Rhopalomastix johorensis

Nests are often found in the bark of fruit trees, such as mango, close to human dwellings. Also in various species of trees in secondary forest, disturbed forest, and both abandoned and operational plantations.

Identification
Wang, Yong, and Jaitrong (2021) - Monomorphic workers with broad size variation (types and non-types: HL 0.44–0.58; HW 0.42–0.56). Head of large worker slightly more rectangular than small worker; posterior margin with shallow median depression. In profile, anterior clypeal face either weakly and broadly convex or almost entirely flat or straight; dorsum and lateral face of head finely striate; ventral surface of head reticulate with smooth and shiny interspaces; outer surface of femora and first gastral tergite superficially reticulate with smooth and shiny interspaces. Dorsal outline of mesosoma weakly and broadly convex with propodeal dorsum gently downward-sloping. Anterior margin of pronotal disc broadly convex, humeral corners roundly obtuse or indistinct. Lateral margins of propodeum in dorsal view broadly convex and posteriorly converging, posterior margin very weak, posterolateral corners usually indistinct, sometimes bluntly and shallowly rounded; propodeal junction in lateral view indistinct and broadly rounded; posterior propodeal face incompletely marginate with obscure dorsal edge, and almost flat or shallowly concave in upper half. Head and dorsum of mesosoma darker brown compared to rest of yellowish-brown body; legs and tips of antennae slightly paler in tone. Large workers generally darker brown compared to small workers.

Queen. Relatively large size (non-types: HL 0.54–0.56, HW 0.51–0.53), head not much more rectangular compared to large worker. In profile, clypeus very weakly convex and almost entirely flat. In dorsal view, anterior margin of pronotal disc strongly convex and sometimes angulate at median point, humeral corners obtusely angulate. Propodeal outline in lateral view roundly convex; propodeal dorsum smoothly rounds into steep posterior face at indistinct rounded posterior edge; posterior face of propodeum weakly convex; in dorsal view propodeum generally sub-semicircular in shape. Anterior half of head finely striate; posterior half mostly smooth and shining with scattered punctures. Entire dorsum of mesosoma finely striate and shiny; lateral face of mesosoma striate-reticulate with interspaces smooth and shining. Overall darker blackish brown as compared to worker, with identical tones.

Male. Relatively large size (non-types: HL 0.44, HW 0.4–0.42). Head in full-face view broad ovate and somewhat narrower posteriorly, in lateral view narrowed posteroapically with steep dorsum. Clypeus in profile projected strongly from dorsal margin of head, short dorsal margin separated from longer anterior margin at roundly angulate edge, anterior clypeal face weakly convex or almost flat. Median ocellus subcircular, lateral ocelli suboval. Petiole subtriangular with bluntly rounded apex, apex rounding into posterior face forming a continuous convex slope. Anterior half of head dorsum rugulose-reticulate, interspaces punctured and weakly shining; punctures on posterior face of head more dense than rest of head; area enclosed by ocelli rugulose-punctate and weakly shining. Mesosoma mostly superficially substriate-reticulate with smooth and shining interspaces. Body colour generally dark grey-brown, head blackish; antennae and joints of legs pale light brown.

There is broad internidal variation in morphology for this species in Thailand, mostly in terms of minor characters that were also observed to vary within different colonies, such as: intensity of striations on posterior half of head, and extent of flatness of anterior clypeal face. Based on COI barcodes, most Thai colonies could be grouped into a large cluster with < 1% uncorrected p distances among individuals; colonies collected from East Thailand, however, formed a separate cluster divergent from the rest by 4.5 %. The East Thai specimens are morphologically indistinguishable from other specimens of R. johorensis, despite the relatively high underlying genetic difference between the two clusters. This result may imply that East Thai populations of the species were established via an introduction event different from populations in all other parts of Thailand (Bergsten et al. 2012). Alternatively, there could have been a deep genetic split in Thai R. johorensis in the past for reasons unknown, leading to the development of two separate lineages (i.e., East Thai vs all other parts of Thailand) of the same species. More empirical evidence is required to draw a definite conclusion.

