Ants of the Monomorium monomorium species-group (Hymenoptera: Formicidae) in the Arabian Peninsula with description of a new species from southwestern Saudi Arabia

We revise the taxonomy of the myrmicine ants of the Monomorium monomorium species-group for the Arabian Peninsula. Six species are recognized: Monomorium aeyade Collingwood & Agosti, 1996, M. clavicorne André, 1881, M. exiguum Forel, 1894, M. holothir Bolton, 1987, M. mohammedi sp. n., and M. sarawatense Sharaf & Aldawood, 2013. On the basis of the worker caste, we describe Monomorium mohammedi sp. n. from the southwestern region of the Kingdom of Saudi Arabia (KSA). We designate a neotype for Monomorium aeyade Collingwood & Agosti and redescribe and illustrate the worker caste. Furthermore, we provide a worker-based species identification key, distribution maps for the treated species, and ecological and biological notes, if available. Monomorium holothir is recorded for the first time from the KSA. Also, we propose M. clavicorne var. punica Santschi, 1915a as a junior synonym of M. clavicorne, as well as M. dryhimi Aldawood & Sharaf, 2011 and M. montanum Collingwood & Agosti, 1996 to be treated as junior synonyms of Monomorium exiguum.


INTRODUCTION
The ant genus Monomorium is one of the most diverse genera in the subfamily Myrmicinae, with 388 described species and subspecies (Bolton, 2017). It is distributed worldwide throughout all zoogeographic regions, with most species occurring in the Old World tropics and temperate zones (Brown, 2000). Considering how diverse and widespread the genus is, very little information on the natural history of most species exists, especially for the M. monomorium species-group (Bolton, 1987). Apparently, most species inhabit the topsoil layer or leaf litter and seem to have a rather generalist diet. The taxonomic foundation for the genus as a whole is in a moderate state based on some regional revisions (e.g., Bolton, 1987;Heterick, 2001;Heterick, 2006), as well as faunistic treatments providing local or regional keys (e.g., Collingwood & Agosti, 1996;Terayama, 2009;Sarnat & Economo, 2012). The available revisionary contributions on the fauna of the M. monomorium species-group are scarce, and basically consist of Bolton (1987) for the Afrotropical fauna and Heterick (2001) and Heterick (2006) for the Australian and Malagasy faunas respectively. However, these include many species now removed from the genus.
The taxonomic history of the M. monomorium species-group in the Arabian Peninsula is a set of single distribution records and single species descriptions scattered through the literature. The first published work on the Arabian Monomorium fauna (Collingwood, 1985) recorded a couple of species of the M. monomorium species-group from the Kingdom of Saudi Arabia (KSA), namely M. clavicorne André, 1881and M. montanum Collingwood & Agosti, 1996(initially misidentified as M. zulu Santschi, 1914in Collingwood, 1985. In a later faunistic contribution Collingwood & Agosti (1996) listed eight species, and described five new species, M. aeyade Collingwood & Agosti, 1996, M. baushare Collingwood & Agosti, 1996, M. desertorum Collingwood & Agosti, 1996, M. montanum Collingwood & Agosti, 1996, and M. qarahe Collingwood & Agosti, 1996 The first record of the species M. exiguum Forel was published by Aldawood & Sharaf (2009) from the Asir Mountains (KSA).  described M. dryhimi, based on the worker caste from the southwestern Mountains of the KSA, and provided a key to the Arabian species. El-Hawagryi et al. (2013) described M. sarawatensis Sharaf & Aldawood from the Al Baha Province based on the worker caste, and provided a key to the Arabian species of the M. monomorium species-group. Recently, M. desertorum Collingwood & Agosti, 1996 was synonymized with M. exiguum Forel, 1894(Sharaf et al., 2015, and Sharaf et al. (2017) produced a key to the Monomorium fauna of the Socotra Archipelago and described M. elghazalyi Sharaf & Aldawood, 2017, and synonymized M. baushare and M. qarahe under M. exiguum. In the Malagasy region, M. exiguum is the most abundant species in leaf litter samples (Heterick, 2006). However, this group of ants is taxonomically difficult due to lack of revisionary work, in addition to the small body size and pale body colors that make the ants frequently overlooked by collectors and consequently poorly represented in the regional museums and collections. Ants of the M. monomorium-group are among the most abundant Monomorium in the Arabian Peninsula, commonly represented in ecological and biodiversity research projects of the region.
In this study, we provide the first comprehensive revision of the M. monomorium speciesgroup for the Arabian Peninsula. We describe one new species from the southwestern region of the KSA and re-describe the five previously known species. For all species we present detailed descriptions, diagnoses, high-quality montage images, and distribution maps. Furthermore, we provide a new illustrated identification key to the members of the species group on the basis of the worker caste.

