Gymnobisium Beier, 1931

Neethling, J. A. & Neethling, C., 2023, A systematic revision of the South African Gymnobisiidae (Pseudoscorpiones: Neobisioidea), Zootaxa 5256 (6), pp. 501-543 : 505-508

Genus Gymnobisium Beier, 1931

Gymnobisium Beier, 1931: 304 ; Beier, 1932: 161–162; Vachon, 1934: 413; Beier, 1947: 289–290; Vitali-di Castri, 1970: 1, 7–8; Harvey, 1992: 1409; Harvey, 1996: 258; Harvey et al. 2016, 75–80.

Type species: Ideobisium quadrispinosum Tullgren, 1907

Diagnosis. The genus Gymnobisium can easily be distinguished from the other gymnobisiid genera in that both sexes lack any ventral depressions or protuberances on their chela. Adults range in size from around 1.75–3.14 mm, with females being overall slightly larger than males.

Further diagnostic characters include a carapace that is subquadrate, overall smooth and without furrow; when present, two pairs of eyes situated anteriorly on the lateral margins of the carapace; tergites and sternites smooth and undivided, tergite and sternite XI merging around tergite and sternite XII; pedipalps smooth and coloured orange- to red-brown; the lightly sclerotized, juxtadentate teeth and lamellae translucent yellow in colour; terminal tooth of fixed chelal finger adapted to an extended, multi-toothed structure, consisting of five to eight strongly sclerotized teeth; pleural membrane faintly papillate.

Description. Gymnobisium colouration follows the same basic pattern among all species and the pedipalps appear orange- to red-brown throughout, the carapace being darker or the same colour as the tergites, the sternites being mostly the same colour as the tergite at the posterior of the abdomen and then transitioning to slightly lighter towards the coxae. The chelicerae are somewhat lighter than the carapace. The legs and leg coxae are always lighter than the abdominal plates, while the pedipalpal coxae are mostly the same colour as the rest of the pedipalps.

Chelicerae have five to six acuminate setae on the cheliceral palm ( Fig. 1A ), es originating at the base of the movable finger, is originating dorsally at the base of the fixed finger, while bs, sbs and ls are situated dorsally on the palm. Seta gs is situated basal of the galea on the movable finger. Lamina exterior absent. Serrula interior present on the fixed cheliceral finger, consisting of multiple lamellae; two basal lamellae elongated and containing a small protrusion each terminally, most distal lamellae displaying faintly serrated margins ( Fig. 1D ). Serrula exterior present on movable finger, also consisting of numerous lamellae; most distal lamellae somewhat elongated and possess faintly serrated margins ( Fig. 1C ). The rallum is comprised of seven to ten long, acuminate blades, and the galea of both sexes divide and terminate in six to ten small rami ( Fig. 1B ).

Carapace smooth with multiple acuminate setae. Two pairs of eyes, when present, are located anteriorly on the lateral margins; a small seta is present on the anterior dorsal surface of the posterior eye pair.Abdominal tergites and sternites smooth, undivided and uniseriate, tergite and sternite XI merging around tergite and sternite XII. The tergal setae are located on the posterior edge of each tergite. Sternites I and II are fused and hardly distinguishable. Female operculum with setae loosely clustered centrally at the posterior edge of sternite II, genital opening not visible ( Fig. 2A ). Males with multiple setae at the operculum cluster, a row of setae located along the posterior margin of sternite II at the genital opening, and additional setae located anteriorly next to these, loosely grouped in the centre of sternite II. Male sternite III with setae located on the anterior margin of the sternite, at the genital opening which is enlarged ( Fig. 2B ). Pleural membrane cream to light yellow in colour, faintly papillate. All legs diplotarsate with subterminal tarsal setae dentate distally, claws simple with arolium shorter than claws. The suture line between femur and patella of leg IV perpendicular to long axis.

Pedipalp segments smooth, trochanter cone-shaped and rounded anteriorly, curved slightly posteriorly with setae present on the anterior surface. Femur very slender, much longer than wide, somewhat constricted basally, widening just distal of base with disto-prolateral excavation present. Setae absent from base, though distributed evenly over rest of surface, no sensory trichobothria present. Patella constricted and slightly angled at base, widening markedly distal at around a third segment length, longer than wide. Several small lyriform fissures present on the dorsal surface, just distal of base. Two larger lyriform fissures present latero-dorsally from these, where the segment starts to widen. Disto-prolateral excavation present. Setae sparse at base, more numerous on wide part of patella, no sensory trichobothria present.

Chelae in both sexes smooth, female hand can be larger and markedly more bulbous than male in some species. Retrolateral condyle small but distinct and rounded in both sexes. Fingers narrow in most species, thickest in Gymnobisium cuneatum sp. nov., and depending on species, range in length from shorter than the chelal hand (without pedicel) to longer than the chelal hand (without pedicel). Fingers curved slightly prolaterally in both sexes. Fixed finger with eight trichobothria, movable finger with four. Trichobothria eb, esb, ib, isb and ist clustered at the base of the fixed finger, while est, it and et form a subterminal group ( Fig. 3 ).

Venom apparatus present on movable finger only, venom duct short, nodus ramosus present and distinct. Fixed finger lacking venom apparatus, terminal tooth adapted to an extended, highly sclerotized, multi-toothed structure. This structure usually consists of a large terminal tooth, followed by a row of four to seven smaller and pointed teeth ( Fig. 4A, B ), except in Gymnobisium inukshuk where the structure consists of a series of 13 roughly equalsized teeth ( Harvey et al. 2016). Both fingers with rounded, juxtadentate teeth situated on a flexible lamella. Two thickened, spine-like setae present on distal end of movable finger in both sexes ( Fig. 4C, D ).

Remarks. Beier (1931) based his original description of the genus on what he erroneously thought was an adult type of Ideobisium quadrispinosum , in which the fixed chelal finger contained only six trichobothria and the movable finger four. This type was a deutonymph, as noted by Vachon (1934). Beier finally updated the genus description in 1947, noting the erroneous use of a deutonymph, by using an adult female of what he thought was Gymnobisium quadrispinosum from the Knysna forests of the Western Cape Province ( Beier 1947, specimen SAMC B6911). That specimen has now been revealed to belong to G. capense sp. nov., and thus, although the genus description was now accurate, the updated description for G. quadrispinosum was still in error.

Species included: Gymnobisium capense sp. nov., G. cuneatum sp. nov., G. hogsbackense sp. nov., G. inukshuk Harvey & Giribet, 2016 , G. megalodontum sp. nov., G. octoflagellatum Beier, 1947 , G. prionotogladiatum sp. nov. and G. quadrispinosum (Tullgren, 1907) .

Figures

FIGURE 1. General morphology of gymnobisiid chelicerae. A. Right chelicera dorsal view; B. Same, prolateral view; C. Serrula exterior; D. Serrula interior. For abbreviations of cheliceral setae see text.

FIGURE 2. General morphology of gymnobisiid genital opercula. A. Female: genital area with spiracles; B. Male: same.

FIGURE 3. Right chela of male Gymnobisium megalodontum sp. nov., retrolateral view, displaying the general chelal morphology and trichobothrial arrangement found in the genus.

FIGURE 4. Morphology of moveable and fixed chelal finger terminal structures. A. Fixed chelal finger of male Gymnobisium megalodontum sp. nov., displaying modified anterior tooth structure; B. Same, male G. prionotogladiatum sp. nov.; C. Moveable chelal finger of male G. megalodontum sp. nov., displaying anterior venom tooth and thick, spine-setae, retrolateral view; D. Same, prolateral view.