The following account compounds and adds on to the characters as already mentioned in Sorger and Zettel (2011). Considering the limited geographic range of specimen sampling and lack of stronger genetic evidence, this diagnosis should be deemed preliminary. The morphological and genetic bases for the treatment of this species group may be confirmed in future when more samples and comprehensive genetic information are made available for assessment. Most of the known species of the O. infandus group, excluding those from the Lesser Sundas, New Guinea and Fiji, were examined to clarify morphological characters that distinguish O. malignus and the new species from other members of the group. The O. malignus group is recognised by the following set of characteristics in the worker caste; those peculiar to the group are shown in italics.

Head rather elongate, with CI 79–82. Temporal prominences and ocular furrow distinct. Vertex posteriorly with a protuberance on each side of median furrow. Mandible with a series of teeth throughout the length of its masticatory margin; apical, intercalary and subapical teeth elongate, none apically truncate, all somewhat acute; palp formula 4, 4. Pronotal dorsum anteriorly with two or more long erect setae that are clearly discernible from short appressed or suberect hairs around them (this condition is occasionally seen in some members of the O. infandus group). Mesopleuron with anteroventral margin strongly developed, looking like a tubercle when seen from above. Petiolar spine rather short, not very sharply pointed apically. First gastral tergite not flattened dorsally, without anteromedian pit. Valviceps of male genitalia lacking dorsolateral carina.

Holotype. Worker, SINGAPORE, Sungei Buloh Wetland Reserve (1.44676N, 103.73018E), back mangrove, nest in Thalassina mound, 28 Mar 2018, W. Wang & M.S. Foo leg. ZRC_ENT00013883 (ZRC).

Paratypes. Twenty workers, 3 queens, 1 male (NHMW, SKYC, ZRC), same data as holotype, colony no. WW-SG18-Odonto1, ZRC_ENT00000917.01–24.

Singapore: 3 workers, Sungei Buloh Wetland Reserve, near Coastal Path, mangrove, nest in soil mound among roots of uprooted mangrove tree, 5 Dec 2018, W. Wang leg., colony no. WW-SG18-Odonto2, ZRC_ENT00007634 (ZRC); 4 workers, 1 queen (alate), 1 male, Sungei Buloh Wetland Reserve (1.44629N, 103.73066E), mangrove back forest, nest in abandoned Thalassina mound, 5 Dec 2018, W. Wang & M.S. Foo leg., colony no. WW-SG18-Odonto3, ZRC_ENT00007635 (ZRC); 4 workers, 2 males, Sungei Buloh Wetland Reserve (1.44632N, 103.73057E), mangrove back forest, nest in abandoned Thalassina mound, 5 Dec 2018, W. Wang, G.W. Yong & M.S. Foo leg., colony no. WW-SG18-Odonto4, ZRC_ENT00007636 (ZRC); 2 workers, Sungai [sic] Buloh Wetland Reserve, Pulau Buloh, mangrove, 7 May 2004, T.M. Leong leg. ZRC_ENT_00000779 (ZRC); 2 workers, Mandai mangroves, 1 Apr 2009, S.P. Goh and D. Pitta de Araujo leg., ZRC_ENT_00000774 (ZRC); 2 workers, Lim Chu Kang mangrove, in Thalassina mound, 1987, Serena Teo leg., ZRC_ENT_00000778. MALAYSIA: East Malaysia (Borneo), Sarawak, Bako (1.72382053N, 110.4451713E), ITZ mangrove forest, nest in rotten/alive roots of mangrove, 31 Mar 2012, D.M. Sorger leg., colony no. BOR12-098, MJ19771.

Worker. With features mentioned for the O. malignus species group. Large-sized, with moderate intranidal size variation. Head in full-face view with posterior margin weakly concave, median furrow deep and rather broad; almost entire head extensively striate to rugose, only faintly shining. Pronotum entirely with dense, fine sculpture and matte; mesonotum strigate and weakly shiny while propodeal dorsum generally more finely strigate and matte; metapleuron distinguished from propodeal dorsum by shallow longitudinal furrow; propodeal junction distinctly angular with strong transverse carina separating propodeal dorsum and declivity; posterior face of propodeum strongly marginate laterally. Basal portion of anterior face of petiolar node with strongly strigate triangular area that is rather distinctly defined; lateral face of node smooth in upper area including spine and striate in lower area. Head and petiole orange-brown; antenna, mesosoma and gaster dark reddish brown; coxae light dull-yellowish brown, femora in apical portion and tibiae orange-brown, tarsi dark brown. Legs covered with dense yellowish pubescent hairs.

