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Redescription of Plesiochactas mitchelli (Scorpiones: Euscorpiidae): a rare scorpion from Central America.

There is little information available about scorpions of the genus Plesiochactas Pocock 1900. This genus consists of only two species: Plesiochactas dilutus Karsch 1881, the type species which is known from four specimens (Soleglad 1976; Soleglad & Sissom 2001), and Plesiochactas mitchelli Soleglad 1976. The male is undescribed for this genus, and P. mitchelli is known only from a single immature specimen.



Pocock (1902) mentioned a young specimen from "Guatemala" nearly allied to P. dilutus. Soleglad (1976) revised the scorpion subfamily Megacorminae, with limited material available. He described P. mitchelli without a precise locality from the same juvenile female reported by Pocock. To this date, no additional specimens of this species have been reported in the literature. In the present paper, we redescribe P. mitchelli on the basis of an adult female from southeastern Chiapas, Mexico, from a locality not too distant from the border with Guatemala (see Remarks).


The new specimen examined for this study is lodged in the Coleccion Nacional de Aracnidos (CNAN), Instituto de Biologia, Universidad Nacional Autonoma de Mexico, Mexico, D.F. Nomenclature and mensuration primarily follow Stahnke (1970), with the following exceptions: metasomal carinal terminology after Francke (1977), pedipalp carinae terminology after Acosta et al. (2008), trichobothrial designations after Vachon (1974), and chelal finger dentition terminology after Soleglad & Sissom (2001). Measurements were made with an ocular micrometer at 10X on a Nikon SMZ 800 stereomicroscope; drawings on the same scope with a camera lucida attached; photographs also on the same scope with a Nikon Coolpix 20 camera.


Family Euscorpiidae Laurie 1896 Subfamily Megacorminae Kraepelin 1899 Genus Plesiochactas Pocock 1900

Plesiochactas Pocock 1900:470.

Type species.--Plesiochactas dilutus (Karsch 1881), by monotypy.

Plesiochactas mitchelli Soleglad 1976

Plesiochactas mitchelli Soleglad 1976:251, 263, 286, 288, 294-298, Table 4, Figs. 17, 21, 86, 87, 117, 118, 121, 123-134; Soleglad & Sissom 2001:29, 58, 67, 92, Figs. 141, 196.

Plesiochactas dilutus (in part) Pocock 1902:16, 17, Table 4, Figs. 5a-f.

Type data.--GUATEMALA: Holotype juvenile female from "Guatemala (Sarg)," date unknown (deposited in Natural History Museum, London).

Additional specimen.--MEXICO: Chiapas: 1 adult [female], Santa Rosa, [Municipio] La Trinitaria, August 1974, [col.] A. Ramirez V. (CNAN).

Diagnosis.--Adult female 53.3 mm long. Pectines quite reduced; 5 teeth for female; fulcra absent. Median carina of sternite VII obsolete. Stigmata elongate oval. Patella with 23 trichobothria on external face: 5 each in em and et series; 8-10 ventral trichobothria in a straight row along posterior margin. P. dilutus, the only other species in the genus has 8-9 pectinal teeth in females, 10 in the male; the pectines have well developed fulcra; and median carina of sternite VII weak to moderately strong, smooth.

