Rhopalomastix tenebra

The type series of R. tenebra was found deep within bark of Durio zibethinus (durian tree) in an abandoned plantation, just a few metres away from another durian tree colonized by Rhopalomastix murphyi. Unlike R. murphyi, the colony of R. tenebra seemed more dispersed throughout a relatively broad area, workers less numerous; individuals were often obscured or well-hidden under bark layers; few or none were found on the bark surface.

Identification
Wang, Yong, and Jaitrong (2018) - Workers medium sized; generally monomorphic with moderate size variation (HL 0.44– 0.53, HW 0.4–0.48). In profile view, clypeus evenly and broadly convex, in full face view, posterior margin with deep median concavity; anterior half and vertex of head substriate and shining, posterior half of lateral head surface largely finely reticulate with smooth and shiny interspaces; ventral head surface superficially reticulate with smooth and shiny interspaces; outer faces of femora and first gastral tergite superficially reticulate with smooth and shiny interspaces; anterior rising face of petiole almost straight, superficially reticulate and shiny. Head and dorsum of mesosoma very dark brown, rest of body relatively uniform lighter brown; joints and tarsi lighter and more yellowish in colour. Larger worker generally darker in colour relative to smaller worker.

Queen. Medium size (HL 0.5–0.54, HW 0.44–0.47), head more rectangular than that of worker, posterior margin with a relatively deep and broad median concavity; posterior half of head largely smooth and shiny, vertex, occipital lobe and ventral surface superficially reticulate; anterior margin of pronotum strongly convex and angulate at its median point; mesoscutum wider than long; anterior face of petiole weakly concave and almost straight, petiole subtrapezoidal with blunt apex. Head, mesosoma, gaster dark blackish-brown, legs, petiole and postpetiole lighter brown.

Male. Medium size (HL 0.44, HW 0.41–0.42). Head in profile view broad-ovate, ventral margin roundly convex; clypeus in profile convex and sub-semicircular, projecting strongly from dorsal margin of head; petiole relatively high and subtrapezoidal; dorsal outline of postpetiole weakly convex, cinctus with gaster distinct; anteroventral extension of subpetiolar process very small and subtriangular. Entire head mostly substriate-reticulate, with smooth and shining interspaces and scattered deep punctures; clypeus weakly reticulate-punctate and shiny; dorsum of mesosoma largely superficially reticulate. Largely uniform grey-brown, head blackish brown; legs and antennae paler in tone, joints of legs pale yellowish brown.

Rhopalomastix tenebra is most similar to Rhopalomastix javana in having the posterior half of lateral head surface weakly sculptured (weak striations or striate-reticulate with smooth and shiny interspaces). However, R. tenebra can be separated from R. javana based on the following characteristics: posterior half of lateral head surface largely superficially reticulate with smooth and shiny interspaces (finely striate-reticulate and shining in R. javana); in full-face view, median depression of posterior margin of head deeply concave (relatively shallow in R. javana); in profile, anterior rising margin of petiole almost straight (weakly concave in R. javana); larger variation in size, larger worker slightly larger than that of R. javana.

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

Nomenclature

 *  tenebra. Rhopalomastix tenebra Wang, Yong & Jaitrong, 2018: 332, figs. 64-75 (w.q.m.) SINGAPORE.

Worker
Holotype: TL 2.1; HL 0.5; HW 0.46; SL 0.18; ML 0.64; PronW 0.36; EL 0.1; EW 0.06; CI 92; REL 21.74; SI. 39.13. Paratype workers (n = 9): TL 1.92–2.36; HL 0.44–0.53; HW 0.4–0.48; SL 0.17–0.21; ML 0.52–0.68; PronW 0.30–0.38; EL 0.08–0.12; EW 0.04–0.07; CI 88–94; REL 18–25; SI 39–46.