Male. Body relatively smaller than that of worker and queen; body sculpture well-marked. Mandible subrectangular or quadrate. Furrow separating metapleural and propodeal bulges broad and deep, lined with transverse carinae along its length; petiolar node in anterior view broadly rounded apically and nearly truncate, largely microsculptured and matte, with only area around apex smooth. Maximum diameter of lateral ocellus slightly shorter than minimum distance between lateral ocellus and eye. Basal disc of abdominal sternite IX subpentagonal with broadly rounded posterolateral corners; posterior lobe of sternite IX almost as long as basal disc and slightly tapering apicad, with apical corner rounded but angulate; anterodorsal margin of valviceps strongly produced; dorsolateral carina of valviceps absent; ventral margin of valviceps broadly concave with ca. 23 denticles. Body almost entirely light dull-yellowish brown; areas around ocelli and pronotum somewhat darker; tibiae and tarsi darker than coxae and femora.

Worker measurements. Holotype: EL 0.65; EW 0.50; HL 3.35; HW 2.78; IFLW 0.78; MDL 2.25; PTH 1.55; PTL 0.95; SL 3.35; WL 4.80; CI 82; MDI 67; PTHI 163; SI 122. Three paratypes, 7 non-types (N = 10): EL 0.55–0.65; EW 0.40–0.50; HL 2.75–3.35; HW 2.18–2.75; IFLW 0.65–0.75; MDL 1.80–2.25; PTH 1.20–1.55; PTL 0.75–0.95; SL 2.80–3.40; WL 4.10–4.90; CI 79–82; MDI 65–70; PTHI 156–168; SI 122–130.

Queen measurements. Paratypes (N = 2): EL 0.70–0.75; EW 0.58–0.60; FWL 9.30–9.90; HL 3.35; HW 2.85–2.90; IFLW 0.85–0.90; MDL 2.20–2.30; OL 0.18–0.20; PTH 1.70–1.75; PTL 1.00–1.05; SL 3.25; WL 4.95–5.00; CI 85–87; MDI 66; PTHI 162–175; SI 112–114.

Male measurements. 1 paratype, 2 non-types (N = 3): EL 0.75–0.80; EW 0.45–0.50; FWL 6.20–6.89; HL 1.10–1.15; HW 1.45–1.50; OL 0.20–0.25; PTH 0.70–0.75; PTL 0.75–0.80; SL 0.28–0.30; WL 3.50–3.75; CI 132–135; PTHI 93–94; SI 19.

Worker. Relatively large compared to male, with moderate variation in size (HL 2.75–3.35; WL 4.10–4.90). Head in full face view with posterior margin broadly and shallowly concave; occipital carina well-developed and dark-pigmented; median furrow deep and rather broad, with a fine longitudinal carina that is stronger anteriorly; area along each side of furrow slightly swollen; vertex area in front of occipital carina with a pair of conspicuous protuberances, each located at same distance from occipital carina and median furrow (Fig. 2A, C); temporal prominence low; extraocular furrow broad and shallow; ocular ridge narrow, widened towards median furrow; frons – taken as the medial area of head dorsum posterior to frontoclypeal suture and above antennal sockets in full-face view – clearly separated from vertex especially laterally; frontal carinae very short and weak, slightly diverging posterad; distance between anterior margin of ocular ridge and anterior margin of eye around half of major axis of eye. Mandible somewhat slender; masticatory margin distinctly dentate with 10–14 teeth/denticles, dentition being often not uniform between left and right mandibles and variable among individuals; apical tooth apically acute but often worn in aged individuals, bearing sharp intercalary tooth at mid-length; subapical tooth at least 1.5 times as long as broad, apically acute in young individuals but apex often worn and truncate in aged individuals. Palp formula 4, 4. Mesosoma relatively slender (more slender in smaller workers) compared to rest of body, in lateral view constricted at mesonotum; pronotum including its anteromedian lobe (neck) rather long, in lateral view with anterior and dorsal faces continuous and weakly convex, in dorsal view with lateral margins roundly convex; mesopleuron with conspicuous carinate anteroventral ridge, which appears like a tubercle posterior to promesonotal articulation in dorsal view (Fig. 2B); mesopleuron demarcated from mesonotum and metapleuron by more or less distinct dorsal and posterior carinae respectively; metapleuron delineated from dorsum of propodeum by a shallow longitudinal furrow spanning between basalar lobe and propodeal spiracles. Propodeum in lateral view with almost straight dorsal outline, with angular and carinate junction between dorsum and declivity, and steeply sloping posterior face with carinate lateral margins (Fig. 2D). Petiolar node in lateral view (Fig. 2E) conical, distinctly tapering apically, with anterior slope excluding apical spine almost straight to very shallowly convex, posterior slope weakly convex and steeper than anterior slope, apical spine short and apically sharply pointed, sometimes slightly curved posterad; subpetiolar process subtriangular with rounded apex, slightly longer than high, with shallowly convex anterior margin and weakly concave posterior margin. In dorsal view, gaster with tergite I large, almost as long as tergites II–IV combined; anterior face of tergite I not clearly demarcated from dorsal face, short and vertical.

