New synonyms of two Arabian ants of the genus Monomorium Mayr, 1855 (Hymenoptera, Formicidae)

Abstract Synonymy of two Arabian Monomorium Mayr, 1855 species is proposed: Monomorium exiguum Forel, 1894 = Monomorium desertorum Collingwood & Agosti, 1996, syn. n.; Monomorium subopacum Smith, 1858 = Monomorium mintiribe Collingwood & Agosti, 1996, syn. n. A lectotype for Monomorium venustum Smith, 1858 is designated. Information on nesting habits of Monomorium exiguum and Monomorium venustum in the Kingdom of Saudi Arabia are provided for the first time. Recently collected records for Monomorium exiguum, Monomorium subopacum, and Monomorium venustum from the Kingdom of Saudi Arabia and United Arab Emirates are listed.


Introduction
The first published work on the ant genus Monomorium Mayr, 1855 for the Kingdom of Saudi Arabia (KSA) was by Collingwood (1985), who listed and keyed 20 species from the country. The genus was subsequently treated comprehensively for the Arabian Peninsula by Collingwood and Agosti (1996). The authors reported 53 species, 32 of which were described as new, (including 15 from the KSA, 10 from Oman, five from Yemen, and two from Kuwait). Collingwood et al. (2011) treated the myrmecofauna of the United Arab Emirates (UAE) and reported 29 Monomorium species. Three species were recorded from Socotra Island (Collingwood et al. 2004) and a new species, M. nimihil Collingwood, 2004 was described. Recently the Monomorium fauna of KSA has received renewed attention, with the first record of M. exiguum Forel, 1894 (Aldawood andSharaf 2009) and descriptions of three new species, M. moathi Sharaf & Collingwood, 2010(Aldawood et al. 2010; M. dryhimi Aldawood & Sharaf, 2011(Aldawood and Sharaf 2011) and M. sarawatensis Sharaf & Aldawood, 2013(in El-Hawagry et al. 2013. During two visits to the World Museum, Liverpool, United Kingdom, two new synonyms were discovered for Arabian Monomorium.

Abbreviations of museums BMNH
The Natural History Museum, London, United Kingdom. MHNG Museum d'Histoire Naturelle, Geneva, Switzerland.
The numbers between parentheses in material examined refer to individual workers. Remarks. Only a single paratype specimen with the same data as the holotype exists at WMLC. The holotype and other paratypes are considered lost.

Monomorium exiguum
The description of M. desertorum in Collingwood and Agosti's (1996) indicated that the eyes are located anterior to the midlength of head, the scapes when retracted back do not reach the posterior margin of head, the antennae are 11 segmented, and the body is not sculptured except for the metanotal cross-ribs. Comparison was made of the single available paraype worker of M. desertorum with the lectotype worker of M. exiguum was carried out. We here treat M. desertorum as a junior subjective synonym of M. exiguum.
Habitat. The vast majority of M. exiguum nests that were collected in KSA were found to be associated with leaf litter and topsoil layers where workers foraged. Frequently nests were directly in the soil. The nesting habits of M. exiguum however, are diverse. In a site located in the southwestern mountains of the KSA, the species was found nesting in loose sandy soil with high moisture content and among roots of small Portulaca oleracea L. (Portulacaceae) plants beneath a date palm tree, Phoenix dactylifera L. (Arecaceae). Several worker series were nesting in a humid clay soil under banana trees. Other worker series were collected under a rock next to Juniperus procera Hochst. ex Endlicher (Cupressaceae) and Acacia spp. (Mimosaceae) trees. Another nest was found in thick layer of leaf litter under a large and old Ficus benghalensis L. (Moraceae) tree where the soil was rich in decayed organic matter. Some nests were found in leaf litter under Calotropis procera (Aiton) W.T.Aiton (Asclepiadaceae) and next to a mango tree (Mangifera sp., Anacardiaceae).

Monomorium subopacum (Smith, 1858) Figs 5-8
For full synonymy, see Heterick (2006) Remarks. The holotype and 10 paratypes of M. mintiribe seem to be lost. Extensive searches at both WMLC and NHMB failed to locate type material except for a single paratype specimen labeled in red at the WMLC. In addition, despite the fact that the label information for the paratype specimen in WMLC does not exactly match the in-formation in Collingwood and Agosti (1996), we consider this specimen as part of the original type series. Collingwood and Agosti (1996) indicated the following paratypes: 1 male, 2 queens, 2 workers, "Oman, Bilad Bani, 20°03'N, 59°17'E, coll. W. Buttiker", whereas the data on the paratype specimen in WMLC is "Bilad Ban, Oman, W. Buttiker, 17.i.1986." The second author (C. A. Collingwood) confirms that the single remaining specimen is an originally designated paratype. The original description of M. mintiribe did not compare this taxon with related congeners. The single paratype is identical to the paralectotypes of M. subopacum and the original description agrees with this. Therefore, M. mintiribe is treated here as a junior subjective synonym of M. subopacum.  Remarks. Originally, M. venustum was described based on syntypes of the worker caste from Syria. Here we designate a lectotype with the following data, "M. venusta Smith, type, BMNH (E), 1015257". The Lectotype is deposited at BMNH.

Monomorium venustum (Smith, 1858)
Habitat. Workers of M. venustum build nests directly into the ground under stones and rocks, directly into the ground. This species apparently prefers to nest in soil with high moisture content as observed in many locations in KSA. In the southwestern mountains of the KSA, nests were constructed next to Juniperus procera Hochst. ex Endlicher (Cupressaceae) and Acacia spp. (Mimosaceae) trees. In addition, the species is usually foraging in areas with dense green flowering grasses that covering the ground. A single nest was found existing next to Mentha longifolia (L.) Huds. (Lamiaceae). Myrmecophilous arthropods (e.g. small beetles, isopods and millipedes) were found inside some nests.