This species is similar to both Rhopalomastix impithuksai and Rhopalomastix javana, but can be distinguished from both mainly by the condition of posterolateral corners of propodeum, in relation to inner posterior propodeal face. In R. johorensis, posterolateral corners of propodeum are bluntly rounded but ill-defined and not distinctly differentiated from the inner posterior propodeal face; whereas in R. impithuksai and R. javana, posterolateral corners of propodeum are bluntly angulate, distinctly projecting posteriorly and differentiated from the inner posterior propodeal face, thus the posterior propodeal margin for the latter two species also appears more distinctly (though still weak) concave in dorsal view.

Previously in Wang et al. (2018b), it was surmised that workers of R. javana and R. johorensis may be differentiated based on strength of sculpture on the posteriormost vertexal strip close to the occiput in posterodorsal view. However, upon examination of additional material from Thailand, it is evident that this character is highly variable within both R. javana and R. johorensis – the posteriormost vertexal strip can range from being finely striate to almost entirely smooth and shining. Thus, in this revision we do not recommend using this character to distinguish between the two species.

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

Biology
In Singapore, four genera of diaspidid scale insects have been found in the tunnels chewed under bark by this ant (Matile-Ferrero & Foldi 2018, [[Media:Rhopalomastix Yong IS2019 &Supplementary.pdf|Yong et al. 2019]]).

Wheeler (1929) - Viehmeyer cites Overbeck as having taken this species from several nests in the dead twigs of mango (Mangifera) and in the bark of durion trees (Durio zibethinus).

Wang, Yong & Jaitrong (2018) - Often found in bark of common relatively large native tree species in secondary or disturbed forests, and abandoned or operational plantations. Colonies have frequently been collected from fruit trees, in particular mango trees (in both Singapore and Thailand).

Wang, Yong & Jaitrong (2021) - Colonies in Thailand are often found in, but not limited to, bark of cultivated mango trees usually on roadsides and/or plantations, close to human dwellings. Actual extent of nests per respective host tree is often unclear, sometimes nest cavities have been uncovered in bark very close to tree base, concurrently with occurrence more than one vertical metre from the base. Nests have also been found near or in the crowns of some mango trees. This species has also been collected in bark of wild Azadirachta excelsa trees, in primary evergreen and peat swamp forests of southern Thailand.

Nomenclature

 *  johorensis. Rhopalomastix rothneyi subsp. johorensis Wheeler, W.M. 1929d: 96 (w.) SINGAPORE.
 * Status as species: Wang, Yong & Jaitrong, 2018: 316.
 * janeti. Rhopalomastix janeti Donisthorpe, 1936a: 55 (w.q.m.) THAILAND.
 * Junior synonym of johorensis: Wang, Yong & Jaitrong, 2018: 316.

Worker
Length 2-2. 6 mm.

Distinctly larger than the typical rothneyi, which measures only 1.7- 1.8 mm. Head scarcely longer than broad (l lj6longer than broad in rothneyi) and with somewhat smaller eyes (13 to 15 facets, instead of about 20). Head and thorax rich ferruginous red; appendages, abdomen and sides and declivity of epinotum clear brownish yellow. I have recently received another series of specimens, comprising all three phases of a second subspecies of rothneyi from Java, which may be described as: Rhopalomastix rothneyi subsp. javana subsp. nov.

Wang, Yong, and Jaitrong (2018) - syntypes (n = 3): TL 2.11–2.20; HL 0.50–0.52; HW 0.48–0.50; SL 0.18–0.20; ML 0.50–0.52; PronW 0.32–0.36; EL 0.08; EW 0.06; CI 96 ; REL 16–17; SI 38–40. non-types (n = 18): TL 1.84–2.43; HL 0.44–0.58; HW 0.42–0.56; SL 0.17–0.22; ML 0.52–0.74; PronW 0.28–0.42; EL 0.08–0.12; EW 0.05–0.08; CI 92–97; REL 16–21; SI 37–45. Wang, Yong, and Jaitrong (2021) - Non-type workers (n = 18): EL 0.08–0.12; EW 0.05–0.08; HL 0.44–0.58; HW 0.42– 0.56; ML 0.52–0.74; PronW 0.28–0.42; PtH 0.13–0.20; PtL 0.15–0.23; SL 0.17–0.22; TL 1.84–2.43; CI 92–97; PtHI 83–90; REL 16–21; SI 37–45.