Head in full face view extensively striate and only faintly shiny; frons and frontal lobes with longitudinal to weakly diverging striae, with interspaces microsculptured; ocular ridge, extraocular furrow and area anterior to vertex protuberances with weaker striae; vertex lobe distinctly strigate, temple and gena very finely microsculptured and more shiny; anterolateral portion of antennal fossa with very fine striation; entire ventral face of head covered with superficial microsculpture; median disc of clypeus superficially sculptured and shiny, with feeble striation on border with frontal lobe; mandible superficially microsculptured on dorsal and ventral faces, smooth and shiny on outer face. Pronotum in dorsal view with very fine and dense striae arranged roughly concentrically, and stronger transverse striae in anterior portion including anteromedian lobe; lateral face irregularly rugulose in anterior and posterior sections, densely microcolliculate in median portion. Mesonotum densely and finely strigate, with interspaces micropunctate; mesopleuron extensively smooth to superficially sculptured and shiny, with anterior and posterior areas transversely striate; metapleuron coarsely striate with microsculptured interspaces and weakly shining. Propodeum in dorsal view mostly with dense strigae that are weaker than those on mesonotum and metapleuron; posterior declivitous face with a few coarse transverse carinae. Anterior face of petiolar node strigate in its basal triangular area; lateral face with median section striate; remainder of anterior and lateral faces superficially microsculptured and weakly shiny; posterior face smooth and shiny. Gaster largely smooth and shining, sometimes mildly pruinose but still shiny. Legs entirely covered with microsculpture and faintly shiny.

Entire dorsal, lateral and ventral faces of head covered with numerous but scattered minute standing or decumbent hairs; frons posteriorly with pair of long erect setae, which may sometimes be lost during specimen processing; mandible covered with scattered short whitish pubescent hairs, ventral face lined with multiple long yellowish setae along masticatory margin. Dorsum of mesosoma with sparse short suberect, decumbent or appressed hairs; pronotal disc with a few sparse and long erect setae. Anterior face of petiolar node covered with short appressed or decumbent hairs; posterior face without hairs. Entire gaster with sparse fine appressed pubescence and sparser long erect setae. Legs covered with fine but dense yellowish pubescent hairs; anterior and posterior faces of procoxa and ventral faces of pro- and meso-femora with sparse erect hairs; ventral face of basal segment of protarsus with dense yellowish and stiff bristle-like hairs.

Head and petiole orange-brown, mandible somewhat darker and more reddish brown; antenna, mesosoma and gaster uniformly dark reddish brown, with apical portion of gaster yellowish; legs including coxae lighter dull-yellowish brown to orange brown; tarsus darker brown than rest of leg, but colour obscured by thick yellowish pubescence.

