A survey of the Sumatran Ctenidae (Araneae). 1. Two new Acantheis species Author Fomichev, Alexander A. Altai State University, Lenina Pr., 61, Barnaul, RF- 656049, Russia & Tomsk State University, Lenina Pr., 36, Tomsk, RF- 634050, Russia Author Omelko, Mikhail M. Federal Scientific Center of East Asia Terrestrial Biodiversity, Far Eastern Branch, Russian Academy of Sciences, Vladivostok 690022, Russia Author Marusik, Yuri M. Altai State University, Lenina Pr., 61, Barnaul, RF- 656049, Russia & Department of Zoology & Entomology, University of the Free State, Bloemfontein 9300, South Africa & Institute for Biological Problems of the North, Magadan 685000, Russia text Zootaxa 2023 2023-10-09 5353 2 117 130 http://dx.doi.org/10.11646/zootaxa.5353.2.2 journal article 10.11646/zootaxa.5353.2.2 1175-5326 8427062 7A24DCBF-BC93-4B25-BB8A-DEAE75167DB4 Genus Acantheis Thorell, 1891 Acantheis Thorell, 1891: 61 . Simon 1897: 118 . Type species Acanthoctenus variatus Thorell, 1890 , from Nias Island . Note. The genus Acantheis was described in a brief footnote, with the three species being assigned to it ( Thorell, 1891 ): viz., A. variatus , A. dimidiatus and A. laetus . The type species was not indicated in that paper. Simon (1897) was the first who considered the genus and selected the generotype ( A. variatus ), although it was described from a juvenile specimen (probably lost according to Lehtinen [1967] ), while two others were known from the males. It is worth noticing that Simon (1897) considered the genus in Acantheae, which later were called the subfamily Acantheinae by Simon (1897) . Initially, we identified our specimens as Acantheis based on similarity of their male palps with that of A. laetus , which was depicted by Lehtinen (1967) . The following features, highlighted by Simon (1897) , support their belonging to this genus: high clypeus, long palpal tibia in the males (length/width ratio 4+) and 8–9 pairs of ventral spines on tibiae I (cf. Simon 1897 : figs 109–111). Our specimens from Sumatra possess all these characters. Acantheis nipponicus from the heavily isolated South Iwo Jima Island has the cymbium with a shortened tip and a slightly modified proximal part and the embolus originating from the proximal part of bulb, which casts doubt on the accuracy of the generic assignment of this geographically isolated species. It could belong to another, yet undescribed genus. Africactenus unumus Sankaran & Sebastian, 2018 , described based on males from the south of India ( Sankaran & Sebastian 2018 ), is another species with doubtful placement. This species possesses several features which we have identified as distinctive for Acantheis , namely: 1) presence of clusters of stiff setae on the dorso-posterior part of opisthosoma; 2) elongated cymbial tip as long as the bulb; 3) the bifurcated embolic tip is divided into dorsal and ventral branches pointing in the same direction (cf. Sankaran & Sebastian 2018 : figs 1A, 3). Due to this there is a possibility that Africactenus unumus actually belongs to Acantheis . However, Africactenus unumus has features not typical for Acantheis (except A. nipponicus ): modified cymbium with lamella-like outgrowth retro-proximally (vs. cymbium with no modifications) and straight and thick embolus ( vs . thin and strongly curved). So, we refrain from transferring of this species as long as Acantheis remains unrevised. A series of photographs of four Acantheis morphospecies, which are most probably new to science, were published in the photographic field guide ( Koh & Bay 2019 ). This account, as well as our findings, suggests that the real species diversity of Acantheis in South-East Asia is much higher than the currently known. We are confident that a large number of Acantheis species will be discovered and described in the near future. Diagnosis. Acantheis differs from all the ctenid genera known from South-East Asia ( Amauropelma Raven, Stumkat et Gray, 2001 ; Anahita Karsch, 1879 ; Bowie Jäger, 2022 and Sinoctenus Marusik, Zhang et Omelko, 2012 ) by the presence of clusters of stiff setae ( CS ) on the dorso-posterior part of opisthosoma ( Fig. 7 ). Males of Acantheis differ in having an elongated cymbial tip, which is as long as the bulb ( vs . the cymbial tip significantly shorter than the bulb, or even absent, cf. Figs 14, 19 and Polotow & Brescovit 2014 : fig. 6b; Jäger 2012 : fig. 32; Jäger 2022 : fig. 338; Marusik et al. 2012 : figs 9–11), and the bifurcated embolic tip, which is divided into dorsal and ventral branches pointing in the same direction ( Figs 30, 31 ; vs. undivided apical part of embolus or divided into prolateral and retrolateral branches). Additionally, Acantheis males can be distinguished from all south-east Asian ctenids but Anahita by the proximal part of the cymbium with no modifications (vs. the presence of prolateral cymbial bulge or retro-proximal cymbial outgrowth). Males of Acantheis can be separated from those of Anahita by the embolus starting from the prolateral side of the bulb ( vs . the retrolateral or posterior side, cf. Figs 23, 27 and Jäger 2012 : fig. 32). Females of Acantheis differ from those of other Oriental ctenid genera, except Bowie , in having the receptacles subdivided into two chambers ( vs . undivided, cf. Fig. 34 and Polotow & Brescovit 2014 : fig. 15b; Jäger 2012 : fig. 26). The endogynes of Bowie are polymorphic, representing both subdivided and undivided receptacles (cf. Jäger 2022 : figs 6, 529, 532). However, the Bowie species with subdivided receptacles always have the receptacle chambers of different sizes: one could be 2–4 times (or even more) larger than the second one. In Acantheis , such receptacle chambers are about equal in size.