Wang, Yong, and Jaitrong (2021) - Workers monomorphic with broad intranidal size variation. Head in full-face view sub-rectangular, longer than wide (only slightly in small worker), slightly less wide anteriorly in large workers; median furrow between frontal lobes short, not extending past transverse midline of head; posterolateral corners smoothly rounded, lateral margins broadly and weakly convex, almost parallel to each other, posterior margin with shallow median depression. Eye relatively large, with 17–29 ommatidia (10–15 in smaller workers). In profile, clypeus projecting forward slightly from dorsal margin of head, anterior clypeal face largely flat or weakly convex. Antennal scape short and thin, extending slightly beyond midlength of head when lain backwards. Mesosoma in lateral view box-shaped, dorsal outline weakly and broadly convex with propodeal dorsum gently downward-sloping, propodeal junction indistinct and broadly rounded; propodeal dorsum rounding into posterior declivitous face at indistinct rounded edge, posterior face of propodeum weakly and incompletely marginate with obscure dorsal edge, almost flat or sometimes shallowly concave in upper half. In dorsal view, mesosoma subcylindrical, anterior margin of pronotal disc broadly convex, sometimes slightly angulate at median point in large worker, humeral corners roundly obtuse and often indistinct, dorsolateral margins almost straight and nearly parallel to each other, not much narrower posteriorly than anteriorly, lateral sides of propodeum broadly convex and posteriorly convergent, posterior propodeal margin very weak and indistinct; posterolateral corners of propodeum usually indistinct and ill-defined, sometimes bluntly and shallowly rounded, indistinctly differentiated from inner posterior propodeal face. In lateral view, petiole inclined posteriorly and bell-shaped, slightly longer than high, anterior face steep and weakly concave, apex rounded, about as high as postpetiole, posterior face less high than anterior face and slightly steeper; dorsal outline of postpetiole weakly convex. In lateral view, anteroventral extension of subpetiolar process short, subtriangular, variable in size across individuals, posterior margin of subpetiolar process usually almost straight or weakly and broadly convex. In dorsal view, both petiole and postpetiole oblong or elliptical, both wider than long; postpetiole larger and wider than petiole.

Dorsum and lateral face of head finely striate and shiny; median clypeal face coarsely striate and weakly shining; ventral face of head striate-reticulate with interspaces smooth and shining. Mandible mostly smooth and shining but densely punctate near masticatory margin, basally with weak striations. Lateral face of mesosoma substriate-reticulate and shiny with interspaces punctate and weakly shining, coarser sculpture near dorsal margin. Petiole and postpetiole largely superficially reticulate with interspaces smooth and shining, apicodorsal face of petiole sometimes weakly striate. Gaster largely superficially reticulate with interspaces smooth and shining, and scattered punctures.

Head and antenna with abundant short standing and suberect hairs; sparse long erect hairs mainly on clypeus, antennal scape and faint outline of antennal scrobe depression. Mesosomal dorsum with abundant but scattered short standing hairs, paired long erect hairs present along dorsolateral margin, with one pair each flanking anterior margin of pronotal disc, pronotum, mesonotum and propodeum. Anterior petiolar face with dense standing and/or appressed hairs; apical faces of petiole, postpetiole each with dense short standing hairs of nearly uniform length, and 1–2 pairs of long erect hairs. Dorsal and ventral faces of gaster with abundant and dense erect, suberect and decumbent short hairs, these hairs much denser and longer than those on head and mesosoma, interspersed with sparse long erect hairs.