Queen. Similar to worker in general appearance, except for characters of reproductive caste. Area around ocelli not swollen; distance between lateral ocelli slightly longer than that between lateral and median ocelli, and longer than major axis of median ocellus; with head in profile median ocellus protruding dorsad; with head seen in posterodorsal view, lateral ocelli directed laterad. Mandible slightly broader relative to head than in worker; masticatory margin strongly dentate with 12–14 denticles; dentition not equal between left and right mandibles. Mesosoma in dorsal view (Fig. 3B) subcylindrical and stout, broadest around wing bases, not constricted at mesothorax. Pronotum in dorsal view with lateral margins less convex than in worker, subparallel in their posterior half; in lateral view pronotum with almost straight dorsal outline; mesonotum in dorsal view large and round; mesoscutum anteriorly roundly convex, delimited from pronotum by a deep sulcus, in posterior half margined laterally with sharp carinae that are close to forewing bases; deep furrows present between the carinae and mesoscutal disc; median line weak; notauli absent; parapsidal line distinct and reaching midlength of mesoscutal disc; mesoscutum separated from mesoscutellum by rather broad (anteroposteriorly) scutoscutellar sulcus that is laterally margined with distinct carinae; mesoscutellum much narrower and smaller than mesoscutum, broader than long, delineated from metanotum by a sharp sulcus; lateral face of pronotum posteriorly demarcated from mesopleuron by indistinct faint groove; mesopleuron distinctly divided into “anepisternum” and “katepisternum” by shallow oblique median furrow, ventrally with a carina separating main katepisternal area from narrow area right above mesocoxa (Fig. 3C). Metanotal disc much smaller than mesoscutellum, broader than long, narrowed posterad; metapleuron demarcated from mesopleuron by a fine sulcus; upper metapleural area not clearly separated from lower metapleural area, much smaller than the latter; lower metapleural area with conspicuous endophragmal pit just below upper section; spiracular sclerite distinct. Propodeum in dorsal view delineated from metanotum by distinct sulcus, gradually narrowed posterad; dorsal face evenly rounded into lateral face, which is separated from metapleuron by a shallow longitudinal furrow; posterior propodeal declivity dorsally margined with strong transverse carina, in posterior view broader than high. Petiole and gaster similar to worker (Figs 3D, E).

Head in full-face view extensively striate and similarly sculptured as in the worker, but with interspaces generally more weakly microsculptured than in the latter. Pronotum dorsally with dense transverse striae with shiny interspaces; lateral face densely and more irregularly striate, with interspaces punctate and weakly shiny; mesonotum longitudinally and rather regularly striate with interspaces punctate; “anepisternum” largely finely striate with posteroventral area smooth; oblique median furrow separating “anepisternum” from “katepisternum” weakly scrobiculate; “katepisternum” extensively minutely punctate and weakly shiny, posteriorly with coarse transverse rugae; scutoscutellar sulcus strongly scrobiculate; mesoscutellar disc largely smooth and shiny. Propodeum except for declivity transversely and regularly striate with interspaces punctate and weakly shiny; declivitous face with several widely spaced and strong transverse carinae. Anterior basal triangular area of petiolar node transversely striate with interspaces minutely punctate and weakly shiny; lateral face of node smooth and shiny except for median section with longitudinal rugae; entire petiolar spine and posterior face of node smooth to very superficially sculptured and shiny. Gaster largely smooth and shiny.

Head excluding mouthparts almost lacking erect setae; only ocellar region with pair of long erect setae; upper frons with pair of long erect hairs; entire dorsal, lateral and ventral faces of head with scattered minute standing hairs. Mandible covered with scattered fine appressed hairs; masticatory margin ventrally lined with multiple long yellowish setae. Dorsum of mesosoma with sparse short erect hairs; pronotum posteriorly bearing at least two long erect setae; propodeum with fine standing hairs mainly on dorsolateral and posterior margins but dorsum almost hairless. Petiolar spine, anterior and lateral faces of petiolar node with fine short decumbent or appressed hairs; anterior face slightly more pilose than lateral; posterior face of node without hairs. Entire gaster with scattered fine appressed pubescent hairs, and numerous but scattered long yellowish setae. Legs covered with fine but dense yellowish pubescence; ventral and lateral faces of procoxa with sparse suberect hairs; coxae and femora of mid- and hind-legs with fewer and shorter suberect hairs; ventral face of basal segment of protarsus with dense bristle-like and yellowish hairs.

Overall body colouration much as in worker. Head and petiole dark orange-brown, head sometimes more blackish in tone; mandibles slightly darker and more reddish brown; mesosoma and gaster dark reddish brown; coxae and femora yellowish to orange brown; rest of legs more reddish brown, increasingly darker towards tarsi.

