Taxonomy of Arabian Temnothorax Mayr (Formicidae: Myrmicinae) with description of a new species enhanced by x-ray microtomography

Temnothorax elmenshawyi sp. n., a new ant species from the Asir Mountains of the southwestern region of the Kingdom of Saudi Arabia, is described based on the worker caste. The new species is a member of the T. exilis species group and is distinguished from the other species included in this group by the impressed metanotal groove, the short, acute and broadly-based propodeal spines, the finely punctate posterior half of cephalic surface, and absence of a median clypeal carina. Despite extensive collecting by the authors at the type locality, only two specimens are available for description, suggesting that this species may be rare and likely endemic to the Asir Mountains. The species description is complemented by still images of volume renderings of a 3D model and a 3D rotation video of the holotype based on x-ray microtomography (micro-CT), allowing remote in-depth examination of the specimen. The virtual micro-CT data is provided as cybertype dataset and freely available online 10.5061/dryad.4gg39k6, as well as 3D surface model (Sketchfab.com, https://skfb.ly/6HYRz). An updated identification key to the Arabian species is presented.

The ant genus Temnothorax Mayr 1 is one of the largest genera in the Family Formicidae with 450 valid species and subspecies known worldwide from all zoogeographic regions [2][3][4][5] . Species of this genus inhabit a wide range of habitats from deserts to rainforests 6 . The taxonomic history of the genus is relatively long and complex including the synonymy with Leptothorax Mayr, 1855 2,7 and subsequent removal from synonymy [8][9][10] . The complete nomenclatorial history of the genus is available in Bolton 3 . Recently, Prebus 5 reviewed the evolution, biography, and natural history of the genus postulating a Nearctic origin during the Eocene-Oligocene with a shift to arboreal nesting habits during the Oligocene.
The genus Temnothorax is diagnosed for the Arabian Peninsula by the following characters for the worker caste 11 : Antennae 12-segmented with a 3-segmented club; mandibles armed with five teeth; palp formula 5, 3; clypeus broadly inserted posteriorly between frontal lobes; anterior clypeal margin convex in full-face view, not forming an apron over mandibular surface in profile; promesonotal suture absent; propodeum bidentate. Members of the T. exilis species group can be characterized by the following characters 12 : colour usually dark brown or black; promesonotal suture distinct; propodeal spines short and acute; petiolar node in profile triangular; body sculpture feeble.
The field of insect taxonomy has advanced at great pace within the last decades through the implementation of new tools and methods, such as DNA barcoding (e.g. 13,14 ), molecular phylogenetics (e.g. 15,16 ), morphometrics 17 , or integrative approaches combining different lines of evidence 18 . Recently, interactive and three-dimensional (3D) imagery, such as x-ray microtomography (micro-CT), is gaining popularity and momentum within arthropod systematics. Micro-CT is a cutting-edge imaging technology that generates high-resolution, virtual, and interactive 3D reconstructions of whole specimens or particular body parts, thus allowing a maximum of morphological accuracy and fidelity [19][20][21] . Furthermore, a crucial benefit of applying micro-CT for insect taxonomy is the use of openly available cybertype datasets linked to the original, physical type material 19,22,23 .
Sharaf et al. 24 provide the sole treatment of Temnothorax for the Arabian Peninsula and recognize and key out three species: T. arabicus Sharaf & Akbar, 2017, T. liviae (Agosti & Collingwood, 2011), and T. megalops (Hamann & Klemm, 1967). Sharaf et al. 24 described T. arabicus from the Asir Mountains, Kingdom of Saudi Arabia (KSA), based on the worker caste and reviewed the available regional records for the genus. In the present work, another new species of the Temnothorax is described from the Asir Mountains (KSA) based on the worker caste.

