Key to species of Butheolus and Xenobuthus gen. n. examined in this study (based on adults)

1 Small to medium sized scorpions, 35–50 mm long; metasomal segments sparsely granular on lateral and ventral sides; basal lobe of hemispermatophore a broad, shallow, curved scoop; pedipalp fingers with 8–10 subrows of denticles; males with macrosetae similar in length to those of females; soles of telotarsi I–III often with single row of setae ..….... Xenobuthus gen. n. .... 5

– Small scorpions, <36 mm long; metasomal segments densely granular on lateral and ventral sides; basal lobe of hemispermatophore a small, narrow, hook-like process; pedipalp fingers typically with 6–7 subrows of denticles; males with shorter macrosetae than females; soles of telotarsi I–III bearing two rows of setae .........…….......................... Butheolus Simon, 188 2 ..... 2

2 Pedipalp patella L/W <2.3, chela manus strongly carinate; telson with large subaculear tubercle, posterior vesicle slope forming acute angle with aculeus base ...………………...... Butheolus thalassinus Simon, 1882

– Pedipalp patella L/W> 2.3, chela manus smooth or with very weak carinae; telson with subaculear tubercle small or indistinct, posterior vesicle slope forming obtuse angle with aculeus base ...……………………...... 3

3 Metasoma and telson densely pilose, with abundant macrosetae; basitarsus III with 11–12 retrosuperior setae (bristle-comb) .................... B. villosus Hendrixson, 2006

– Metasoma and telson with sparse setation; basitarsus III with 4–10 retrosuperior setae (bristle-comb) …. 4

4 Carapace and tergites coarsely granulated; color of metasoma I–II yellow to light brown; pedipalp patella with complete dorsomedian carina .... B. harrisoni sp. n.

– Carapace and tergites finely granulated; color of metasoma I–II dark brown; pedipalp patella with dorsomedian carina restricted to distal half of segment ...........…………….............. B. gallagheri Vachon, 1980

5 Carapace, mesosoma and metasoma I–III dark brown to black; metasoma IV with 10 carinae, lateral median carinae distinct; ventromedian carina of metasoma V bifurcate; metasoma V in males with ventral intercarinal surface granulated posteriorly ...........…........ ………..……… Xenobuthus anthracinus (Pocock, 1 985)

– Carapace, mesosoma and metasoma I–III bright yellow; metasoma IV with 8 carinae, lateral median carinae obsolete; ventromedian carina of metasoma V not bifurcate; metasoma V in males with ventral intercarinal surface smooth posteriorly .... Xenobuthus xanthus sp. n.

The above key omits two species, Butheolus arabicus Lourenço & Qi, 2006, and B. hallani Lourenço & Rossi, 2017 (= B. pallidus Lourenço & Duhem, 2012), because the author was not able to loan and examine the types of these two species.

From the published description, and the habitus shown in images on the MNHN website, B. arabicus does not fit the diagnosis of Butheolus given here. Instead, it complies with certain characters of Xenobuthus gen. n.: i.e., 1) large size and general proportion and shape of body and appendages resemble X. anthracinus and X. xanthus; 2) 8–9 denticle subrows on pedipalp chela fingers. It is hereby provisionally transferred to that genus: Butheolus arabicus Lourenço & Qi, 2006 = Xenobuthus arabicus (Lourenço & Qi, 2006) comb. n. .

The type locality “East of Khamis Mushayt” suggests the possibility that X. arabicus is a junior synonym of X. anthracinus, since the latter is also known from Khamis Mushayt (by the AMNH female, examined here and by Hendrixson, 20 06). Lourenço & Qi (2006) distinguished X. arabicus from X. anthracinus as follows:

1) “much larger overall size”, i.e. paratype female 40.4 mm vs. 25.3 mm length of a female X. anthracinus (cf. their Tab. 1, cited as a ‘paratype’): this character is not applicable because: (i) adult female X. anthracinus can be up to 40–5 0 mm long (Figs. 1 98–199, Tab. 7), and the AMNH female is ca. 35 mm long (Hendrixson, 2006). The female studied by Lourenço & Qi (2006) is the same size as the lectotype female (♀ 25 mm, photograph examined) of X. anthracinus in ZMUH designated by Kovařík (2004), and is thus likely to be immature or juvenile. It is not a paratype, but either a paralectotype, or possibly the lectotype itself. (ii) the holotype male of X. arabicus (total L 35.6 mm, carapace L 4.7 mm) is about the same size as the holotype male of X. anthracinus (total L 36.5 mm, carapace L 4.4 mm, cf. Pocock, 1895: 295).