Small workers generally uniform pale yellowish-brown in colour; head and mesosoma of slightly darker tone, especially the dorsal surface of the mesonotum. Large workers darker brown or more reddish brown compared to small workers, tonal patterns identical.

Queen
Wang, Yong, and Jaitrong (2021) - non-type queens (n = 7): EL 0.16–0.18; EW 0.12–0.13; HL 0.54–0.56; HW 0.51–0.53; ML 0.8–0.88; MsW 0.42–0.46; PtH 0.22–0.25; PtL 0.25–0.29; SL 0.2–0.22; TL 2.53–3.1; CI 94–96; PtHI 84–89; REL 31–35; SI 38–43.

Relatively large compared to worker. Head in full-face view subrectangular, posterolateral corners roundly angulate, posterior margin nearly straight with a weak median depression. Eye very large (13–18 ommatidia in the longest axis); posteriormost point of eye located just below transverse midline of head in full-face view. In profile, clypeus projected slightly forward from dorsal margin of head, very weakly convex or almost straight and flat. Scape short, barely surpassing posteriormost point of eye when lain back on head. Ocelli small and suboval, generally similar in size and equidistant from each other. In lateral view, mesosoma subcylindrical, more elongate than large worker, dorsal outline broadly and weakly convex, propodeal outline roundly convex; sulci on lateral face of mesosoma distinct. Propodeal dorsum not distinctly differentiated from and smoothly rounding into steep posterior face at indistinct rounded posterior edge; posterior face of propodeum weakly convex. In dorsal view, mesosoma subcylindrical, anterior margin of pronotal disc strongly convex and sometimes angulate at median point, humeral corners obtusely angulate, mesoscutum almost as wide as long; sides of propodeum broadly convex and posteriorly converging, posterior propodeal margin indistinct, entire propodeum generally sub-semicircular in shape. In lateral view, petiole subtrapezoidal, slightly inclined posteriorly, anterior petiolar face weakly concave and steep; apex of petiolar node rounded and slightly higher than postpetiole; posterior petiolar face less high and steeper than anterior face. In dorsal view, petiole suboval, less than twice as wide as long; postpetiole globular, broader and larger than petiole.

Anterior half of head including frontal lobes largely finely striate and shiny, posterior half of head mostly smooth and shining with scattered punctures, vertexal area with feeble fragmented striations; ventral face of head mostly smooth and shining. Median clypeal triangle coarsely striate-reticulate but less coarse than that of worker, interspaces punctate but shiny. Mandible mostly smooth and shining with very feeble striations. Lateral face of mesosoma weakly striate-reticulate, interspaces smooth and shining; dorsum largely finely striate and shiny, median section of pronotum for Thai queens mostly smooth and shiny and less striate than that of Singapore queens. Petiole and postpetiole largely superficially reticulate with interspaces smooth and shining; apical faces of petiole and postpetiolar dorsum entirely smooth and shining. Gaster superficially reticulate with interspaces smooth and shining, and scattered punctures.

Dorsal and ventral faces of head with dense short suberect and decumbent hairs, interspersed with sparse long erect hairs; antennal scape with uniform short standing hairs, and two long erect hairs near base and median of scape respectively. Dorsum of mesosoma with sparse short standing hairs, paired long erect hairs sparsely present along dorsolateral margin, with one pair each flanking the anterior margin of pronotal disc, pronotum, mesoscutum, and propodeum; mesoscutum and mesoscutal disc each one pair of long erect hairs. Lateral face of pronotum with decumbent short hairs, pilose relatively to rest of mesosoma. Petiole and postpetiole with dense short standing hairs, and sparse long erect hairs. Gaster more pilose compared to mesosoma, with dense appressed and decumbent hairs on both dorsal and ventral faces; long erect hairs sparsely present along posterior margins of sternites and tergites.

Body generally uniformly dark-brown; legs lighter brown, becoming progressively paler from femur to tarsus. Antennal funiculus and lateral surfaces of petiole, postpetiole also lighter brown. Generally darker in colour relative to worker.