MATERIAL AND METHODS
The species names follow the online catalogue of ants of the world (Bolton, 2017). Distribution maps were made using DIVA-GIS (version 7.5.0.0). Digital color images of lateral and dorsal views of the entire body and full-face views of the head of each species were created using a Leica DFC450 digital camera with a Leica Z16 APO microscope and LAS (v3.8) software. These images are also available online on AntWeb (http://www.AntWeb.org) and are accessible through unique specimen identifiers attached to each pin (e.g., CASENT0922329). Throughout the text, 'w' stands for 'worker' or 'workers'. Morphometric measurements are shown in Figs. 1-3.

Nomenclatural acts
The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:58913E59-AEE4-43BD-881E-C23CD61F44F9. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

Diagnosis of Arabian ants in the Monomorium monomorium species-group
Within the genus Monomorium, workers of the M. monomorium species-group can be easily recognized by the following combination of characters (Bolton, 1987): monomorphic, with size variation; median clypeal portion raised, projecting anteriorly and longitudinally bicarinate; anterior clypeal margin without a pair of teeth; dorsal surface of mandibles unsculptured and masticatory margin armed with four teeth, decreasing in size from apex to base; antennae with 10-12 segments, terminating in a well-defined three-segmented club; eyes present but variable in size, situated in front of the midlength of the sides in full-face view, and with four or more ommatidia in the longest row; head longer than broad; cephalic dorsum smooth and shining; metanotal groove impressed, with distinct cross-ribs; propodeal spiracle circular to subcircular; propodeal dorsum meeting declivity in a rounded angle; promesonotum and propodeal dorsum unsculptured; body pilosity variable in distribution but usually conspicuous, rarely absent from mesosomal dorsum; petiole, postpetiole and gastral tergites usually unsculptured.

Synoptic species list of Arabian ants in the Monomorium monomorium species-group
Monomorium aeyade Collingwood & Agosti, 1996 Monomorium clavicorne André, 1881 = Monomorium clavicorne punicum Santschi, 1915a; syn. nov. Monomorium exiguum Forel, 1894 = Monomorium exiguum var. bulawayensis Forel, 1913b = Monomorium faurei Santschi, 1915b = Monomorium exiguum r. flavescens Forel, 1916 = Monomorium montanum Collingwood & Agosti, 1996 syn. nov. = Monomorium dryhimi  syn. nov. Monomorium holothir Bolton, 1987 Monomorium mohammedi Sharaf & Hita Garcia sp. n. Monomorium sarawatense  Identification key to the Arabian species of the Monomorium monomorium-group Mesosoma. In profile with flat promesonotal dorsum, which slopes posteriorly to a welldefined metanotal groove; propodeal spiracles small and pinhole-like; propodeal dorsum evenly sloping posteriorly to short declivity. Petiole. Node massive, rounded dorsally, and little higher than postpetiolar node in profile; anterior peduncle short. Postpetiole. Node low and convex dorsally. Sculpture. Entire body surfaces smooth and shining except for  Note This species was originally described based on two worker specimens, the holotype and one paratype. During an extensive search in the WMLC collection it was not possible to locate the holotype, which is presumably lost. However, the paratype specimen was available for examination. The original description given by Collingwood & Agosti (1996) was brief and the diagnostic differentiation was unclear, therefore, we designate a neotype to unequivocally ascertain the identity of the species and re-describe the species.

Biological and ecological notes
Nothing is known of the biology or ecology of the species.

Geographic range
Monomorium aeyade is only known from the type locality in Oman (Table 1).    (Collingwood, 1985). In full-face view distinctly longer than broad with nearly parallel sides and feebly concave posterior margin; median clypeal portion without carina or anterolateral angles, anterior clypeal margin feebly concave; antenna 11-segmented; terminal funicular segment enlarged, more than twice longer than the two preceding segments; scapes long (SI 79-86); eyes oval, small, (EL 0.17-0.21 × HW) with a ring of ommatidia encircling two inner short rows of 2-3 ommatidia; frontal lobes farther apart in full-face view. Mesosoma. In profile with a feebly convex promesonotal dorsum, which slopes posteriorly to a well-defined metanotal groove; propodeal spiracles small and pinhole-like; propodeal dorsum evenly sloping posteriorly to short declivity. Petiole. Node massive, narrowly rounded above, and slightly higher than postpetiolar node in profile, anterior peduncle short. Postpetiole. Node small and convex dorsally. Sculpture. Cephalic surface smooth and shining; mandibles smooth and shining, with faint striations; mesosoma, petiole, postpetiole, and gaster smooth and shining; metanotal cross ribs distinct. Pilosity. Cephalic surface with scattered minute hair-pits; anterior clypeal margin and mandibles with longer hairs; antennae with abundant appressed pubescence; pronotal angles with a pair of long hairs; propodeal dorsum with one pair of hairs; petiole and postpetiole each with one pair of backward directed hairs; gaster with few longer hairs on distal half. Color. Overall uniform clear yellow.

