Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Isometrus maculatus
(lesser brown scorpion)



Isometrus maculatus (lesser brown scorpion)


  • Last modified
  • 27 September 2018
  • Datasheet Type(s)
  • Documented Species
  • Preferred Scientific Name
  • Isometrus maculatus
  • Preferred Common Name
  • lesser brown scorpion
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Arthropoda
  •       Subphylum: Chelicerata
  •         Class: Arachnida
  • Summary of Invasiveness
  • Isometrus maculatus, commonly known as the lesser brown scorpion, is a native species of Asia which now has an extended pantropical range. Its spread is thought to have been facilitated by human activity, and i...

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Isometrus maculatus (lesser brown scorpion); adult.  Museum set specimen. National Museum of Nature & Science, Tokyo, Japan. September 2013.
CaptionIsometrus maculatus (lesser brown scorpion); adult. Museum set specimen. National Museum of Nature & Science, Tokyo, Japan. September 2013.
CopyrightPublic Domain, released by Daderot-2013/via wikipedia - CC0 1.0
Isometrus maculatus (lesser brown scorpion); adult.  Museum set specimen. National Museum of Nature & Science, Tokyo, Japan. September 2013.
AdultIsometrus maculatus (lesser brown scorpion); adult. Museum set specimen. National Museum of Nature & Science, Tokyo, Japan. September 2013.Public Domain, released by Daderot-2013/via wikipedia - CC0 1.0
Isometrus maculatus (lesser brown scorpion); adult. Tama Zoological Park, Japan. October 2009.
CaptionIsometrus maculatus (lesser brown scorpion); adult. Tama Zoological Park, Japan. October 2009.
Copyright©OpenCage.Info-2009 - CC BY-SA 2.5
Isometrus maculatus (lesser brown scorpion); adult. Tama Zoological Park, Japan. October 2009.
AdultIsometrus maculatus (lesser brown scorpion); adult. Tama Zoological Park, Japan. October 2009.©OpenCage.Info-2009 - CC BY-SA 2.5


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Preferred Scientific Name

  • Isometrus maculatus (De Geer)

Preferred Common Name

  • lesser brown scorpion

Other Scientific Names

  • Buthus filum Ehrenberg, 1828
  • Isometrus americanus Thorell, 1876
  • Isometrus europaeus (Linneaus, 1758) sec Borelli 1909
  • Isometrus filum Ehrenberg, 1828
  • Isometrus maculates
  • Isometrus madagassus Roewer 1943
  • Lychas gabonensis Lucas, 1858
  • Lychas guineensis Lucas, 1858
  • Lychas paraensis Koch, 1845
  • Scorpio americanus Herbst, 1800
  • Scorpio dentatus Herbst, 1800
  • Scorpio europaeus Linneaus, 1758
  • Scorpio gabonensis Lucas, 1858
  • Scorpio guineensis Lucas, 1858
  • Scorpio maculatus de Geer, 1778

Local Common Names

  • Cook Islands: spotted house scorpion; spotted scorpion; striped scorpion
  • Tonga: sokopio

Summary of Invasiveness

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Isometrus maculatus, commonly known as the lesser brown scorpion, is a native species of Asia which now has an extended pantropical range. Its spread is thought to have been facilitated by human activity, and it is often found near human habitation in the areas where it has been introduced. Information on the effects it may have on native species where it has been introduced is scarce. It has been introduced to the islands of Hawaii, where it is a potential predator and a threat to some native cave dwelling arthropods.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Arthropoda
  •             Subphylum: Chelicerata
  •                 Class: Arachnida
  •                     Order: Scorpiones
  •                         Family: Buthidae
  •                             Genus: Isometrus
  •                                 Species: Isometrus maculatus

Notes on Taxonomy and Nomenclature

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There is some confusion over whether Scorpio europaeus Linneaus, 1758 is a synonym of Isometrus maculatus De Geer 1778, with the original type specimen for S. europaeus being lost (Fet et al., 2002).


