Marbled scorpion Lychas marmoreus, copyright Reiner Richter.

Belongs within: Buthoidea.

The Buthinae are a pantropical group of scorpions with oblique, non-overlapping rows of tooth-like granules on the movable finger of the pedipalp. The genus Isometrus is found primarily in the Old World tropics, except for the pantropical I. maculatus.

The world’s scorpion
Published 14 July 2014
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.

Systematics of Buthinae
    |--Isometroides Keyserling 1885K77
    |    `--*I. vescus (Karsch 1880) [=Isometrus vescus; incl. Isometroides angusticaudus Keyserling 1885]K77
    |    |--B. cyaneusD57
    |    |--B. eupeus [incl. B. eupeus var. cognatus]K01
    |    |--B. flavicruris Rainbow 1897R97
    |    |--B. gibbosusF04
    |    |--B. grammurus Thorell 1889T89
    |    |--B. groutii Wood 1869W69
    |    |--B. hottentotaeT89
    |    |--B. martensi Karsch 1879 [incl. B. confucius Sim. 1880]K01
    |    |--B. occitanusGK04
    |    |--B. reticulatusD57
    |    `--B. tamulusK77
    |--Isometrus Hemprich & Ehrenberg 1828 [=Atreus Walckenaer & Gervais 1844]K77
    |    |--I. maculatus (De Geer 1778)SP10 (see below for synonymy)
    |    |--I. acanthurus Pocock 1899K77
    |    |--I. antillanusT89
    |    |--I. assamensis Oates 1888K77
    |    |--I. atomariusT89
    |    |--I. basilicus Karsch 1879K77
    |    |--I. brachycentrus Pocock 1899K77
    |    |--I. feae Thorell 1889T89
    |    |--I. formosus Pocock 1893K77
    |    |--I. fuscusT89
    |    |--I. madagassus Roewer 1943K77
    |    |--I. melanodactylus (Koch 1867) (see below for synonymy)K77
    |    |--I. perfidus Koch & Keyserling 1884-1885KK90
    |    |--I. phipsoniT89
    |    |--I. rigidulus Pocock 1897K77
    |    |--I. shoplandii Oates 1888T89
    |    |--I. thurstoni Pocock 1893K77
    |    |--I. thwaitesii Pocock 1897K77
    |    `--I. vittatus Pocock 1900K77
    `--Lychas Koch 1850 [incl. Archisometrus Kraepelin 1891, Hemilychas Hirst 1911]K77
         |--*L. scutilus Koch 1845K77 [=Isometrus scutilusT89]
         |--L. albimanus Henderson 1919K77
         |--L. alexandrinus Hirst 1911 (see below for synonymy)K77
         |--L. feae (Thorell 1889)K77
         |--L. flavimanus (Thorell 1888)K77
         |--L. gravelyi Henderson 1913K77
         |--L. guineensis Lucas 1858SP10 [=Scorpio (Lychas) guineensisK77]
         |--L. hendersoni (Pocock 1897)K77
         |--L. infuscatus (Pocock 1890)K77
         |--L. laevifrons (Pocock 1897)K77
         |--L. marmoreus (Koch 1845) (see below for synonymy)K77
         |--L. mucronatus (Fabricius 1798)K77 (see below for synonymy)
         |--L. nigrimanus (Kraepelin 1898)K77
         |--L. nigristernis (Pocock 1899)K77
         |--L. perfidus (Keyserling 1887)K77
         |--L. rugosus (Pocock 1897)K77
         |--L. scaber (Pocock 1893)K77
         |--L. shelfordi (Borelli 1904)K77
         |--L. shoplandi Oates 1888K77
         |--L. tweediei Kopstein 1937K77
         `--L. variatus (Thorell 1877) (see below for synonymy)K77

Isometrus maculatus (De Geer 1778)SP10 [=Scorpio maculatusK77, Lychas maculatusK77; incl. S. americanus Herbst 1800K77, Centrurus (Isometrus) americanusK77, Lychas americanusK77, Scorpio dentatus Herbst 1800K77, Buthus (*Isometrus) filum Hemprich & Ehrenberg 1828K77, S. (*Atreus) filumKK90, K77, S. (Lychas) gabonensis Lucas 1858K77]

Isometrus melanodactylus (Koch 1867) [=Lychas melanodactylus; incl. I. gracilis Thorell 1877, I. melanodactylus inflatus Glauert 1925, I. papuensis Werner 1916]K77

