Overview

Brief Summary

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).[1]

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,[2] 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 species are known to have venom capable of killing a human being.[3]

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Distribution

There are 12,00 described extant species of scorpions, most commonly found in the tropics and subtropics, and appear on all continents except Antarctica (Víquez 1999).

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Physical Description

Morphology

Scorpions are large arachnids, with a body length ranging from 3 to 9 cm. The body is divided into the two main sections of the cephalothorax (head-like structure) and abdomen (Víquez 1999). In the middle of the cephalothorax is a pair of large, elevated median eyes, in addition to two to five pairs of small lateral eyes closer to the margin (Ruppert et al. 2004). The eyes have been lost in some cave-dwelling species. The head broadly connects to the abdomen, which consists of a seven-segmented preabdomen and a tail-like five-segmented postabdomen, ending in the characteristic telson, or sting, with a curved barb for injecting venom from nearby glands into prey. On the anterior part of the body are the characteristic three-segmented chelicerae, with the end segment modified into pincers, and three more pairs of legs. There are variable degrees of sexual dimorphism in different species, but generally, adult males are thinner and longer than females (Víquez 1999). When shone under ultraviolet light, scorpions show a striking green fluorescence, making them observable at night (Ruppert et al. 2004).

The body has hardly changed since the Silurian period. The earliest forms were aquatic, with gills, and lacking specialized claws on the legs (Ruppert et al. 2004).

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Development

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 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 cannibalised by the female, although sexual cannibalism is infrequent with scorpions.

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Ecology

Habitat

Scorpions are generally elusive and nocturnal, hiding by day under logs, bark, stones, and in crevices (Vísquez 1999). They can inhabit forests, grasslands, deserts, and most other terrestrial habitats. Different species are associated with the ground, trees, caves, or even the intertidal zone.

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Trophic Strategy

Scorpions generally sit in an alert position, pedipalps open, waiting for approaching prey. They feed upon invertebrates, especially insects and other arachnids. The postabdomen is raised over the body to curve forward and stab the prey, which is then caught and held by the pedipalps, but the pedipalps alone can also subdue the prey (Bub and Bowerman 1979).

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Associations

Known predators

Scorpiones (scorpion, spiders) is prey of:
Talpinae
Aporosaura
Typhlosaurus
Red racer
Pituophis
Crotalus
Urocyon cinereoargenteus
Mephitinae
Onychomys
Geococcyx velox

Based on studies in:
Namibia, Namib Desert (Desert or dune)
USA: Arizona, Sonora Desert (Desert or dune)

This list may not be complete but is based on published studies.
  • E. Holm and C. H. Scholtz, Structure and pattern of the Namib Desert dune ecosystem at Gobabeb, Madoqua 12(1):3-39, from p. 21 (1980).
  • P. G. Howes, The Giant Cactus Forest and Its World: A Brief Biology of the Giant Cactus Forest of Our American Southwest (Duell, Sloan, and Pearce, New York; Little, Brown, Boston; 1954), from pp. 222-239, from p. 227.
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Known prey organisms

Scorpiones (scorpion, spiders) preys on:
Orthoptera
Tenebrionidae
Curculionidae
Aclerda
Thysanura
Isoptera
Scarabaeidae
Pogonomyrmex
cactus weevils
Moneilema
Palo Verde weevil
Diptera

Based on studies in:
Namibia, Namib Desert (Desert or dune)
USA: Arizona, Sonora Desert (Desert or dune)

This list may not be complete but is based on published studies.
  • E. Holm and C. H. Scholtz, Structure and pattern of the Namib Desert dune ecosystem at Gobabeb, Madoqua 12(1):3-39, from p. 21 (1980).
  • P. G. Howes, The Giant Cactus Forest and Its World: A Brief Biology of the Giant Cactus Forest of Our American Southwest (Duell, Sloan, and Pearce, New York; Little, Brown, Boston; 1954), from pp. 222-239, from p. 227.
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Feed upon invertebrates, especially insects and other arachnids, including scorpions (Ruppert et al. 2004).

Preyed upon by vertebrates, such as birds, snakes, and amphibians.

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Life History and Behavior

Reproduction

Scorpions use spermatophores for indirect sperm transfer (Ruppert et al. 2004). Though there are differences between species, males and females engage in an elaborate courtship ritual in which the male and female walk backward and forward “hand-in-hand”, until the male finds an appropriate place to deposit the spermatophore, which the female then takes up into the gonopore.

Scorpions are born viviparously and resemble adults, and parents invest an unusually long amount of time investing care in their offspring, compared to other terrestrial arthropods (Ruppert et al. 2004). Development continues for a year, and eventually molt four to seven times in their 25-year lifespan. Scorpion populations’ intrinsic growth rates (r) can be lower than that of whales and elephants.

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Physiology and Cell Biology

Physiology

Scorpions have among the lowest metabolism known for animals, and can very efficiently convert prey biomass into scorpion tissue (Bub and Bowerman 1979). Desert scorpions have evolved means to cope with desiccating temperatures, such as a nearly impervious exoskeleton (Ruppert et al. 2004). The venom delivered from a sting out of self-defense may be painful for humans but very rarely fatal.

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:2,010Public Records:1,146
Specimens with Sequences:1,261Public Species:61
Specimens with Barcodes:991Public BINs:320
Species:163         
Species With Barcodes:118         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Scorpiones

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