The Buthidae are the largest family of scorpions, containing about 80 genera and over 800 species as of mid-2008. Its members are known as, for example, fat-tailed scorpions and bark scorpions. A few very large genera are known, but a high number of species-poor or monotypicones also exist. New taxa are being described at a rate of several to several dozen new species per year. They occur in the warmer parts of every major landmass on Earth, except on New Zealand. Together with four other families the Buthidae make up the superfamilyButhoidea. The family was established by Carl Ludwig Koch in 1837. Around 20 species of medically important scorpions are known, and all but one of these are members of the Buthidae. In dead specimens, the spine beneath the stinger, characteristic for this family, can be observed.
Few Buthidae scorpions are among the larger scorpions; on average the members of this family are mid-sized tending towards smallish. Microtityus and Microbuthus barely reach 2 cm. The largest members are found among Androctonus, Apistobuthus, and Centruroides; and can reach 12 cm. Most of them have between two and five pairs of eyes. Some resemble the Vaejovidae. The Chaerilidae and Chactidae have one pair of eyes at most, and the former show a yellowish spot between and to the rear of these. Their vernacular name refers to the thick tails found in many Buthidae, especially in the Old World. The pedipalps, though, tend to be weak, slender, and tweezer-like. The Buthidae are generally rather cryptically colored, quite uniformly ochre to brown, but some are black or more vividly colored. More conspicuous patterns and shapes occur, e.g., in Isometrus or Lychas.
Toxicity and relationship with humans
A handful of sometimes species-rich genera – fattail scorpions, Centruroides, Hottentotta, Leiurus, Parabuthus, and Tityus – are notorious for their strong venom. Human fatalities have been recorded from fewer than two dozen species; identification of a particular Tityus is likely problematic and detailed data on the venom exists only for a small fraction of the Buthidae.
Mechanism of Toxicity
Studied Buthidae toxins have been found to hyperpolarize nerves and slow inactivation, causing painful nerve firing that can last for hours. Many toxins trigger Na+ channels to open prematurely, causing excess sodium ion leakage that triggers nerve signal transduction. Other toxins have been found to bind K+ leak channels to prevent the flow of potassium ions, disallowing the hyperpolarized nerve membrane from depolarizing and returning to its resting state. The Na+ channel toxins of both Old World and New World Buthids have a similar protein homology and mechanism of action. It is speculated that toxins from these two groups are derived from the evolutionary ancestor before divergence 150 million years ago.
