The World's Scorpion

Pair of lesser brown scorpions Isometrus maculatus in captivity, from here. The more elongate individual at lower left is a male; his stouter companion is female.

With their elongate, sting-tipped tails, scorpions are instantly distinguishable from any other arachnid. Depending on how you look at it, they are either charismatic or infamous (not many invertebrates have constellations named after them). Yet though the world diversity of scorpions is not unrespectable (about 1750 species have been described so far), distinguishing one scorpion species from another can be a challenging prospect. They tend, as a whole, to be a morphologically conservative group.

As a result, new species of scorpion continue to be described, at a rate limited only by the relatively small number of people taking up the challenge. Isometrus is a genus of about thirty species of scorpion found mostly from in southern Asia and Australasia, from Pakistan to northern Australia and New Caledonia. A good third of those species have only been described since 2000, and probably more remain to be described. They are small, relatively slender scorpions, often with the 'fingers' of the chelae noticeably darker than the 'palms' (as in the photo above). They have a sting that has been referred to as painful, but doesn't seem to have lead to recorded fatalities in humans.

Isometrus has been divided between two subgenera, Isometrus sensu stricto and Reddyanus (Kovařík 2003). The two subgenera are distinguished primarily by the locations of trichobothria, long sensory hairs, on the chelae. The higher diversity of species belong to Reddyanus, but Isometrus is by far the more widespread subgenus. This is due to its inclusion of one particular species: I. maculatus, commonly given the rather underwhelming name of 'lesser brown scorpion'. For some reason, Isometrus maculatus has proven itself very amenable to transport by humans. It may have been originally native to Sri Lanka (as cited here) but from there it has spread to tropical regions around the world. It is found in North America, South America, Africa and Australia, and on various oceanic islands such as Hawaii, Saint Helena and the Seychelles. In Europe, it is only known from southern Spain, though it may have been the species originally indicated by Linnaeus' 'Scorpio europaeus' (Fet et al. 2002; some authors have consequently used the name 'Isometrus europaeus' for this species, but Linnaeus' name was declared invalid by the ICZN due to the uncertainty of its identity). Currently, I. maculatus is regarded as the world's most widespread scorpion species. No other Isometrus species has been subject to the same degree of spread, though I am personally inclined to wonder about the distribution of I. heimi, recorded by Kovařík (2003) from both New Guinea and New Caledonia.

REFERENCES

Fet, V., M. E. Braunwalder & H. D. Cameron. 2002. Scorpions (Arachnida, Scorpiones) described by Linnaeus. Bull. Br. Arachnol. Soc. 12 (4): 176-182.

Kovařík, F. 2003. A review of the genus Isometrus Ehrenberg, 1828 (Scorpiones: Buthidae) with descriptions of four new species from Asia and Australia. Euscorpius 10: 1-19.