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Information in this section is from Gerlach and Marusik (2010) unless indicated otherwise.

I. maculatus is 45-60 mm from the anterior carapace margin to the tip of the aculeus, and is a uniformly mottled, pale yellowy-brown colour over the abdomen, pedipalps and legs. It has enlarged pedipalps appearing clawlike, and the true jaws are small and partly hidden from the top by the carapace (Yates, 1993). It has four pairs of terminally clawed legs (Yates, 1993).

It has a triangular sternum, a subaculear tubercle on the telson (segment bearing the prominent sting), and a small, slender pedipalp chelae. The dorsosubmedian carinae of metasomal segments I–IV terminate in small, inconspicuous spiniform granules.

I. maculatus has a Type A trichobothrial pattern, a long tapering hemispermatophore, and apoikogenic development. Marked adult sexual dimorphism is displayed in the species, with male pedipalpal and metasomal segments being much longer than those of females.

I. maculatus fluoresce under ultraviolet light, except when emerging from an old exoskeleton when the new soft exoskeleton is not fluorescent (Daintree Forest, 2016). Presumably this prevents detection by some predators when they are at their most vulnerable during a molt (Daintree Forest, 2016). Scorpions may require 5-7 instars to reach maturity, and fluorescence increases in intensity with each successive instar (Daintree Forest, 2016).


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I. maculatus is thought to be the most widely distributed scorpion in the world (Lourenço and Cloudsley-Thompson, 2012).

Itis distributed in tropical areas worldwide (Yates, 1993). It is probably native to Sri Lanka as it is found inland there under natural conditions. Human activity has potentially allowed it to spread (Lourenço and Cloudsley-Thompson, 2012).

I. maculatus is mostly absent from Europe, although is present on the Spanish mainland and Alboran (Fauna Europaea, 2013). It is also present in the following regions: Afro-tropical, Australia, Nearctic, Neotropical and Oriental (Fauna Europaea, 2013).

I. maculatus is distributed in tropical areas worldwide and is reported on the Hawaiian Islands of Oahu, Maui and Hawaii (Yates, 1993). It has also been introduced to Kauai in Hawaii where it is inhabiting the same caves as endemic amphipods, upon which it most likely preys (US Fish and Wildlife Service, 2006).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes


ChinaPresentEncyclopedia of Life, 2016
IndonesiaPresentPresent based on regional distribution.
-JavaPresentWhitten et al., 1996Lives in houses
JapanPresentMiyashita et al., 2010
SingaporePresentKoh et al., 1990; Tan and Mong, 2013; Bugs and Insects of Singapore, 2014Found in kampongs, usually under rotten tree trunks
Sri LankaPresentNativeLourenço and Cloudsley-Thompson, 2012; Akilan et al., 2013; Encyclopedia of Life, 2016Sri Lanka, Matala, Mannar, Anuradhapura, Jaffna. Rare on Jaffna Peninsula. Not found in arid areas


CameroonPresentIntroducedLourenço, 2013
KenyaPresentGerlach and Marusik, 2010
MadagascarWidespreadIntroducedLegendre, 1972Ubiquitous on Grande Ile, found near or in houses and described as domesticated
MauritiusPresentIntroducedGerlach and Marusik, 2010; iNaturalist, 2016Bois des Amourettes and Grand Port
MozambiquePresentGerlach and Marusik, 2010
RéunionWidespreadGerlach and Marusik, 2010; Faune de La Reunion, 2016Common in low altitude areas up to 400m
Saint HelenaPresentIntroduced Invasive St Helena National Trust, 2013Found in the drier areas on the island
SeychellesPresentGerlach and Marusik, 2010Aldabra, Alphonse, Aride, Astove, Bird, Cousin, Denis, Mahé, North, Praslin, Silhouette
SomaliaPresentGerlach and Marusik, 2010
TanzaniaPresentGerlach and Marusik, 2010