Medium sized, generally monomorphic with moderate size variation. Head in full face view subrectangular, slightly longer than wide, posterior margin with a relatively deep and concave median depression, posterolateral corners very smooth and rounded, lateral margins of head broadly convex and almost parallel with each other. Eye with 12–18 ommatidia. In profile, mesosoma subcylindrical, dorsal margin weakly convex, propodeal declivity steep and almost vertical. In dorsal view, mesosoma subrectangular, anterior pronotal margin strongly convex, almost angulate at its median point, humeral angle obtuse but smooth and rounded, lateral margins almost entirely parallel with each other except for a gentle depression where the metanotal groove occurs, dorsolateral sides of propodeum rounded and smooth, posterior propodeal margin weakly concave. In large workers, mesosoma more robust, pronotal width in dorsal view very broad, posterior propodeal margin almost as broad as pronotum. In profile, petiole slightly inclined posteriorly, not much longer than high, anterior rising margin almost entirely straight and steep, petiolar node bell-shaped, apex smooth and rounded, posterior petiolar declivity slightly shorter and steeper than anterior margin. In the same view, petiole slightly higher and much longer than postpetiole, postpetiole quadrant in shape, dorsal margin broadly convex. In dorsal view, petiole globular, less than twice as broad as long; in small worker, petiole sub-oval, nearly twice as broad as long. Postpetiole globular, larger and wider than petiole, lateral margins rounded, about twice as wide as long.

Surfaces of head substriated and shining, clypeus with dense punctures and striae but still weakly shining. Striations on head strong anteriorly but somewhat weaker posteriorly, posterior half of lateral head surface including occipital lobe mostly superficially reticulate with smooth and shiny interspaces; ventral surface of head finely reticulate with smooth and shiny interspaces. Mandible mostly smooth and shining, punctures concentrated to small areas close to masticatory margin and basal insertion. Lateral surface of mesosoma mostly striate-reticulate and shining, upper part of mesopleuron, i.e. anepisternum, rugulose-reticulate and weakly shiny; dorsum of mesosoma striated, dorsolateral margin wholly lamellate and distinct. Lateral and dorsal surfaces of petiole, postpetiole superficially reticulate, weakly punctate, interspaces shining. Gastral tergites mostly smooth and shining, with feeble microreticulate sculpture.

Surfaces of head including gena with scattered but abundant short erect hairs, one or two long erect hairs; antennal scape also lined with evenly distributed short standing hairs and sparse longer erect hairs. Mesosoma dorsum with scattered short standing hairs, and sparse longer erect hairs limited to dorsolateral sides. Anterior rising face and posterior declivity of petiole with abundant decumbent hairs, dorsoapical surface of petiole with many uniform short standing hairs and one pair of much longer erect hairs. Dorsum of postpetiole also covered with abundant short erect hairs of uniform length, and one pair of long erect hairs. Gaster pilose, both dorsal and ventral surfaces covered with dense short erect, suberect hairs, interspersed with sparse longer erect hairs.

Head and dorsum of mesosoma very dark brown, rest of body relatively uniform lighter brown; joints and tarsi relatively lighter and more yellowish in colour. Large worker much darker in colour, head, dorsum of mesosoma and gaster almost opaque blackish-brown, rest of body lighter brown, also paler in tone towards distal parts of antennae and legs, i.e. antennal tips and tarsi.

Queen
Three paratypes and one non-type (n = 4): TL 2.56–2.8; HL 0.50–0.54; HW 0.44– 0.47; SL 0.20; ML 0.68–0.70; MsW 0.41–0.42; EL 0.16–0.18; EW 0.10–0.12; CI 85–90; REL 36–39; SI 43–45.

Medium size. Head in full face view subrectangular, more rectangular than that of worker, posterolateral corners smooth and rounded, lateral margins almost entirely straight and weakly converging anteriorly, posterior margin of head clearly wider than anterior clypeal margin, with a relatively deep and broad median concavity. Posterior margin of eye located roughly at midline of head in full face view. In profile, clypeus very weakly but evenly convex. Scape short, not exceeding midline of head when lain back. In profile, mesosoma subcylindrical, more elongate than that of worker, dorsal outline broadly and weakly convex, propodeal junction rounded and smooth, posterior face of propodeum steep and smoothly flat. In dorsal view, mesosoma sub-elliptical, propodeum distinctly less wide than pronotum, anterior margin of pronotum strongly convex and angulate at its median point; humeral corners more angular than that of worker, mesoscutum wider than long. In profile view, petiole subtrapezoidal, inclined posteriorly, longer than high, anterior face steep and weakly concave, almost entirely straight, apex bluntly rounded, slightly higher than postpetiole; dorsal margin of postpetiole nearly entirely straight. In dorsal view, petiole suboval, less than twice as wide as long, postpetiole globular, much larger and wider than petiole.