Biological and Ecological notes
Little is known of the biology of the species. The single specimen available was found in a cultivated area with a sewage water stream. It was coexisting with Tapinoma simrothi Krausse, 1911, Trichomyrmex mayri (Forel, 1902), and Tetramorium caespitum (Linnaeus, 1758). Due to the relatively broad regional distribution of the species it might be an introduction to the KSA.

Geographic range
This species was described from Palestine (André, 1881) and recorded from many countries in the Middle East including Iran, Lebanon, Sudan, Syria, Turkey, the KSA (Collingwood, 1985;Collingwood & Agosti, 1996), the UAE (Collingwood et al., 2011), and several North African countries including Egypt (Sharaf, 2006), Morocco and Tunisia (Table 1).     In full-face view distinctly longer than broad with nearly parallel sides and feebly concave or straight posterior margin; clypeal carinae feebly developed, broadly separated and clearly divergent anteriorly; anterior clypeal margin feebly concave, without anterior sharp angles; antennae 11-segmented; scapes, when laid back straight from their insertions, failing to reach posterior margin of head; eye size variable (EL 0.19-0.22 × HW), in profile consisting of an outer ring of ommatidia encloses one or two longitudinal rows of 2-4 ommatidia; in few specimens one or two ommatidia within the ring present; eyes distinctly situated in front of midlength of head sides in full-face view. Mesosoma. Promesonotum feebly convex in profile; metanotal groove distinctly impressed, with short cross-ribs; propodeal dorsum and declivity meeting in a rounded convexity. Petiole. Petiolar peduncle short, with a small anteroventral process in profile; petiolar node low-subconical in profile. Postpetiole. Postpetiole smaller than petiole, lower and broadly convex dorsally in profile. Sculpture. Entire body smooth and shining, except for metanotal cross-ribs on sides of metanotal groove. Pilosity. All body surfaces with standing hairs, pronotum with a single pair on anterior margin between humeral pair; promesonotum usually with four pairs of hairs but in many specimens a fifth pair present; propodeum without hairs or with one or two pairs of hairs. Color. Variable, from uniform yellow to uniform dark brown, frequently with a pair of brown patches or a darker band apically on first gastral tergite. Note  described M. dryhimi from Al Baha Province (KSA) based on the worker caste. Comparing the type material of M. dryhimi and M. montanum with M. exiguum revealed that they have similar body size and color, in addition to the possession of the following characters: eyes of moderate size, with an outer ring of ommatidia encloses one or two longitudinal rows of 2-4 ommatidia; metanotal groove impressed, with distinct cross-ribs; all body surfaces with standing hairs, pronotum with a single pair on anterior margin between humeral pair; posterior half of first gastral tergite brown; body smooth and shining. Herein, we treat M. dryhimi and M. montanum as junior synonyms of M. exiguum.

Biological and ecological notes
This species is by far the most common Arabian species of the M. monomorium-group, and it appears to be very flexible in its ecological requirements since it occurs in numerous habitats throughout the Arabian Peninsula. It was found living in humid soil, leaf litter, under rocks, and under bark. Once it was even collected from inside galleries of a Camponotus sp. colony. Furthermore, M. exiguum was found in a variety of agricultural landscapes and human settlements, very often in close proximity to trees or other vegetation.

