Chthonius C.L. Koch, 1843

Zaragoza, Juan A., 2017, Revision of the Ephippiochthonius complex in the Iberian Peninsula, Balearic Islands and Macaronesia, with proposed changes to the status of the Chthonius subgenera (Pseudoscorpiones, Chthoniidae), Zootaxa 4246 (1), pp. 1-221 : 20-21

Genus Chthonius C.L. Koch, 1843

Diagnosis. Chelal hand with fusiform shape, constricted between trichobothria ib / isb and base of fixed finger ( Fig. 8 ); bases of chelal fingers without sclerotized condylar complex; without medial protuberance (ip) between chelal condyles; hand without distoventral hollow; base of hand abruptly constricted paraxially; proximal portion of hand with 4 setae in adults and tritonymphs, seta ph3 present. Axis of fixed chelal finger apically forming a widely obtuse angle; some species from eastern and southeastern Europe with sigmoid fixed chelal finger; chelal fingers usually with more or less close-set teeth, distinctly reclined backwards, dental row of fixed finger without a modified subdistal tooth (mt) ( Fig. 9 ); tip of fixed finger without an antiaxial accessory tooth (td); tip of fixed chelal finger of male, tritonymph and deutonymph with neither a deep hollow on paraxial face nor a subapical protuberance (sp); base of movable chelal finger without an enlarged sclerotized condyle, internal proximal apodeme very poorly developed; coupled sensilla pc usually distad of trichobothrium sb, rarely between sb and b (e.g. C. dacnodes , C. tenuis ). All chelal lyrifissure groups present in standard numbers. Anterior margin of carapace more strongly dentate between median macrosetae, usually prominent and frequently with epistome. Coxae II and III with coxal spines; bisetose intercoxal tubercle present between coxae III and IV; distal marginal seta of pedipalpal coxa distinctly longer and thicker than that of coxa I; in addition to the stigmatal setae, sternite III in males and females usually with 10 posterior setae, of which the lateral one on each side is reduced in size (microseta). Male genitalia without a median hiatus between setae of each row of guard-setae.

Type species. Obisium orthodactylum Leach, 1817 .

Distribution. Widespread in Europe, but also present in Macaronesia, North Africa, from Turkey to Central Asia, and U.S.A. (Harvey 2013).

Remarks. The genus Chthonius C.L. Koch, 1843 previously comprised four subgenera: Chthonius s. st., Ephippiochthonius , Globochthonius and Hesperochthonius . Hesperochthonius is easily distinguishable by the presence of a median hiatus dividing each row of the male genital guard-setae into two groups of two setae. The other three subgenera have mainly been distinguished in the literature by a different shape of the chela, the chelal dentition and the presence/absence of a long apodeme at the base of the movable chelal finger.

The arrangement of the coxal spines on coxae II and III and the presence of an intercoxal tubercle have apparently been the reasons for maintaining these subgenera within the genus Chthonius . During this study some new morphological characteristics have been considered and have led to the conclusion that the current known genus Chthonius represents a complex of different genera, as discussed below.

Ephippiochthonius was created by Beier (1930) as subgenus of Chthonius . In the original description, the characteristics used to distinguish it from Chthonius s. str., were the dorsal depression of the chelal hand distad of trichobothria ib and isb, the strong sclerotization of the base of movable chelal finger, and the chelal fingers with upright and well-spaced teeth. Ephippiochthonius also differs from Chthonius s. str. in the presence of an accessory tooth (td) on antiaxial face at the tip of fixed chelal finger, an enlarged sclerotized condyle at the base of the movable chelal finger and its large proximal apodeme. The presence of a well developed apodeme seems constant within genera and is considered a good characteristic for distinguishing closely related genera, as in the case of the tyrannochthoniines Lagynochthonius and Tyrannochthonius . The presence of the following characteristics distinguishes Ephippiochthonius s. st. from other Chthonius -related genera: 3 proximal setae on the chelal hand, lacking ph3 (versus 4 in the other genera); tip of fixed chelal finger of male, tritonymph and deutonymph with a deep hollow on paraxial face and subapical protuberance (sp) (absent in the other genera, except for Spelyngochthonius and, in part, for Globochthonius ); presence of a prominent medial bow-like protuberance (ip) between chelal condyles (except for Cantabrochthonius ). Apart from those concerning external morphology, few studies have been carried out on the subgenera, but the results are significant: Weygoldt (1968) found important differences in the development of the protonymph between Chthonius (C.) ischnocheles and C. (E.) tetrachelatus ; Legg (1975) described differences in the internal genital structures of Chthonius s. st. and Ephippiochthonius ; Šťáhlavský & Král (2004), in their karyological analysis of Chthoniidae , observed a more pronounced tendency in Ephippiochthonius towards reduced chromosome numbers with respect to Chthonius s. st.; Murienne et al. (2008: 174) obtained molecular sequences for one species of Ephippiochthonius and another of Chthonius s. st. in their phylogenetic analysis of pseudoscorpion families, their results reveal a high level of divergence (21.3% COX1 pdistance, based on an analysis of the GenBank sequences), consistent with their belonging to separate genera (Carles Ribera, in litt.). Taken together, the above characteristics differentiating Ephippiochthonius from the other Chthonius -related genera are important enough to support the elevation of Ephippiochthonius to full generic rank, as proposed herein.

Figures

FIGURES 8 – 9. Chthonius orthodactylus (auct.), female from Blackmoor Copse, Wiltshire, England, left chela. 8, antiaxial view; 9, dorsal view. See Material and methods for abbreviations.