North America

USAPresentPresent based on regional distribution.
-CaliforniaPresentBarceloux, 2008
-FloridaPresentIntroducedBarceloux, 2008; Azscorpion, 2014
-HawaiiPresentIntroducedYates, 1993; US Fish and Wildlife Service, 2006; Hawaii Biological Survey, 2016Oahu, Maui, Hawaii, Kauai

Central America and Caribbean

Cayman IslandsPresentHounsome, 2012Grand Cayman
CubaPresentIntroducedTeruel, 2009Accidentally introduced
Saint Vincent and the GrenadinesPresentSimmons et al., 1944

South America

BrazilPresentPresent based on regional distribution.
-PernambucoPresentFreitas and Vasconcelos, 2008
ChilePresentPresent based on regional distribution.
-Easter IslandPresentVachon, 1972; Encyclopedia of Life, 2016


SpainPresentFauna Europaea, 2013


AustraliaPresentKaltsas et al., 2008; Fauna Europaea, 2013
-QueenslandPresentDaintree Forest, 2016Common in Daintree Forest
Cook IslandsPresentThe Cook Islands Natural Heritage Trust, 2007Penrhyn Tongareva, on land and in dwellings
French PolynesiaPresentEncyclopedia of Life, 2016
KiribatiPresentSimmons et al., 1944Phoenix Islands

Risk of Introduction

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Where goods are imported from areas inhabited by I. maculatus, and where it is imported live (for example, in the pet trade), there is the potential for introduction if conditions are favourable for the species.


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Scorpions prefer to live in warm areas, resting during the day, hiding in damp places, beneath loose rocks, loose bark of fallen trees, boards, piles of lumber, floors of outbuildings, and debris (Yates, 1993).

I. maculatus prefers tropical and moistened habitats and does not thrive in arid and desert areas (Lourenço and Cloudsley-Thompson, 2012).

Habitat List

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Terrestrial – ManagedDisturbed areas Principal habitat
Urban / peri-urban areas Principal habitat
Buildings Principal habitat
Terrestrial ‑ Natural / Semi-naturalLand caves Present, no further details
Scrub / shrublands Present, no further details

Biology and Ecology

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White (1977) concluded that I. maculatus has a chromosome number of 2n=12.

Studies on I. maculatus and other Buthidae in Australia found that these scorpions exhibit a unique combination of cytogenetic features, including achiasmate meiosis, holocentric chromosomes, and interchange heterozygosity (Shanahan, 1989). These studies concluded that the basic diploid number was 2n=14.

Reproductive Biology

I. maculatus has high reproductive ability (Lourenço and Cloudsley-Thompson, 2012).

Male and female scorpions go through a courtship similar to that of spiders, and after mating, the female may eat the male (Yates, 1993). The male grasps the pedipalps of the female, leads her to a place to deposit his spermatophore, and then guides the female to the spermatophore so that it enters her genital aperture (Grzimek, 2012). Sperm are then catapulted to the gonopore and internal fertilization takes place (Grzimek, 2012).

Scorpions do not lay eggs, instead the young are born alive and climb onto the back of the mother where they remain until the first molt (Yates, 1993), which occurs a few days after birth (Bugs & Insects of Singapore, 2014). Immature scorpions are nourished by yolk material stored in their bodies (Yates, 1993). The gestation period is 2.5 months (Punzo, 2000).

I. maculatus has a long tapering hemispermatophore, and is apoikogenic (produces eggs with high yolk content that complete their development in the ovarian tube rather than the follicle) (Gerlach and Marusik, 2010).  

It is thought that males and females may locate and identify each other via pheromones, vibrational communication and due to the fluorescence in UV light (Daintree Forest, 2016).


I. maculatus can live for 3-5 years (Yates, 1993).

Activity Patterns

I. maculatus is a nocturnal species, that hunt and feed at night and rest during the day, hiding beneath rocks, bark, boards, piles of lumber, floors of outbuildings and debris (Yates, 1993). They have been described as a synanthropic species, and are often found in and around houses (Gerlach and Marusik, 2010) where they are found under floors, in attics, kitchens, washrooms and bathrooms, being especially attracted where water is available (Yates, 1993).

