Introduction

The ant genus Monomorium was established by Mayr (1855) for the newly described species Monomorium minutum Mayr (which was given the new name Monomorium monomorium Bolton (1987:287)). This genus includes more than 300 species and subspecies (Bolton 1995; Bolton et al. 2007) found in all zoogeographic regions with most species occurring in the Old World tropics and temperate regions (Brown 2000). Taxonomic revisions of the Australian and Malagasy Monomorium fauna were carried out by (Heterick (2001, 2006) respectively. The Afrotropical Monomorium fauna was comprehensively revised by Bolton (1987). Two new South American species of Monomorium were described by Fernández (2007) and notes on the genus were presented. The Monomorium fauna of Arabian Peninsula was reviewed and listed giving 53 species for the region (Collingwoood and Agosti 1996). Most species of Monomorium nest in rotten wood, under stones, or directly in the soil.

Members of the genus Monomorium can be distinguished by the following characters: monomorphic to polymorphic; antennae 10–12 segmented (most frequently 12), usually with a conspicuous 3-segmented club; mandibles with 3–5 teeth, median clypeal seta conspicuous; median portion of clypeus raised, the raised section longitudinally bicarinate; the carinae usually distinct; metanotal groove present, commonly impressed; propodeal dorsum usually unarmed and rounding into the declivity; propodeal spiracle usually circular, located at about the midlength of the sclerite (Bolton 1987).

The Monomorium monomorium-group (Bolton 1987) can be distinguished by the following characters: Monomorphic; mandibles unsculptured; the masticatory margin usually with 4 teeth; palp formula predominantly 2, 2; cephalic dorsum unsculptured and glossy smooth except for scattered hair-pits; eyes always present, size small to large (0.15–0.38×HW), with 4 or more ommatidia in the longest row; head always longer than broad (CI 72–89); metanotal groove moderately to strongly impressed, with distinct cross-ribs; propodeal dorsum rounding into declivity, not angulate or dentate; petiole, postpetiole and gaster usually unsculptured.

The first treatment of the Arabian Monomorium fauna was Collingwood’s (1985) study of the genus in Saudi Arabia where 20 species were recorded, of which a single species was of the monomorium-group, Monomorium clavicorne Andre, 1881. The second and more comprehensive contribution was that of Collingwood and Agosti (1996) for the Monomorium in the Arabian Peninsula. In that study, 53 species were recorded, 17 of which were described from Saudi Arabia including two species belonging to the Monomorium monomorium-group, Monomorium montanum and Monomorium qarahe. Since that time the contributions to the Arabian Monomorium fauna were descriptions of only two new species, Monomorium nimihil Collingwood from Socotra Archipelago (Collingwood et al. 2004) and Monomorium moathi Sharaf & Collingwood from Yemen (Aldawood et al. 2010).

In the present paper a new species of the genus Monomorium, Monomorium dryhimi is described from Saudi Arabia based on worker caste. A key to the four Arabian species of the Monomorium monomorium-group is presented.

Materials and methods

All measurements are in millimeters and follow the standard measurements (Bolton 1987).

TL Total Length; the outstretched length of the ant from the mandibular apex to the gastral apex.

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

HL Head Length; the maximum length of the head, excluding the mandibles.

CI Cephalic Index (HW × 100/HL).

SL Scape Length, excluding basal neck.

SI Scape Index (SL × 100/HW).

EL Eye Length; the maximum diameter of the eye.

ML Mesosoma Length; the length of the mesosoma in lateral view, from the point at which the pronotum meets the cervical shield to the posterior base of the propodeal lobes or teeth.

PRW Pronotal width, maximum width in dorsal view.

PL Petiole Length; the maximum length measured in dorsal view, from the anterior margin to the posterior margin.

PW Petiole Width; maximum width measured in dorsal view.

PPL Postpetiole Length; maximum length measured in dorsal view.

PPW Postpetiole Width; maximum width measured in dorsal view.

Images taken under the scanning electron microscope ((SEM) JSM-6380 LA) were used to record morphological details of the new species (Figs 1–7).

Results Discussion

This new species is a member of the Monomorium monomorium group with closest resemblance to Monomorium holothir Bolton, 1987, which was described from Kenya. Both species sharing the following characters: clypeal carinae sharply developed and distinctly elevated; head sides behind eyes weakly convex; posterior margin feebly concave; in lateral view the maximum diameter of eyes clearly greater than the distance between anteriormost point of the eye and the nearest point of the mandibular articulation; body colour yellow to light brownish yellow; relatively similar body dimensions e.g. HL, HW, SI, and CI.

Monomoroium dryhimi can be easily separated from Monomorium holothir by the following characters: eyes relatively small, their maximum diameter EL 0.19–0.25 × HW and with 6 ommatidia in the longest row, while in holothir eyes larger, their maximum diameter EL 0.30 × HW and with 8–9 ommatidia in the longest row. In Monomorium dryhimi, the median portion of anterior clypeal margin is clearly concave, whereas it is transverse to feebly concave in holothir. Moreover, in Monomorium dryhimi head in profile with a weakly convex dorsal surface and a clearly convex ventral surface, whereas in Monomorium holothir, head in profile dorsoventrally flattened. Furthermore, the promesonotum in Monomorium dryhimi has 3–4 pairs of hairs whereas in holothir the promesonotum have 8 pairs of hairs.

Biology of Monomorium dryhimi

The type locality is a forest called Amadan, Al Mandaq governorate, Al Bahah province, Kingdom of Saudi Arabia, with much wild vegetation including Erica arborea L, Juniperus procera Hochst. Ex Endle.and Acacia gerrardii Benth (Fig. 8). Monomorium dryhimi type was taken from a nest under a stone on hard-packed soil which contained tens of workers and was found in relatively elevated area of a valley which is high enough to avoid direct impacts of flooding. No additional nests were found despite extensive surveys. In addition, we were not able to collect foraging workers near the nest. It appears that members of the Monomorium monomorium group may prefer inhabiting areas of high elevations. All the four Arabian species were found inhabiting elevated localities with more than 1800 m asl., except Monomorium clavicorne which was also recorded from both relatively lower elevated areas including Riyadh and Al Qatif, in Central and Eastern regions of Saudi Arabia, respectively, and also from a much elevated area, Fayfa, Asir province (Collingwood 1985). Apparently this species is endemic to the chain of Asir Mountains which extends to Yemen.

Key to the Arabian species of the Monomorium monomorium-group