Posterior half of head largely smooth and shining; vertex, occipital lobe and ventral surface superficially reticulate; anterior half of head finely striated, interspaces punctate but shiny, median part of clypeus striate-punctate and weakly shining. Mandible mostly smooth and shining, area close to masticatory margin punctate. Lateral surface of mesosoma including pronotum weakly substriate-reticulate, interspaces smooth and shining; dorsum of mesosoma largely finely striate and shining, median section of pronotum superficially reticulate and shiny, with feeble striations; mesoscutellar disc with very weak striations, mostly smooth and shining. Lateral surfaces of petiole, postpetiole reticulate with smooth and shining interspaces, apices superficially reticulate, largely smooth and shining. Gaster superficially reticulate and shining.

Dorsum of head with numerous short erect and suberect hairs with sparse longer erect hairs; antennal scape with scattered short erect hairs. Gena and ventral surface of head with abundant but scattered short appressed hairs. Dorsum of mesosoma with sparse short standing hairs mainly on the mesoscutum and along dorsolateral margin; sparse long erect hairs along dorsolateral margin, one pair on pronotal humerus. Anterior face of petiole with short decumbent hairs, apex posterior declining face abundant short standing hairs, sparse long erect hairs on apex. Dorsum of postpetiole with abundant short suberect hairs and a pair of long erect hairs. Dorsal surface of gaster with abundant suberect and subdecumbent short hairs, ventral surface with many appressed and decumbent short hairs, both surfaces with sparse long erect hairs; hairs denser distally.

Head, mesosoma, gaster dark blackish-brown, petiole and postpetiole lighter brown, legs also lighter brown, paler in tone towards tarsi.

Male
Paratype and one non-type (n = 2): TL 2.17–2.18; HL 0.44; HW 0.41–0.42; SL 0.05–0.06; ML 0.78–0.84; MsW 0.46–0.48; EL 0.22–0.24; EW 0.16–0.20; CI 93–95; REL 54–57; SI 12–15.

Medium size. Head in full face view almost entirely spherical, posterior margin broad and roundly convex; maximum diameter of eye only slightly exceeding midline of head; outline of eye convex and exceeding lateral margin of head. In profile, head broad-ovate, lateral corners smooth and rounded, ventral and posterior margins forming a continuous smooth curve, ventral margin roundly convex; clypeus convex and sub-semicircular, projecting strongly from dorsum of head. In profile, mesosoma sub-oblong, dorsal margin almost straight, propodeal junction rounded and smooth, posterior face of propodeum steep and weakly convex, almost entirely vertical. In dorsal view, mesosoma obovate and narrower posteriorly, notauli absent. In profile view, petiole slightly inclined posteriorly, relatively high and subtrapezoidal, anterior margin weakly concave, apex weakly and broadly convex, forming a continuous convex curve with posterior margin; anteroventral extension of subpetiolar process greatly reduced to a very small, shallow, and subtriangular translucent lamella. Dorsal outline of postpetiole weakly convex, differentiated from gaster by a weak but distinct cinctus.

Entire head mostly substriate-reticulate, with smooth and shining interspaces and scattered deep punctures; clypeus weakly reticulate-punctate and shiny. Lateral surface of mesosoma mostly superficially reticulate with smooth and shining interspaces; dorsum very feebly and superficially striate-reticulate, interspaces smooth and shining, mesoscutum with scattered punctures. Petiole, postpetiole and gaster superficially reticulate with smooth and shiny interspaces.

Dorsum of head with abundant short standing hairs, ventral surface with dense short appressed hairs. Dorsum of mesosoma with numerous but scattered short decumbent and erect hairs, mainly on mesoscutum. Dorsal surfaces of petiole, postpetiole with many uniformly-short erect hairs. Gaster pilose, dorsal surface with abundant decumbent and erect short hairs, ventral surface with numerous appressed and decumbent short hairs, sparse short erect hairs.

Body generally a uniform dark grey-brown, head blackish-brown, legs and antennae lighter in tone; joints of legs pale yellowish brown.

Type Material
Holotype. Worker. SINGAPORE, Upper Thomson Nature Park (1.38653°N, 103.81938° E), nest in bark of Durio zibethinus tree, 24 Oct 2016, G.W. Yong & S.X. Chui leg., colony no. GY-SG16-RhoB, ZRC_ENT_000000877 .Paratypes. Thirty-six workers (, ZRC), 3 queens (ZRC), one male (ZRC), same data as holotype, ZRC_HYM_0000288.

Etymology
The specific name means “darkness” in Latin and alludes to this species’ dark body colours, and observed inclination to hide in dark crevices in tree bark.

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.