Male
Wang, Yong, and Jaitrong (2021) - non-types (n = 12): EL 0.22–0.26; EW 0.18–0.22; HL 0.40–0.48; HW 0.40–0.46; ML 0.82–1.00; MsW 0.42–0.52; PtH 0.17–0.21; PtL 0.20–0.26; SL 0.08–0.09; TL 2.36–2.80; CI 92–96; PtHI 75–84; REL 55–60; SI 18–23.

Relatively large size. Head in full-face view broad ovate, somewhat narrower posteriorly, posterior margin strongly convex. Eye extremely large (20–25 ommatidia in the longest axis), posteriormost point just below transverse midline of head, outline of eye roundly convex and bulging from lateral margin of head in full face view. Median ocellus subcircular and less oval than lateral ocelli; small area of head dorsum immediately anterior to median ocellus moderately depressed. In lateral view, head broad-ovate and lobular, narrowest at ocelli, dorsum steeply sloping, lateral corners indistinct and rounded, ventral margin rounding into posterior margin as a continuous broadly convex curve. In the same view, clypeus projected strongly from anterodorsal margin of head, short dorsal clypeal face transitions to longer anterior face at weakly angular edge, anterior face weakly convex or almost flat. In lateral view, mesosoma subcylindrical and elongate, dorsal outline broadly and weakly convex, anterodorsal face of mesoscutum distinctly higher than anterior face of pronotum; propodeal junction roundly angulate, posterior propodeal declivity steep and weakly convex or flat. Propodeal dorsum rounding into posterior face at indistinct rounded edge. In posterior view, posterior propodeal face emarginate, rounding into and undifferentiated from lateral propodeal face. In dorsal view, mesosoma obovate, narrower posteriorly, notauli absent, parapsidal line weakly present. In lateral view, petiole inclined posteriorly, longer than high, petiolar node subtriangular with bluntly rounded apex, apex rounding into posterior petiolar face as a continuous convex slope, anteroventral extension of subpetiolar process reduced to a short and shallow translucent lamella. Dorsal outline of postpetiole almost entirely flat, slightly differentiated from gaster by an indistinct cinctus.

Anterior half of head dorsum rugulose-reticulate, interspaces punctate and weakly shining, rest of head mostly substriate-reticulate and shiny with scattered punctures, punctures on posterior face of head denser than rest of head; area enclosed by ocelli rugulose-punctate and weakly shining. Lateral face of mesosoma mostly superficially substriate-reticulate with interspaces smooth and shining. In dorsal view, mesoscutum largely finely substriate with interspaces smooth and shining, lateralmost area of mesoscutum in between parapsidal line and parascutellar carina unsculptured, largely smooth and shining; mesoscutal disc largely smooth and shining with feeble superficial striations; propodeum superficially reticulate with interspaces smooth and shining. Strength of sculpture on mesoscutum can vary substantially among different Thai populations, both sympatric and allopatric. Petiole, postpetiole and gaster superficially reticulate with interspaces smooth and shining.

Dorsum of head with fairly dense and uniform short standing hairs, ventral face with dense short standing and decumbent hairs. Dorsum of mesosoma with scattered short standing hairs, most dense on mesoscutum; standing hairs on propodeum limited to posterior half. Dorsums of petiole and postpetiole with dense standing hairs. Gaster pilose, with dense standing, decumbent and appressed hairs of generally uniform length.

Body generally dark greyish brown, head more blackish and darker than rest of body; antennae and joints of legs pale light brown.

References based on Global Ant Biodiversity Informatics

 * Chapman, J. W., and Capco, S. R. 1951. Check list of the ants (Hymenoptera: Formicidae) of Asia. Monogr. Inst. Sci. Technol. Manila 1: 1-327
 * Donisthorpe H. 1936. Rhopalomastix janeti (Hym. Formicidae) a species of ant new to science. Entomologist's Record and Journal of Variation 48: 55-56.
 * Wang W.Y., Yong, G.W.J. & Jaitrong, W. 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.