Results
Synoptic list of Arabian Temnothorax species. Cybertype. Volumetric raw data (in DICOM format), 3D rotation video (in.mp4 format, T.elmenshawyi_ CASENT0790240_video.mp4, see Suppl. material XXX), still images of surface volume rendering, and 3D surface (in PLY format) of the physical paratype (CASENT0790240) in addition to montage photos illustrating head in full-face view, profile and dorsal views of the body. The data is deposited at Dryad 23 (https://doi.org/10.5061/dryad.4gg39k6) and can be freely accessed as virtual representation of the type. In addition to the cybertype data at Dryad, we also provide a freely accessible 3D surface model of the holotype at Sketchfab (https://skfb.ly/6HYRz).
Diagnosis. Temnothorax elmenshawyi can be distinguished from other members of the species group by the impressed metanotal groove, the short, acute and broadly based propodeal spines, the finely punctate posterior half of cephalic surface, and the absence of median clypeal carina.
Description. Head. In full-face view distinctly longer than broad with nearly straight posterior margin, rounded corners and feebly convex sides; anterior clypeal margin entire and convex; frontal carinae short and distinctly failing to reach anterior margin of eyes in full-face view; mandibles armed with five teeth decreasing in size from apex to base; antennae 12-segmented; scape relatively short (SI 60-73) clearly not reaching posterior margin of head by about length of second funicular segment in full-face view; eyes moderately large (EL 0.29-0.32 × HW, OI 30-32) with about 16 ommatidia in the longest row. Mesosoma. Promesonotal suture indistinct; promesonotum flat in profile; metanotal grove distinct; propodeal spines short, acute and broadly based (PSLI 20-29). Petiole. In profile without a peduncle; the anterior face forms a shallow concavity anteriorly; anterior face of petiolar node forms a right angle with posterior face; subpetiolar process reduced to a small denticle. Postpetiole. In profile globular (LPpI 75-100) and relatively lower than the height of the petiole; in dorsal view trapezoidal broadest anteriorly, 1.2-1.3 broader than long (DPpI 129-133). Sculpture. Mandibles longitudinally rugulose; clypeus and cephalic surface behind posterior levels of eyes to posterior margin of head mostly unsculptured medially and shiny, laterally with sparse punctate ground sculpture; cephalic surface starting from posterior margins of clypeus to posterior level of eyes faintly longitudinally irregularly rugulose; dorsal surface of mesosoma densely and finely punctate; lateral sides of mesosoma densely punctate; area between mesopleura and metapleura with distinct longitudinal rugae; promesonotum and mesonotum smooth in dorsal view; propodeum irregularly rugulose; petiole and postpetiole densely and finely punctate; gaster smooth and shining. Pilosity. Anterior clypeal margin with six protrusive setae, two short lateral and four central longer ones; clypeus and cephalic surface with appressed scattered pubescence; posterior margin of head with four pairs of erect setae; antennae with abundant appressed pubescence; promesonotum with seven pairs of blunt stiff, short erect setae; mesonotum and propodeum each with two pairs of setae; propodeal spines with one pair of setae; petiole with three pairs of longer setae directed posteriad; postpetiole with five pairs of setae; gaster with scattered blunt setae. Colour. Uniformly black, tibiae and tarsi pale brown.
Queens and Males: Unknown. Etymology. This new species is named in the honor of the late Egyptian Qur'an reader Mohamed Siddiq El- Menshawy (1920-1969).
Habitat. The type locality (Raydah) (Fig. 3) is located in the Asir Mountains, 10 km west of the city of Abha and with an estimated area of 9 km 2 16 . This area includes one of the last remnants of dense juniper forests (African pencil cedar, Juniperus procera Hochst. ex Endl. (Cupressaceae) remaining on the Arabian Peninsula.
Biology and Ecology. Nothing is known of the biology and ecology of the new species.

Discussion
This new species belongs to the T. exilis species group as defined by Cagniant & Espadaler 12 in the inventory of Moroccan Temnothorax, and will not key successfully to any of the Moroccan or African species 4 . It has to be pointed out though, that the T. exilis species group is not a monophyletic group based on a recent global phylogeny of the genus 5 , which was already suspected by Cagniant & Espadaler 12 . Without any other existing higher-level classification above species level available for the genus, we prefer to place T. elmenshawyi in the T. exilis group, at least temporarily. Temnothorax elmenshawyi superficially resembles T. exilis (Emery) from Italy but can be readily distinguished by the deeply impressed metanotal groove, the finely punctate posterior half of the cephalic surface, and absence of median clypeal carina. Workers of T. exilis have an indistinct metanotal groove, acute propodeal spines, a reticulate-rugulose posterior half of cephalic surface, and a median clypeal carina. With regards to the Arabian Temnothorax fauna, T. elmenshawyi cannot be confused with any of the Arabian species. It can be immediately www.nature.com/scientificreports www.nature.com/scientificreports/ separated by the uniformly very dark brown to blackish colour (vs. yellowish in the other three species), moderately large eyes (vs. extremely large eyes in T. liviae), and the mostly unsculptured posterior half of the head behind the level of the eyes (vs. conspicuously rugulose/rugose in the other three species).
More than 220 pitfall traps were placed in the Asir Mountains during two years of sampling and only two specimens of the new species were trapped. Undoubtedly, this may reflect the scarcity of this species and indicate a high probability of it being a regional endemic. Due to the extension of the Asir Mountains into Yemen, T. elmenshawyi may also occur in that country. It is hoped that future surveys of ants of this region will confirm this distribution, and result in the discovery of additional specimens, especially the male and queen castes.
As discussed in previous studies 19,21 , the use of micro-CT has great advantages for taxonomy. The non-destructive x-ray scan allows the generation of a virtual avatar or cybertype of the physical type material, providing a detailed and almost complete virtual reconstruction of the morphology, thus permitting a high degree of interactive