I. maculatus retreat for cover under white light, but under blacklight (UVA) they do not (Daintree Forest, 2016).

I. maculatus are timid, fast and very good climbers (Liboupat, 2016).

These scorpions can bury themselves in sand or loose earth for up to 6 months without food or water (Yates, 1993).


I. maculatus feeds on insects, spiders, millipedes, small rodents, and will eat other scorpions that are smaller than themselves or those they catch that are in the process of molting when the exoskeleton is still soft (Yates, 1993).

Scorpions use their pedipalps attack and kill prey, and may also use their sting (Grzimek, 2012).


None have been identified.


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Af - Tropical rainforest climate Preferred > 60mm precipitation per month
Am - Tropical monsoon climate Preferred Tropical monsoon climate ( < 60mm precipitation driest month but > (100 - [total annual precipitation(mm}/25]))
As - Tropical savanna climate with dry summer Preferred < 60mm precipitation driest month (in summer) and < (100 - [total annual precipitation{mm}/25])
Aw - Tropical wet and dry savanna climate Preferred < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers
Cw - Warm temperate climate with dry winter Preferred Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
-36 30

Notes on Natural Enemies

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Domestic cats are predators of I. maculatus (Yates, 1993).

Scorpions in general are predated on by small mammals including rodents, birds, snakes, lizards, and other scorpions (Scorpion Worlds, 2016).

Means of Movement and Dispersal

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Accidental Introduction

It is thought that human activity has facilitated the spread of this scorpion from its native range (Lourenço and Cloudsley-Thompson, 2012).

Its use in the pet trade may also have contributed to the spread of the species.

Intentional Introduction

No records of intentional introduction have been identified.

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Botanical gardens and zoosPotential for accidental release to new areas Yes Yes
Pet tradePotential for accidental release to new areas Yes Yes
Research Yes Yes

Impact: Economic

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No information has been identified that describes any significant negative economic impact.

Impact: Environmental

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It is possible that it may affect cave habitats and populations of cave-inhabiting arthropods on the Hawaiian Island of Kauai, particularly the Kauai Cave Amphipod (Spelaeorchestia kolana), an endangered US ESA listed species (US Fish and Wildlife Service, 2006).

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Spelaeorchestia koloana (Kauai cave amphipod)USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiPredationUS Fish and Wildlife Service, 2006

Impact: Social

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Scorpions can sting, although the venom of I. maculatus does not appear to be particularly dangerous to people although it can cause pain, swelling and nausea (Yates, 1993).

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Capable of securing and ingesting a wide range of food
  • Highly mobile locally
  • Benefits from human association (i.e. it is a human commensal)
  • Has high reproductive potential
Impact mechanisms
  • Predation
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Highly likely to be transported internationally deliberately


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Economic Value

It is likely that its use in the pet and zoo trade has a positive economic impact.

Scorpions are eaten in some parts of the world, e.g. Singapore, Thailand and China, so it is possible that I. maculatus could be one of the species consumed (Scorpion Worlds, 2016).

I. maculatus venom has been investigated for potential antimicrobial (Miyashita et al., 2010) and pesticide applications (Miyashita et al., 2010; Kawachi et al., 2013).

Uses List

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  • Pet/aquarium trade
  • Research model

Detection and Inspection

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I. maculatus is active at night, and so could potentially be detected using UV light due to its fluorescent exoskeleton (Bugs and Insects of Singapore, 2014).

Similarities to Other Species/Conditions

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I. maculatus is smaller than Chiromachus ochropus and is a paler colour (Gerlach and Marusik, 2010). It is different from Lycas braueri in that it has no tibial spurs on legs III and IV, is paler, has inconspicuous spiniform granules on metasomal segments I-IV, and displays sexual dimorphism (Gerlach and Marusik, 2010).