Tegula distinct, elongate-ovate, much longer than broad. Wings slightly infuscate, entirely and finely setose. Forewing (Fig. 3F) with costal vein tubular to large and conspicuous pterostigma; costal, basal and subbasal cells closed; marginal cell 1, submarginal cells 1 and 2 closed; medial vein consistently strong and tubular from base to lateral wing margin. Cross-vein 1m-cu present, discal cell 1 and subdiscal cell 1 closed; cubital vein consistently tubular almost throughout entire length but fading out slightly closer to lateral wing margin. Hindwing (Fig. 3G) with jugal lobe present; basal and subbasal cells closed; basal cell roughly divided into two halves by a longitudinal spectral vein; basal one-third of radial vein (anterior margin of hindwing) lined with a series of short, stiff hamuli. With wing positioned on the same plane as body, hamuli projecting distally and dorsoventrally upwards.

Male. Body smaller than workers and queens (HL 1.10–1.15, WL 3.5–3.75), rather robust. Head in full-face view (Fig. 4A) much broader than long when including compound eye (hereafter simply termed ‘eye’), with roundly convex posterior margin; in lateral view dorsal outline convex around antennal insertion and vertex rather steeply sloping to occipital carina (Fig. 4B); in dorsal view strongly narrowed posterad (Fig. 4C); occipital carina thin and low, continuing anteroventrad but weakening slightly beyond midlength of lateral face of head. Clypeus posteriorly weakly demarcated from frons, gradually widened anterad, broadly produced anterad with anterior margin shallowly emarginated, with distance between anterior tentorial pits longer than that between antennal bases. Mandible reduced, subrectangular, either edentate or with 1–2 minute teeth on masticatory margin. Eye large (EL 0.75–0.80; EW 0.45–0.50), with head in full-face view bulging, breaking lateral (outer) margin of head (Fig. 4A), inner margin weakly emarginate; distance between eye and mandibular base as long as antennomere 2 (pedicel); with head in obliquely posterior view, outer margin broadly emarginate. Ocelli large, positioned on raised bases; major axis of median ocellus equal to or slightly shorter than minimum distance between lateral ocelli; maximum diameter of lateral ocellus slightly shorter than minimum distance between lateral ocellus and eye. Antenna 13-merous; antennomere 1 (scape) short, less than 1/3 as long as and slightly wider than flagellomere 1; pedicel slightly shorter than 1/2 of length of scape; flagellomeres each long, narrow and subcylindrical. Palp formula 6, 4; basal palpomere of maxillary palp very short and apical palpomere longest; labial palp with third palpomere shortest. Mesosoma elongate, much longer than high, and also longer than dorsolaterally broad. Pronotum with very short dorsal plane, in lateral view anterior slope rather steep; lateral face large, weakly widened posterad. Mesosoma in dorsal view (Fig. 4E) with mesoscutum almost as long as wide, with strongly convex anterior margin; median line weakly present in anterior half of mesoscutellar disc, in anteriormost portion branching into two short diverging carinae; notauli present as broad but shallow depressions in anterior half of mesoscutum but almost unrecognisable in its posterior half; parapsidal sulcus distinct, starting from posterolateral corner of disc to midlength of disc; scutoscutellar sulcus distinct, widened laterally; mesoscutellum much narrower than mesoscutum, broader than long, with shallow median furrow; in lateral view dorsal outline of mesoscutum largely flat with short anterior slope; mesoscutellum strongly convex dorsally; mesopleuron divided into “anepisternum” and “katepisternum” by broad but shallow furrow that widens ventrad; “katepisternum” with posteroventral carina near metacoxa. Metanotal disc small, anteroposteriorly short (narrow), well-demarcated from mesoscutellum and propodeum by sharp sulci; metapleuron divided into upper and lower areas by shallow furrow; upper area with ill-defined depression posteriorly; spiracular sclerite distinct; metapleural gland orifice occluded, but partially margined with thin walls. Propodeum delimited from metapleuron by shallow furrow, in dorsal view slightly longer than broad; propodeal junction round; posterior propodeal declivity laterally and dorsally margined with discontinuous carinae; spiracle located close to metapleuron, with its opening directed posteriorly; propodeal lobe small but distinct. Petiole (abdominal segment II) (Fig. 4F) sessile, in lateral view almost as long as high; anteriorly margined with visually-pigmented carinae; node in anterior view narrowed apicad with broadly rounded apex, in lateral view subtrapezoidal and posteriorly inclined, posterior slope shorter and steeper than anterior slope; anteroventral subpetiolar process triangular. Gastral segment I longest, in dorsal view gradually widened posterad. Legs somewhat long in proportion to mesosoma, femora longer than tibiae; meso- and meta-tibiae each with two apicoventral spurs; pretarsal claw with an inner dent; arolium relatively small compared to pretarsal claws.

