Chelicerata, a subphylum of Arthropoda that contains sea spiders and horseshoe crabs, alongside 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, unlike those of scorpion which are second appendages. Scorpiones is sister to the Tetrapulmonata, a terrestrial group of pulmonates containing the spiders and whip scorpions.Recent studies place pseudoscorpions as the sister group of scorpions in the clade Panscorpiones, which together with Tetrapulmonata makes up the clade Arachnopulmonata.
Cladogram of current understanding of chelicerate relationships, after Sharma and Gavish-Regev :
Internal
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 during the Devonian and into 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. In 2022, the Caraboctonidae and Hadruridae were treated as superfamilies, separate clades from Iuroidea, while the Vaejovidae were similary treated as a separate clade from Chactoidea.Taxonomy
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". In 1801, Jean-Baptiste Lamarck divided up the "Insecta aptera", creating the taxon Arachnides for spiders, scorpions, and acari, though it also contained the Thysanura, Myriapoda and parasites such as lice. German arachnologist Carl Ludwig Koch created the order Scorpiones in 1837. He divided it into four families, the six-eyed scorpions "Scorpionides", the eight-eyed scorpions "Buthides", the ten-eyed scorpions "Centrurides", and the twelve-eyed scorpions "Androctonides". More recently, some twenty-two families containing over 2,500 species of scorpions have been described, with many additions and much reorganization of taxa in the 21st century. There are over 100 described taxa of fossil scorpions.Geographical distribution
Scorpions are found on all continents except Antarctica. They are unusual among animal groups in that they are most diverse at the subtropics rather than the tropics, and are also less common near the poles. New Zealand, and some of the islands in Oceania, have in the past had small populations of introduced scorpions, but they were exterminated. Five colonies of Euscorpius flavicaudis have established themselves since the late 19th century in Sheerness in England at 51°N, while Paruroctonus boreus lives as far north as Red Deer, Alberta. A few species are on the IUCN Red List; Afrolychas braueri is classed as critically endangered, Isometrus deharvengi as endangered and Chiromachus ochropus as vulnerable.Scorpions are usually xerocoles, primarily living in deserts, but they can be found in virtually every terrestrial habitat including high-elevation mountains, caves, and intertidal zones. They are largely absent from boreal ecosystems such as the tundra, high-altitude taiga, and mountain tops. The highest altitude reached by a scorpion is in the Andes, for Orobothriurus crassimanus. As regards microhabitats, scorpions may be ground-dwelling, tree-loving, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and use any habitat on Socorro Island, Baja California, while others such as Euscorpius carpathicus, endemic to the littoral zone of rivers in Romania, occupy specialized niches.
Morphology
Scorpions range in size from the Typhlochactas mitchelli of Typhlochactidae, to the Heterometrus swammerdami of Scorpionidae. The body of a scorpion is divided into two parts or tagmata: the cephalothorax or prosoma, and the abdomen or opisthosoma. The opisthosoma consists of a broad anterior portion, the mesosoma or pre-abdomen, followed by a thinner tail-like posterior, the metasoma or post-abdomen. External differences between the sexes are not obvious in most species. In some, the tail of the male is slenderer than that of the female.Cephalothorax
The cephalothorax comprises the carapace, eyes, chelicerae, pedipalps 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, which makes it possible for nocturnal species to use starlight to navigate at night. The chelicerae are at the front and below the carapace. They are pincer-like and have three segments and "teeth". The brain of a scorpion is located in the front part of the cephalothorax, just above the esophagus. As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but has a long ventral nerve cord with segmented ganglia which may be a primitive trait.The pedipalp is a segmented, clawed appendage segmented into the coxa, trochanter, femur, patella, tibia and tarsus. Unlike those of some other arachnids, the eight-segmented legs have not been altered for other purposes, though they may occasionally be used for digging, and females may use them to catch emerging young. They are covered with many proprioceptors, bristles and sensory setae. Depending on the species, the legs may have spines and spurs.
Mesosoma
The mesosoma or preabdomen is the broad part of the opisthosoma. In the early stages of embryonic development the mesosoma consist of eight segments, but the first segment disappear before birth, so the mesosoma in scorpions actually consist of segments 2-8. These anterior seven somites of the opisthosoma are each covered by a hardened plate, the tergite, on the back surface. Underneath, somites 3 to 7 are armored with matching plates called sternites. The underside of somite 1 has two covering over the genital opening. Sternite 2 forms the basal plate bearing the comb-like pectines, which function as sensory organs.The next four somites, 3 to 6, all possess two spiracles each. They serve as openings for the scorpion's respiratory organs, known as book lungs, and vary in shape. There are thus four pairs of book lungs; each consists of some 140 to 150 thin flaps or lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles keep the lamellae from touching. A muscle opens the spiracle and widens the atrial chamber; dorsoventral muscles contract to constricts the pulmonary chamber, pushing air out, and relax to allow the chamber to refill. The 7th and last somite lacks any notable structure.
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 blood to the heart; the blood is re-oxygenated by cardiac pores. The mesosoma also houses the reproductive system. The female gonads are three or four tubes which are aligned and have two to four transverse anastomoses connecting them. These tubes create oocytes and house developing embryos. They connect to two oviducts which connect to a single atrium leading to the genital orifice. Male gonads are two pairs of cylindrical tubes with a ladder-like configuration; they contain spermatozoa-producing cysts. Both tubes end in a spermiduct, one on the opposite sides of the mesosoma. They connect to glandular symmetrical structures called paraxial organs, which end at the genital orifice. These create two halves of the chitin-based spermatophore which merge.