Geographic range
Even though this species was originally described from Ethiopia (Forel, 1894), M. exiguum is extremely widespread (Table 1) since it occurs in much of the Afrotropical Region (Bolton, 1987;Sharaf et al., 2017), the Malagasy Region (Heterick, 2006), and the Mediterranean Basin (Gòmez & Espadaler, 2006;Sharaf, 2006). As mentioned above, this species is also broadly distributed throughout the Arabian Peninsula, and is considered as the commonest species of the M. monomorium-group in the region. The first record from the Arabian Peninsula was from the KSA (Aldawood & Sharaf, 2009) while later records from the UAE, Oman, and Yemen (Collingwood et al., 2011) were published under the name M. baushare (Collingwood & Agosti, 1996) that was recently synonymized under M. exiguum (Sharaf et al., 2017). Fig. 8 Monomorium holothir Bolton, 1987: 393 (w.) Type material examined Holotype, pinned worker, KENYA: Lake Baringo, 1.xii.1983 (L. Darlington) (BMNH: CASENT0902243). Paratype, pinned worker with same data as holotype (BMNH). In full-face view distinctly longer than broad with feebly convex sides behind eyes and nearly straight or feebly concave posterior margin; clypeal carinae sharply developed; anterior clypeal margin feebly concave; antennae 12-segmented; scapes failing to reach posterior margin of head; eyes relatively large (EL 0.30 × HW), with 7-9 ommatidia in longest row; in profile eye length clearly greater than distance between anteriormost point of eyes and nearest point of mandibular insertion. Mesosoma. Promesonotum feebly convex in profile; metanotal groove distinctly impressed, with distinct cross-ribs; propodeal dorsum and declivity meeting in a rounded convexity; propodeal spiracle small and pinhole-like. Petiole. Petiolar node high, subconical and with narrowly rounded dorsum in profile. Sculpture. Entire body smooth and shining, except for metanotal cross-ribs on sides of metanotal groove. Pilosity. All body surfaces with abundant long-standing hairs; promesonotum with more than six pairs of hairs; propodeum with four pairs of hairs.

Biological and ecological notes
This species was collected from mangos, Mangifera sp. (Anacardiaceae) imported from Kenya and this provides the context for the present record. It was also found in leaf litter next to Calotropis procera (Aiton) W.T.Aiton (Asclepiadaceae). Some workers were found nesting in a thin layer of clay soil above sandy soil, while several workers were collected from leaf litter under a Conocarpus L. tree (Combretaceae).

Geographic range
Monomorium holothir is a comparatively rare species originally described from Kenya and prior to this study only known from the type locality (Bolton, 1987;Hita Garcia, Wiesel & Fischer, 2013). Our collections represent a new species record for KSA, and it is highly likely an introduction to the country. Diagnosis. Monomorium mohammedi can be readily diagnosed by the combination of the following characters: eyes distinctly small, with 5-6 ommatidia; mesosoma, petiole, and postpetiole without standing hairs; mesopleuron, metapleuron, petiole and postpetiole finely shagreened. Description Worker. Head. In full-face view distinctly longer than broad with shallowly convex or nearly parallel sides and clearly concave posterior margin in full-face view; median clypeal portion without carina or anterolateral angles, anterior clypeal margin feebly concave; antenna 11-segmented; scapes short, when laid straight back, just surpassing midlength of head (SI 88-97); mandibles armed with three teeth, decreasing in size from apex to base; eyes oval, tiny, (EL 0.13-0.15 × HW) with 5 ommatidia, set in front of midlength of head; frontal lobes farther apart in full-face view; underside of head with six scattered short hairs. Mesosoma. In profile with a flat promesonotal dorsum, which slopes posteriorly to a well-defined metanotal groove; propodeal spiracles small and pinhole-like; propodeal dorsum evenly sloping posteriorly to short declivity. Petiole. Node massive, narrowly rounded above, and little higher than postpetiolar node in profile; anterior peduncle short; ventral petiolar surface below node broadly convex extending anteriorly to form a blunt broad dent. Postpetiole. Node small with convex dorsal margin; postpetiole as high as broad. Sculpture. Cephalic surface smooth and shining; mandibles smooth and shining, with faint striations on the outer margin; mesosoma dorsum and propleuron smooth and shining; meso-and metapleuron finely shagreened; metanotal cross ribs distinct; petiole and postpetiole with traces of superficially shagreened sculpture, but never smooth; gaster smooth and shining. Pilosity. Underside of head without hairs; cephalic surface with scattered minute hair-pits; anterior clypeal margin and mandibles with longer hairs; antennae with abundant appressed hairs; mesosoma without hairs, only rare appressed pubescence; petiole and postpetiole without hairs, only few appressed pubescence dorsally; gaster with scattered appressed pubescence, few longer hairs on the last gastral tergites.

Monomorium mohammedi
Color. Overall uniform clear yellow, mandibular teeth light brown.