Lychas alexandrinus Hirst 1911 [=L. (*Hemilychas) alexandrinus; incl. L. annulatus Glauert 1925, L. mjobergi Kraepelin 1916, L. truncatus Glauert 1925]K77

Lychas marmoreus (Koch 1845) [=Tityus marmoreus, *Archisometrus marmoreus, Lychas marmoreus typicus; incl. Isometrus bituberculatus Pocock 1891, Archisometrus bituberculatus, Lychas jonesae Glauert 1925, L. marmoreus nigrescens Kraepelin 1916, L. marmoreus obscurus Kraepelin 1916, L. marmoreus splendens Kraepelin 1916]K77

Lychas mucronatus (Fabricius 1798)K77 [=Scorpio mucronatusT94, Archisometrus mucronatusT94, Isometrus mucronatusT89; incl. Tityus varius Koch 1845T89, I. variusT89]

Lychas variatus (Thorell 1877) [=Isometrus variatus, Archisometrus variatus, Lychas marmoreus variatus; incl. Isometrus armatus Pocock 1890, Archisometrus armatus, Lychas armatus, L. spinatus besti Glauert 1925, L. marmoreus kimberleyanus Kraepelin 1916, L. lappa Glauert 1954, L. spinatus pallidus Glauert 1925, Isometrus variatus papuanus Thorell 1888, L. papuanus, L. spinatus Kraepelin 1916, Isometrus thorelli Keyserling 1885]K77

*Type species of generic name indicated


[D57] Doleschall, C. L. 1857. Bijdrage tot de kennis der Arachniden van den Indischen Archipel. Natuurkundig Tijdschrift voor Nederlandsch Indië, series 3, 3 (5–6): 399–434, pls 1–2.

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

[F04] Fritsch, A. 1904. Palaeozoische Arachniden. Selestverlag: Prague.

[GK04] Gantenbein, B., & P. D. Keightley. 2004. Rates of molecular evolution in nuclear genes of east Mediterranean scorpions. Evolution 58 (11): 2486–2497.

[K77] Koch, L. E. 1977. The taxonomy, geographic distribution and evolutionary radiation of Australo-Papuan scorpions. Records of the Western Australian Museum 5 (2): 83–367.

[KK90] Koch, L., & E. Keyserling. 1884–1890. Die Arachniden Australiens nach der Natur beschrieben und abgebildet vol. 2. Bauer & Raspe: Nürnberg.

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.

[K01] Kulczyński, V. 1901. Arachnoideák [Arachnoidea]. In: Horváth, G. (ed.) Zichy Jenő Gróf Harmadik Ázsiai Utazása [Dritte Asiatische Forschungsreise des Grafen Eugen Zichy] vol. 2. Zichy Jenő Gróf Harmadik Ázsiai Utazásának Állattani Eredményei [Zoologische Ergebnisse der Dritten Asiatischen Forschungsreise des Grafen Eugen Zichy] pp. 311–369. Victor Hornyánszky: Budapest, and Karl W. Hierseman: Leipzig.

[R97] Rainbow, W. J. 1897. Contributions to a knowledge of the arachnidan fauna of Australia. No. 1. Proceedings of the Linnean Society of New South Wales 21 (4): 634–636, pl. 49.

[SP10] Santos, R., & C. E. Prieto. 2010. Los Assamiidae (Opiliones: Assamiidae) de Río Muni (Guinea Ecuatorial), con la descripción de ocho nuevas especies. Revista de Biologia Tropical 58 (1): 203–243.

[T89] Thorell, T. 1889. Viaggio di Leonardo Fea in Birmania e regioni vicine. XXI.—Aracnidi Artrogastri Birmani raccolti da L. Fea nel 1885–1887. Annali del Museo Civico di Storia Naturale di Genova, Serie 2a, 7: 521–729.

[T94] Thorell, T. 1894. Förteckning öfver Arachnider från Java och närgränsande öar, insamlade af docenten D:r Carl Aurivillius; jemte beskrifingar å några sydasiatiska och sydamerikanska Spindlar. Bihang till K. Svenska Vetenskaps-Akademiens Handlingar 20 pt 4 (4): 1–63.

[W69] Wood, H. C., Jr. 1869. On the Phalangia and Pedipalpi collected by Professor Orton in western South America, with the description of new African species, accompanied with a plate. Transactions of the American Philosophical Society 13: 435–442, pl. 17.

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