Description of adult female (Figs. 1, 2).--Coloration: uniformly orange-brown, darkening to reddish brown in carapace, pedipalps, metasomal segments and telson. Prosoma: carapace 1.2 times wider than long, densely covered with random mixture of large, medium, and small granules, anterior margin straight. Lateral eyes, two per side, well developed, posterior eye slightly smaller. Median ocular tubercle very well developed and prominent, one-fifth width of carapace at that point; placed on anterior two-fifths of carapace (Fig. 3). Sternum wider than long with some granules distally, deep median groove (Fig. 4). Mesosoma: tergites rough, minutely and densely granulose, with larger, sparse granules on posterolateral regions. Median carina weak to vestigial in all tergites. Tergite VII with two pairs of crenato-granulose carinae. Genital operculum: relatively small, subtriangular, with complete median longitudinal membranous connection. Pectines (Fig. 4): reduced, three marginal and one middle lamellae well defined; fulcra absent; pectinal teeth 4 times longer than wide, sensorial areas present distally, on 1/3 to 1/2 of tooth; tooth count 5-5. Sternites: III-VI lustrous; VII with a pair of vestigial lateral carinae, hinted by some granules; median carina obsolete. Stigmata: reniform elongate, with a distinctly sclerotized lid hinged distally, which opens inward to the book lung cavity (Figs. 5, 6). Metasoma: intercarinal spaces with sparse granules. Segments I-II wider than long, III as long as wide and IV longer than wide. Dorsolateral and lateral supramedian carinae crenulated to serrated. Posterior spines on dorsal carinae weakly developed, gradually stronger on distal segments (Fig. 7). Lateral infra-median carinae on I, granulose and complete; on II with few granules posteriorly, and on III-IV obsolete. Ventrolateral carinae on I-II, granulose; on III-IV crenulated to serrated. Ventral median carina on I weak, granulose; on II-IV crenulated to serrated. Segment V: dorsolateral and ventrolateral carinae crenulated to serrated; lateral median carina weak, indicate only on proximal region; ventral median carina, serrated; anal arc with subtle granulation. Telson: segment elongated, aculeus-vesicle juncture not sharply defined. Vesicle dorsally smooth, lateral and ventrally covered with low, rounded granules. Aculeus slightly curved. Subaculear tubercle/spine absent. Chelicera: movable finger with two subdistal denticles on dorsal edge; inferior margin with 2-4 small denticles; without serrula (Fig. 8). Pedipalps: dorsal face of patella and femur with scattered granules; ventral face smooth to rough. Femur orthobothriotaxic (Fig. 9): 2.6 times longer than wide; retrodorsal, prodorsal and proventral carinae, strong and crenulated; retroventral carina present only on proximal region, strong and granulose; retrolateral face with granulose to serrated median carina; prolateral face with a median carina indicate by some longer spiniform granules. Patella neobothriotaxic (Figs. 10, 11): retrolateral face with 23 trichobothria (5 et, 4 est, 5 em, 2 esb and 7 eb); ventrally on right side with 9 v trichobothria, left with 10 v; 2.3 times longer than wide; all carinae crenulated to serrated; retrolateral aspect with well developed median carina, crenulated; proximal projection of prolateral face with one medium and one small spine. Chela: Orthobothriotaxic (Figs. 12, 13): movable finger shorter than carapace and slightly shorter than fifth metasomal segment. Digital and retrolateral secondary carinae strong, crenulated; dorsal secondary and prodorsal carinae weak, granulose; retroventral carina very strong and crenulated; proventral carina weak, crenulated; ventral median carina vestigial, only present and granulose proximally; prolateral median carina crenulated. Intercarinal spaces with sparse granules. Chelal finger dentition, based on right movable finger (Fig. 14): median denticle row flanked by groups of one outer denticle plus two inner denticles; in addition, the median denticle corresponding to each group is slightly enlarged, resulting in transverse rows of four denticles each, which divide the median denticle row into distinct sub-rows. Distally there are no accessory denticles; basal to distal group 3 inner accessory denticles appear, and basal to distal group 5 outer accessory denticles are present. Basal to distal group 6 a poorly defined "double row" formed by the median denticle row and the inner accessory denticles can be discerned. Measurements: (in mm; L = length, W = width, D = depth). Total L 53.3; carapace L/W 8.0/9.3; mesosoma L 17.2; metasoma L 19.4: segment I L/W/D 2.3/3.8/3.1, segment II L/W/D 2.6/3.3/3.0, segment III L/W/D 3.0/3.2/3.1, segment IV L/W/D 4.0/3.0/3.1, segment V L/W/D 7.5/2.7/2.8. Telson L 8.7: vesicle L/W/D 5.1/3.0/2.6; pedipalp L 26.7: femur L/W 6.6/ 2.5, patella L/W 6.8/3.0, chela L/W/D 13.3/4.0/4.3, movable finger L 7.0, fixed finger L 5.9.




Remarks.--Francis Charles Sarg (whom we presume collected the holotype) was born in Guatemala; in the 1880s he lived in Coban and around 1902 he owned a farm called "El Chicabal" in Quetzaltenango. It is possible the holotype was collected in or near that farm.

There are four settlements called Santa Rosa in the Municipio of La Trinitaria, Chiapas, and we do not know from which of them the specimen was collected; thus we can not provide geographical coordinates or elevation for the specific location. Quetzaltenango, Guatemala, is approximately 100 km from the Municipio of La Trinitaria.


The principal difference, other than size, between the juvenile holotype and the adult female of P. mitchelli is the patella ventral trichobothrial number: eight in the holotype, nine and ten in the adult female specimen. This character shows similar variability in the other described species of Plesiochactas (P. dilutus: 9-12), and is also variable in species of the genus Megacormus Karsch 1881 (Soleglad 1976; Sissom 1994; Francke pers. obs.). The submedian and lateral carinae of sternite VII are obsolete in the holotype, and on the adult female a pair of vestigial lateral carinae are present, and the submedian carinae are also obsolete. One somewhat large denticle on the distal aspect of the ventral edge of the movable cheliceral finger of the holotype, the adult female has two to four spaced denticles. The movable finger of the pedipalp chela on the adult female is shorter than the carapace and slightly shorter than the fifth metasomal segment; in the holotype it is slightly shorter than the carapace, and longer than segment V of the metasoma. Several differences of mesosomal, metasomal, and pedipalpal carinae between the holotype and the adult female are ascribable to ontogenetic changes. Both the presence or absence of fulcra on the pectines, and the pectinal tooth counts on females, are diagnostic characters that separate the two species of Plesiochactas. Also, the median carina on sternite VII is weak to moderate and smooth on P. dilutus, and it is smooth and vestigial to obsolete on P. mitchelli.