Prevention and Control

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To prevent transfer of these scorpions from one place to another, care should be taken not to leave materials in places where scorpions might hide during the day time when they are less active.

To prevent scorpions from becoming a hazard around homes it is recommended to keep outdoor areas free from clutter and rubbish (Yates, 1993).


Scorpions can be attracted to congregate around damp heavy cloth, and then can be destroyed (Yates, 1993).


Chemical Control

Chemical treatment within dwellings should consist of spot treatment of baseboards, under furniture, in crevices and closets, around plumbing, attics, basements, crawl spaces and any other areas where the scorpions are observed (Yates, 1993).

Outdoor structures (foundations, porches, etc.) that are in contact with the soil should be treated to a height of 2 feet above ground if possible (Yates, 1993).

Gaps in Knowledge/Research Needs

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Further research is needed into the effects that I. maculatus may have on other species in areas where it has been introduced. Research on how to control and eradicate scorpion species where they are invasive is also sparse.


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Akilan VK, Murugananthan A, Eswaramohan T, 2013. Diversity and identification key to the species of scorpions (Scorpiones: Arachnida) from Jaffna Peninsula, Sri Lanka. Journal of Entomology and Zoology Studies, 1(5):70-77.

Azscorpion, 2014. Isometrus maculatus (DeGeer, 1778).

Barceloux DG, 2008. Medical Toxicology of Natural Substances: Foods, Fungi, Medicinal Herbs, Plants, and Venomous Animals. Hoboken, USA: John Wiley & Sons, 621 pp.

Bugs and Insects of Singapore, 2014. Creature of the month - Lesser Brown Scorpion.

Daintree Forest, 2016. Lesser Brown Scorpion (Isometrus maculatus). Queensland, Australia.

Encyclopedia of Life, 2016. Encyclopedia of Life.

Fauna Europaea, 2013. Fauna Europeaea version 2.6.2.

Faune de La Reunion, 2016. Scorpio. Isometrus maculatus. Fauna of the island of La Reunion (Scorpio. Isometrus maculatus. Faune de l'ile de La Reunion).

Fet V, Braunwalder ME, Cameron HD, 2002. Scorpions (Arachnida, Scorpiones) described by Linnaeus. Bulletin of the British Arachnological Society, 12(4):176-182.

Freitas GCC, Vasconcelos SD, 2008. Scorpion fauna of the island of Fernando de Noronha, Brazil: first record of Tityus stigmurus (Thorell, 1877) (Arachnida, Buthidae). Biota Neotropica, 8(2):235-237.

Gerlach J, Marusik YM, 2010. Arachnida and Myriapoda of the Seychelles Islands. Manchester, UK: Siri Scientific Press, 435 pp.

Grzimek, 2012. Scorpions (Scorpiones), Grzimek's Animal Life, Gale Document Number AAA000003805.

Hawaii Biological Survey, 2016. Lesser Brown Scorpion/kopiana. Hawaii, USA.

Hounsome MV, 2012. Terrestrial Invertebrates (Other Than Insects) of the Cayman Islands. In: The Cayman Islands: Natural History and Biogeography [ed. by Brunt MA, Davies JE]. Springer Science & Business Media, 307-323.

iNaturalist, 2016. Lesser Brown Scorpion (Isometrus maculatus)., USA: California Academy of Sciences.

Kaltsas D, Stathi I, Fet V, 2008. Scorpions of the Eastern Mediterranean. In: Advances in Arachnology and Developmental Biology. Monographs, 12:209-246.

Kawachi T, Miyashita M, Nakagawa Y, Miyagawa H, 2013. Isolation and characterization of an anti-insect ss-toxin from the venom of the scorpion Isometrus maculatus. Bioscience, Biotechnology and Biochemistry, 77(1):205-207.