Differential diagnosis
This new species is closest to M. guillarmodi Arnold, 1946 from Lesotho in terms of the small body size, tiny eyes, 11-segmented antennae, lack of hairs on mesosoma, and the smooth body. However, M. mohammedi is readily separated from M. guillarmodi by the following characters: the posterior margin of head without hairs and concave in full-face view, petiole and postpetiole without hairs, median clypeal portion without anterolateral angles or carina, scape relatively longer (SI 81-97), whereas M. guillarmodi has a transverse posterior head margin with 1-2 pairs of hairs, petiole and postpetiole each with a single pair of hairs, median clypeal portion prominent with well-defined anterolateral angles and distinct carina, and scape shorter (SI less than 80). Among the Arabian species of the M. monomorium-group four species have 11segmented antennae: M. mohammedi, M. clavicorne, M. aeyade, and M. exiguum. Monomorium mohammedi is easily separated from clavicorne by its smaller eyes, smaller terminal funicular segment, and lack of mesosomal pilosity, whereas clavicorne has larger eyes, greatly swollen terminal funicular segments, and abundant hairs on the mesosoma.
When comparing M. mohammedi with M. aeyade, both lack hairs on the mesosoma but M. mohammedi can be immediately separated by its smaller eyes with only 5 ommatidia that are situated distinctly farther apart from the mandibular insertions (EM 0.09-0.11), the finely shagreened meso-and metapleuron, the hairless petiole and postpetiole, whereas M. aeyade has larger eyes (EL 0.24 × HW), with a ring of ommatidia encircling a single row of 2 ommatidia that are situated closer to the mandibular insertions (EM 0.05); meso-and metapleuron smooth, and petiole and postpetiole each with one pair of hairs.

Biological and ecological notes
The new species was collected from leaf litter under a Hyphaene tree (Arecaceae) and another nest series was found in a thin layer of clay soil above sandy soil under a Mango tree.  (Collingwood, 1985;Collingwood & Agosti, 1996;Aldawood & Sharaf, 2009;El-Hawagryi et al., 2013;Sharaf et al., 2015;Sharaf et al., 2017). In this study, the number of species remains six due to some taxonomic amendments, additional records, and the description of M. mohammedi sp. n. Within the M. monomorium-group, M. exiguum is the most broadly distributed species (Fig. 11A) due to its ability to inhabit a broad range of habitats in the region. The species is widespread in the central and southwestern regions of the KSA, Oman and the UAE but it is highly likely that it has a similar broad geographical distribution in other unexplored areas of the Arabian Peninsula. In addition, the species has a wide distribution range in the Afrotropical region (Table 1). The recent synonymies of some Arabian species under M. exiguum (Sharaf et al., 2017), as well as the ones suggested in this study, reveal a remarkable size and color variation, which has led to erroneous species descriptions in the past. During the present work, it was observed that some nest series are uniform yellow, or with brown transverse bands on the first and second gastral tergites, whereas other series have brown heads and yellow-brown gasteral tergites. These variations were already noted by Bolton (1987) for the Afrotropical fauna. These morphological variations should be taken into consideration in future studies that will treat the M. monomorium-group in the region, especially those concerning descriptions of new species, in order to avoid possible synonymies. Considering the challenging morphological species delimitations, integrative approaches combining morphological and molecular techniques might provide a more comprehensive taxonomic system (Schlick-Steiner et al., 2010).
We hope that this work will help regional researchers with the identification of this rather challenging group of ants and we also expect adding more material including new records and undescribed species with further surveys of poorly collected areas of the region.

Conclusions
The Arabian ant fauna of the Monomorium monomorium species-group is revised, keyed and illustrated based on the worker caste. Six species are treated with description of a new species M. mohammedi sp. n. from the southwestern region of the KSA.

BMNH
The Natural History Museum, London, U.K.

KSMA
King Saud University Museum of Arthropods, Plant Protection Department, College of Food and Agriculture Sciences, King Saud University, Riyadh, Kingdom of Saudi Arabia.

Measurements and indices Figs. 1-3:
All measurements are in millimeters and follow the standard measurements of previous works on the genus (Bolton, 1987;Sharaf et al., 2017):

EL
Eye Length; maximum diameter of eye in lateral view.

EM
Distance between anterior margin of eye and mandibular insertion in lateral view.

HL
Head Length; maximum length of head, excluding mandibles in full-face view.

HW
Head Width; maximum width of head behind eyes in full-face view.

ML
Mesosoma Length (=Weber Length); length of mesosoma in lateral view; from a point at which pronotum meets cervical shield to posterior base of propodeal lobes or teeth.

TL
Total Length, sum of lengths of head, mesosoma, petiole, postpetiole and gaster in profile.