Head extensively weakly and superficially sculptured, generally shiny; clypeus densely microsculptured and matte; mandible, antennal scape and pedicel superficially sculptured and shiny; entire flagellum densely microsculptured and matte. Mesosoma extensively weakly sculptured and shiny; mesoscutellum and narrow median section of metapleuron smoother and shiny; scutoscutellar sulcus longitudinally striate; shallow furrow separating metapleuron from propodeum with many strong transverse carinae across its length. Propodeum extensively very densely microsculptured, with anterior portion of dorsum strigate. Petiolar node extensively weakly sculptured, but smooth and shiny around apex. Gaster smooth to very superficially sculptured and shiny. Coxae nearly entirely smooth and shiny; femora weakly sculptured and shiny; tibiae slightly more strongly sculptured and less shiny; tarsi with dense microsculpture and matte.

Dorsum of head densely covered with short suberect hairs; longer hairs sparsely present around ocelli; ventral face of head covered with much shorter near-appressed hairs; mandible basally with some suberect hairs and apically a few longer hairs; scape with appressed hairs; pedicel and flagellum densely covered with short pubescence. Mesosoma and petiole almost entirely covered with short appressed to suberect or erect hairs; legs entirely covered with appressed pubescence. Gastral tergites and sternites with appressed to decumbent short hairs and very sparse longer hairs that are slightly denser near the posterior margin of each tergite.

Almost entire body light dull-yellowish brown; areas around ocelli and pronotum somewhat darker; areas where notauli occur present as pale-pigmented bands’; tibiae and tarsi darker than coxae and femora.

Tegula distinct, suboval, longer than broad. Wings (Fig. 4I, J) more weakly infuscate than in the queen, entirely and finely setose. Both forewing and hindwing venation and other characters much as in those in the queen.

Posterior spine of abdominal tergite VIII rather short and robust in lateral view (Fig. 4H). Pygostyle elongate-digitiform with long hairs in its apical two-thirds (Fig. 5A). Basal disc of abdominal sternite IX subpentagonal, a little broader than long; lateral margin weakly and broadly convex, slightly converging posterad with rounded posterolateral corner (Fig. 5B). Posterior lobe of sternite IX about as long as basal disc and slightly tapering apicad with apical margin almost truncated but slightly convex. Carina of lower basimere (BmC in Fig. 5C) distinct. Dorsal margin of telomere feebly concave; telomeral apex strongly produced (Fig. 5C). Ventral apex of digitus somewhat acute in lateral view (Fig. 5C). Apex of cuspis angular in lateral view. Distiventral apex relatively narrow compared to rest of valviceps in lateral view; dorsolateral carina absent; subapical lamina narrow and long; diagonal sclerotisation forming a short semi-ellipse; anterodorsal margin of valviceps strongly produced; ventral margin of valviceps broadly concave with ca. 23 denticles (Fig. 5D).

Singapore, Borneo (Sarawak).

This species inhabits the mangroves, and nests have been found in abandoned mud lobster (Thalassina) mounds located near back forest. It has also been found foraging near mudflats and in mangrove backforest.

The species name alludes to the littoral intertidal zone where this species is exclusively found.

