Rhopalomastix glabricephala

Rhopalomastix glabricephala appears to be a rare species in Singapore, currently found only in its type locality. The type series was collected from a nest within deeper bark layers of a Tembusu tree in a secondary forest.

Identification
Wang, Yong, and Jaitrong (2018) - Small (HL 0.43–0.45, HW 0.39–0.42), monomorphic, with little variation in size. In profile, clypeus projected slightly forward from dorsal margin of head, almost entirely flat or straight; posterior half of head smooth and shiny; ventral surface of head largely smooth and shiny; outer surface of femora mostly smooth and shiny, with very feeble superficial reticulation; petiole rather small, anterior face weakly concave and covered with short appressed hairs. Head and dorsum of mesosoma darker brown compared to rest of yellowish-brown body; legs and tips of antennae pale yellow.

Rhopalomastix glabricephala is most similar to Rhopalomastix murphyi in having small body size, smooth and shining posterior half of head, largely smooth and shining femora and first gastral tergite. However, R. glabricephala can be distinguished from R. murphyi by the following characteristics: 1) deeper, more pronounced central median concavity of the posterior margin of the head, 2) petiolar node small and usually not higher than postpetiole, 3) anterior rising face of petiole pilose with decumbent hairs (anterior petiolar face is largely smooth and shining without hairs in R. murphyi).

Distribution based on Regional Taxon Lists
Indo-Australian Region: Singapore.

Nomenclature

 *  glabricephala. Rhopalomastix glabricephala Wang, Yong & Jaitrong, 2018: 308, figs. 4-7 (w.) SINGAPORE.

Worker
Holotype. TL 1.93; HL 0.44; HW 0.42; SL 0.18; ML 0.54; PronW 0.28; EL 0.09; EW 0.05; CI 95; REL 21; SI 43. Paratype workers (n = 6): TL 1.73–1.86; HL 0.43–0.45; HW 0.39–0.42; SL 0.16–0.18; ML 0.44–0.52; PronW 0.26–0.28; EL 0.06–0.08; EW 0.04–0.05; CI 91– 95; REL 15–20; SI 38–46.

Monomorphic with little variation in size. Head in full face view subrectangular, slightly longer than wide, posterior margin with a weak median concavity, posterolateral corner of head roundly convex, lateral margin broadly convex. Eye with 13–17 ommatidia. Median portion of anterior clypeal margin broadly convex and lined with minute denticles. In profile, clypeus projected slightly forward from dorsal margin of head, almost entirely flat or straight. Antennal scape short, gradually swollen distally, reaching midlength of head when laid backwards. In profile, mesosoma box-shaped, subcylindrical, dorsal outline weakly convex, propodeal declivity shallowly concave. In dorsal view, mesosoma subrectangular, anterior pronotal margin strongly convex and rounded, posterior propodeal margin weakly concave. In profile, petiole inclined posteriorly, much longer than high, anterior rising margin weakly concave, forming a smooth continuous curve with rounded apical margin, junction between both margins indistinct; petiolar node bell-shaped, apex convex, posterior declivity roughly half as long and as steep as anterior rising margin. Petiole as high as and longer than postpetiole, dorsal margin of postpetiole broadly convex in profile view. In dorsal view, petiole elliptical in shape, about twice as broad as long in large workers, less broad and more oval in smaller workers; postpetiole globular and sub-oblong in shape, lateral margin rounded, wider than long, larger than petiole.

Posterior half of head including dorsum, lateral surfaces and venter mostly smooth and shiny, anterior half of head including antennal scrobe and area surrounding eye with feeble striations. Median clypeal surface carinate and punctate but still shiny. Mandible mostly smooth and shining with sparse and feeble striations, area close to masticatory margin lightly punctate. Lateral surface of mesosoma weakly striate-reticulate and shining; dorsum of mesosoma striated. Lateral surfaces of petiole, postpetiole weakly punctate, largely smooth and shiny. Dorsal surfaces of petiolar and postpetiolar nodes weakly strigate with sparse punctures, still shining. Gaster mostly smooth and shining, with sparse and weak transverse sculpture and scattered punctures. Dorsal surface of head with numerous short erect and suberect hairs, interspersed by sparer and much longer erect hairs; ventral surface and gena with abundant short appressed and decumbent hairs. Mesosoma dorsum with sparse, scattered short standing hairs mainly aligned close to dorsolateral margin, sparer and longer erect hairs on the same margin. Petiole and postpetiole relatively more pilose, anterior face of petiole with dense decumbent hairs, petiolar and postpetiolar nodes with abundant short standing hairs, and sparer and longer erect hairs. Gaster pilose, both ventral and dorsal surfaces with dense erect and suberect hairs. Head and dorsum of mesosoma relatively dark brown in colour, rest of body lighter and uniformly yellowish-brown, legs and tips of antennae pale yellow.

Type Material
Holotype. Worker, SINGAPORE, Mandai Road (1.41379°N, 103.79820°E), nest in bark of Tembusu tree (Fagraea fragrans), 16 Nov 2016, G.W. Yong & K.H. Pwa leg., ZRC_ENT_00000873. Paratypes. Thirty-six workers (,, , ZRC), same data as holotype, colony no. GY-SG16- RhoE, ZRC_HYM_0000508.

Etymology
The specific name refers to the smooth and shining, i.e. glabrous, posterior half of head in this species.

References based on Global Ant Biodiversity Informatics

 * Wang W. Y., G. W. J. Yong, and W. Jaitrong. 2018. The ant genus Rhopalomastix (Hymenoptera: Formicidae: Myrmicinae) in Southeast Asia, with descriptions of four new species from Singapore based on morphology and DNA barcoding. Zootaxa 4532: 301-340.