Messor mcarthuri

Described from Greece and western Turkey. In Greece, prefers lowlands and highland habitats up to 820 m a.s.l. Nests were observed in various open habitats like pastures, stone hills, roadsides, sides of oak forests, olive plantations, ruderal areas in tourist resorts but also in limestone gorges or stream valleys with open deciduous forests (Salata & Borowiec, 2019).

Identification
Steiner et al. (2018) – Worker and queen. Colour generally lighter as in Messor muticus and Messor ponticus. Discrimination from all species based on very regular costate sculpture of head and body generally, petiole included. Discrimination from M. muticus and Messor structor by base of scape lacking lobe, short 1st funicular segment, from M. muticus, M. ponticus, and M. structor by imbricate surface of 1st gastral tergite, and from M. ponticus by side of head covered abundantly with standing setae.

Worker. Similarly sized as M. ponticus, usually larger than Messor ibericus, M. muticus, and M. structor. Safe discrimination from M. structor only feasible by morphometrics. Discrimination from all other species based on the regular costate surface of body including petiole and postpetiole. Also using morphometrics, Messor mcarthuri can be separated safely from any other species treated in this revisiona using a linear discriminant function.

Queen. Larger than M. ibericus, similarly sized as M. muticus, M. ponticus, and M. structor. Discrimination from M. ibericus by reduced reticulation and rugosity of anepisternum and katepisternum.

Distribution based on Regional Taxon Lists
Palaearctic Region: Greece, Turkey.

Biology
mtDNA: Six haplotypes of COI (1375 bp) known (GenBank).

Nomenclature

 *  mcarthuri. Messor mcarthuri Steiner et al., 2018: 401, figs. 2, 5ae, 5be, 5ce, 5de, 5ee, Table S1 (w.q.) TURKEY.

Taxonomic Notes
Salata & Borowiec (2019) (misidentifications):
 * As Messor muticus: Borowiec and Salata, 2012: 514 (part).
 * As Messor orientalis: Borowiec and Salata, 2012: 515 (part), 2017b: 214; Bračko et al., 2016: 23.

Description
Worker and queen. Colour: Head and mesosoma brown to brownish red, gaster dark brown. Major workers can be dark brown entirely. Size: Large. Head: Regularly costate throughout, interstices reticulate. In major and most medium workers, costae present on much of postocular region. Erect setae abundant on side of head from occiput to mandibular insertion. Scape: Base without lobe. Laterally directed, tooth-like process in major worker present, less distinct in minor worker. 1st funicular segment short and flattened, longer than 2nd segment, but clearly shorter than 2nd and 3rd segment together. Clypeus: Median notch well defined; can be shallow in minor worker. Pronotum: Middle mostly shining, laterally irregularly running costae. Mesonotum: Regularly costate entirely. Mesopleuron: With slightly irregular transverse costae, well developed punctuation among costae. Propodeum: Mostly smoothly or obliquely rounded, never angled even in major worker; with well-developed distinct longitudinal carinae on both sides. Petiole: With regular costae. Surface of 1st gaster tergite: Entire surface imbricate. Covered with thick whitish, sparse hairs, some of which decumbent or subdecumbent.

Queen: Metanotum: Regularly costate. Anepisternum: Smooth and shining. Katepisternum: Sides covered with fine costulae, middle smooth and shining.

Type Material
Turkey TR271 Mugla, Köycegiz 25.09.2008 leg. Csősz [–] Holotypus Messor” “mcarthuri” [on the reverse side: Top specimen design. Csősz 2016].

Paratypes: 8 workers labelled as the holotype: Turkey TR271 Mugla, Köycegiz  25.09.2008 leg. Csősz (3 paratype workers: ; 5 paratype workers: HNHM).

Etymology
Named in honour of ant taxonomist Archie J. McArthur who we remember for his outstanding enthusiasm, dedication, and warmth.