2) “strongly marked” (= strongly developed) carinae on tergite VII and metasoma, and granulation of carapace tergites and coxapophysis: this character may not be applicable because adult X. anthracinus also have strong carination on tergite VII and metasoma, and strong granulation on carapace, tergites and coxal endites I–II (= coxapophyses) (Figs. 194–201, 252–257). The carination and granulation may not be fully developed in the probably immature or juvenile female used for comparison by Lourenço & Qi (2006).

3) “metasomal segments I to III with 10 carinae in both sexes”: this character is not applicable because adult X. anthracinus can also have 10 carinae on metasoma I–III in both sexes (Figs. 252–257). Either this character is variable, or the carination is not fully developed in the probably immature or juvenile female used for comparison by Lourenço & Qi (2006).

Another potential difference is in coloration, as the base color of X. arabicus was described as “yellow to reddish-yellow”, consistent with color photographs of the holotype posted on the MNHN website, which contrasts with the dark reddish-brown to black color of X. anthracinus . However, biological pigments can suffer fading or wash out after long storage in alcohol, and coloration is not a very reliable character in old specimens that were not fixed by specific methods that preserve coloration, e.g. heat shock and aldehyde fixation (Williams, 1968). The problem was well demonstrated by the holotype of Butheolus gallagheri (also in MNHN), that was loaned and examined by the author in 1994, only 14 years after its description by Vachon (1980) as “brun chocolat” (= dark brown natural color, exhibited by live animals and by the aldehyde-fixed materials studied here). By 1994, the holotype had already faded to a pale yellow color on the carapace and tergites, and a pale orange brown color on the metasoma, and the base of the pedipalp fingers had lost the diagnostic melanic pigmentation that was noted by Vachon (1980). The holotype of X. arabicus was presumably stored much longer in alcohol (34 years) before it was described.

Although X. arabicus and X. anthracinus seem similar and were both collected in the vicinity of Khamis Mushayt, they could nevertheless be distinct species. In Dhofar, Oman, similar but distinct species of Butheolus and Xenobuthus exist in relatively close proximity. Lourenço & Qi (2006) noted that X. arabicus only bears 8 carinae on metasoma IV, which is a diagnostic character separating X. xanthus from X. anthracinus . The question might be addressed by restudying the types of X. arabicus, or by further sampling the Xenobuthus populations in the region of Khamis Mushayt.

The taxonomic status of B. hallani, based on a single holotype male from the United Arab Emirates or adjacent Oman, is also uncertain. It is not clear from the published description, and the general habitus shown in images posted on the MNHN website (reproduced in Lourenço & Rossi, 2017), whether it fits the diagnosis of Butheolus given here. In their description, Lourenço & Duhem (2012) noted:

1) “posterior margin of sternites III to VI without denticulations”: however, such denticulations are proposed to be diagnostic for both Butheolus and Xenobuthus (Figs. 332–335), and were present and in all species examined here. Although the sternite denticulations are weaker and may be overlooked in females, they are conspicuous in males, and the holotype of B. hallani is male.

2) metasoma V with “lateroventral carinae with a few slightly spinoid granules in the distal region”: their Fig. 4 depicts an irregularly dentate posterior carina with two enlarged denticles that are considerably larger than the anterior dentition, a pattern that differs from that seen in Butheolus and Xenobuthus, which have numerous regular denticles gradually increasing in size posteriorly.

These characters, together with the small body size (18 mm), and presence of only 5–6 subrows of denticles on the pedipalp fingers, are more suggestive of Neobuthus, than Butheolus (Kovařík & Lowe, 2012; Lourenço, 2001, 2005; Lourenço & Qi, 2006; Lowe & Kovařík, 2 016). Other key taxonomic characters that could be relevant to its generic affiliation (e.g. position of trichobothrium V 2 on pedipalp manus, number of denticles on ventral aspect of cheliceral fixed finger) have yet to be specified. In any case, the presence of either Butheolus or Neobuthus in the southeastern corner of the Arabian Peninsula would be a significant range extension for either genus.