The taxonomic status of this genus and its species remains uncertain. Originally, Plesiochactas was separated from Megacormus by the presence of distinct fulcra in the pectines, but P. mitchelli lacks the pectinal fulcra. Plesiochactas dilutus and P. mitchelli share having five trichobothria on the external terminal series of the patella, as opposed to only 3-4 on Megacormus, and on that basis P. mitchelli was placed in Plesiochactas by Soleglad (1976). However, the number of trichobothria on that region of the patella is quite variable in the family Euscorpiidae Laurie 1896 (Soleglad & Sissom 2001), and thus the similarity between the two species currently placed in Plesiochactas could be a symplesiomorphy.


Soleglad (1976) and Soleglad & Sissom (2001) also indicate that Plesiochactas and Megacormus can be separated on the basis of chelal finger dentition: the former with 45+ small, inner accessory denticles, arranged row-like; the later with 35+ small outer accessory granules, scattered, irregular row-like. A direct comparison of the denticle pattern in P. mitchelli (Fig. 14), against those of Megacormus gerstchi Diaz 1966 (Fig. 15), and P. dilutus (Fig. 16) clearly indicate a stronger similarity between the first two, which share transverse "tetrads" along the movable finger length. In P. dilutus, however, the tetrads tend to disappear after distal group 3 and only enlarged inner and outer denticles are present, and a "double row" of median plus inner accessory denticles is quite distinct on the basal 2/3 of the movable finger. The second author has undertaken a revision of the tribe Megacormini Kraepelin 1899 [Megacormus + Plesiochactas], including a cladistic analysis of the phylogenetic relationships of all included taxa, which should help resolve the current uncertainty (ms. in preparation).



Our thanks to Dr. Carlos R. Beutelspacher Baigts for donating the specimen to the CNAN, Belen Chavez Galvan kindly researched the information on Mr. Francis C. Sarg, Alejandro Valdez Mondragon helped with the photographs, and Carlos Santibariez Lopez kindly reviewed the manuscript. Financial support was provided by FOMIX-Chiapas through the project with key: CHIS-2006-C06-45752.

Manuscript received 12 January 2009, revised 22 July 2009.


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Pocock, R.I. 1902. Arachnida: Scorpiones, Pedipalpi and Solifugae. In Biologia Centrali Americana (F.D. Goodman &O. Salvin, eds.). Taylor and Francis, London.

Sissom, W.D. 1994. Systematic studies on the genus Megacormus (Scorpiones, Chactidae, Megacorminae), with descriptions of a new species from Oaxaca, Mexico and the male of Megacormus segmentatus Pocock. Insecta Mundi 8:265-271.

Soleglad, M.E. 1976. A revision of the scorpion subfamily Megacorminae (Scorpionida: Chactidae). Wasmann Journal of Biology 34:251-303.

Soleglad, M.E. & W.D. Sissom. 2001. Phylogeny of the family Euscorpiidae Laurie, 1896: a major revision. Pp. 25-111. In Scorpions 2001. In Memoriam Gary A. Polis. (V. Fet & P.A. Selden, eds.). British Arachnological Society, Burnham Beeches, Buckinghamshire, UK.

Stahnke, H.L. 1970. Scorpion nomenclature and mensuration. Entomological News 81:297-316.

Vachon, M. 1974. Etude des caracteres utilises pour classer les familles et les genres de Scorpions (Arachnides). 1. La trichobothriotaxie en Arachnologie. Sigles trichobothriaux et types de trichobothriotaxie chez les Scorpions. Bulletin du Museum National d'Histoire Naturelle, Paris, 3e serie 140 (Zoologie 104):857-958.

Kaleb Zarate-Galvez: Coleccion de Aracnidos, Facultad de Ciencias Biologicas, Universidad de Ciencias y Artes de Chiapas. Libramiento Norte Poniente s/n, Colonia Lajas Maciel, C.P. 29039. Tuxtla Gutierrez, Chiapas, Mexico. E-mail:

Oscar F. Francke: Coleccion Nacional de Aracnidos, Departamento de Zoologia, Instituto de Biologia, Universidad Nacional Autonoma de Mexico, Apartado Postal 70-153, Ciudad Universitaria, Mexico, D.F. 04510, Mexico
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Author:Zarate-Galvez, Kaleb; Francke, Oscar F.
Publication:The Journal of Arachnology
Article Type:Report
Date:Sep 1, 2009
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