Koh J, Gopalakrishnakone P, Hun K, 1990. A Colour Guide to Dangerous Animals [ed. by Gopalakrishnakone P]. Kent Ridge, Singapore: Singapore University Press, Ridge Books.

Legendre R, 1972. The Arachnids of Madagascar. (Les Arachnids de Madagascar.) In: Biogeography and Ecology in Madagascar [ed. by Battistini R, Richard-Vindard G]. Dordrecht, Netherlands: Springer Science & Business Media, 427-458.

Liboupat, 2016. Isometrus maculatus, Buthidae family (Isometrus maculatus, famille des Buthidae).

Lourenço W, Cloudsley-Thompson JL, 2012. Introduction and adaptation of Isometrus maculatus (De Geer, 1778) (Scorpiones, Buthidae) in arid and desert formations. Newsletter of the British Aarchnological Society, 125. 8-9.

Lourenço WR, 2013. A new species of babycurus Karsch, 1886 from northern Cameroon (Scorpiones: Buthidae). Arthropada Selecta, 22(4):343-348.

Miyashita M, Sakai A, Matsushita N, Hanai Y, Nakagawa Y, Miyagawa H, 2010. A novel amphipathic linear peptide with both insect toxicity and antimicrobial activity from the venom of the scorpion Isometrus maculatus. Bioscience, Biotechnology and Biochemistry, 74(2):364-369.

Punzo F, 2000. Desert Arthropods: Life History Variations. Berlin, Germany: Springer Science & Business Media, 230 pp.

Scorpion Worlds, 2016. Scorpion Predators.

Shanahan CM, 1989. Cytogenetics of Australian scorpions. I. Interchange polymorphism in the family Buthidae. Genome, 32(5):882-889.

Simmons JS, Whayne TF, Anderson GW, 1944. India and the Far East: a Geography of Disease and Sanitation. London, UK: William Heineman Ltd. 530 pp.

St Helena National Trust, 2013. Isometrus maculatus. Bugs on the Brink - Our Inverterbrates.

Tan HH, Mong RP, 2013. Scorpion stings presenting to an emergency department in Singapore with special reference to Isometrus maculatus. Wilderness and Environmental Medicine, 24(1):42-47.

Teruel R, 2009. Morphology, ecology and distribution of Isometrus maculatus (DeGeer 1778) in Cuba (Scorpiones: Buthidae). (Morfología, ecología y distribución de Isometrus maculatus (Degeer 1778) en Cuba (Scorpiones: Buthidae)). Boletin de la S.E.A, No.45:173-179.

The Cook Islands Natural Heritage Trust, 2007. Isometrus maculatus, Spotted Scorpion Species Page.

US Fish and Wildlife Service, 2006. In: Kauai Cave Amphipod (Spelaeorchestia koloana). 5-Year Review: Summary and Evaluation. US Fish and Wildlife Service, 10 pp.

Vachon M, 1972. Observations on the scorpions belonging to the genus Isometrus H. et E. (Buthidae) with respect to the species Isometrus maculatus (Geer) occurring on Easter Island [Pacific Ocean]. (Remarques sur les scorpions appartenant au genre Isometrus H. et E. (Buthidae) a propos de l'espece Isometrus maculatus (Geer) habitant l'Ile de Paques [Pacific Ocean]). Cahiers du Pacifique, No. 16. 169-180.

White MJD, 1977. Animal cytology and evolution. Cambridge, UK: Cambridge University Press. 961 pp.

Whitten T, Soeriaatmadja RE, Afiff SA, 1996. The Ecology of Java and Bali. The Ecology of Indonesia Series, 2. Dalhousie University, Hong Kong: Periplus Editions Ltd., 969 pp.

Yates JRIII, 1993. Isometrus maculatus (De Geer), Lesser Brown Scorpion. Urban Knowledge Master, Extension Entomology & UH-CTAHR Integrated Pest Management Program. Hawaii, USA: University of Hawaii.


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07/05/2016 Original text by:

Vicki Cottrell, Consultant, UK

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