The worker of the new species O. litoralis (OL-w) is similar to O. malignus workers from Singapore (OMSG-w) and the Philippines (OMPH-w), but has consistently stronger body sculpture and generally darker body colour than the latter. Some important morphological differences are as follows: 1) Propodeal junction in OL-w is strongly angulate, with the dorsum separated from the declivity by a strong transverse carina, whereas in OMSG-w and OMPH-w the propodeal junction is round with a much weaker carinate edge. 2) Propodeal declivity in OL-w is distinctly marginate laterally with raised ridges and clearly differentiated from the lateral faces of propodeum, while in OMSG-w and OMPH-w it is not distinctly margined laterally, instead rounding into the lateral faces. 3) Metapleuron in OL-w is delineated from propodeum by a shallow but broad longitudinal furrow spanning between the basalar lobe and propodeal spiracle, while in OMSG-w and OMPH-w the furrow is very weak or not recognised, thus the metapleuron is not clearly distinguished from propodeum. 4) Sculpture on the lateral face of pronotum in OL-w is always relatively coarser than that on the dorsal face and less shiny, while in OMSG-w and OMPH-w, sculpture on the lateral face of pronotum is either similar to or weaker than that on the dorsal face, with interspaces mostly smooth and shining. 5) Mandible in OL-w is longer (albeit slightly) relative to head length as compared to that of OMSG-w and OMPH-w (i.e., OL-w MDI 65–70, OMSG-/OMPH-w MDI 63–64). 6) Mesosoma is uniformly dark reddish brown in OL-w, but the pronotum has a lighter shade of brown compared to the rest of the mesosoma in OMSG-w and OMPH-w. 7) In OL-w the anterior and lateral faces of petiolar node are generally more matte with stronger superficial sculpture, while in OMSG-w and OMPH-w these are mostly smooth and shining, with only few rugae in the median section of the lateral face. Condition of sculpture on the lateral petiolar face, however, may vary between different geographic populations of the same species, for example OL-w from Sarawak has a smoother and shinier lateral petiolar face compared to OL-w from Singapore.

Males of the two species (i.e., OL-m, OMSG-m) are also morphologically similar to each other, but OL-m has consistently stronger body sculpture than OMSG-m. Males of the two species can be distinguished based on the following characters: 1) Mandible is subrectangular or quadrate in OL-m, while it is falcate and apically bluntly pointed in OMSG-m. 2) In OL-m, furrow separating the metapleuron from propodeum is broad and deep, lined with transverse carinae along its length, whereas in OMSG-m the furrow is shallow, though also lined with transverse carinae along its length. 3) Petiolar node in anterior view is apically broadly rounded in OL-m, but sharply or bluntly pointed in OMSG-m. 4) Petiolar node in OL-m is largely microsculptured and matte, with only area around the apex smooth, while in OMSG-m, the petiolar node is almost entirely smooth to superficially sculptured and shiny. 5) Maximum diameter of lateral ocellus in OL-m is slightly shorter than the minimum distance between lateral ocellus and eye, whereas it is equal to or slightly longer than the same distance for OMSG-m.

In addition, male genitalia of the two species may be differentiated using the following traits: 1) Basal disc of abdominal sternite IX is subpentagonal with broadly rounded posterolateral corners in OL-m, but with angular posterolateral corners in OMSG-m. 2) Posterior lobe of abdominal sternite IX is almost as long as basal disc, slightly tapering apicad with almost truncated apex, apical corners rounded but angulate in OL-m, but a little longer than disc, with almost parallel sides, and apex broadly and weakly convex in OMSG-m. 3) Anterodorsal margin of valviceps is strongly produced in OL-m, but weakly produced in OMSG-m. 4) Ventral margin of valviceps is broadly concave with around 23 denticles in OL-m, but undulate with ca. 33 denticles in OMSG-m.

SINGAPORE: 1 worker, Lim Chu Kang mangrove, 23 Sep (23/9-1), D.H. Murphy leg., ZRC_HYM0000902 (ZRC); 8 males, Pulau Semakau Old Fragment, 1.20664N, 103.76044E, mangrove, malaise trap SMO3, Jul-Nov 2012, M.S. Foo, P. Grooteart & J. Puniamoorthy leg., ZRC_BDP0014432/14442/14515/14516/14535/14676/14677/14712 (ZRC); 1 male, Pulau Semakau Old Fragment, 1.20489N, 103.76047E, mangrove, malaise trap SMO1, 30 Aug 2012, M.S. Foo, P. Grooteart & J. Puniamoorthy leg., ZRC_BDP0014733(ZRC); 1 male, Pulau Semakau New Fragment, 1.20125N, 103.76281E, replanted mangrove, malaise trap SMN2, 13 Dec 2012, M.S. Foo, P. Grooteart & J. Puniamoorthy leg., ZRC_BDP0016086 (ZRC). PHILIPPINES: 1 worker, Luzon, Batangas, Mabini, Mainit, 8.1.2013, C.V. Pangantihon leg., P467.

Worker. With features mentioned for the O. malignus species group. Body sculpture much weaker and coloration somewhat paler than in O. litoralis. Head in full-face view with posterior margin weakly concave; median furrow deep and rather broad; head extensively and finely (often indistinctly) striate, frontal lobes and frons with stronger striae; frontal carinae short and only slightly divergent posterad. Sculpture on pronotum much weaker than in O. litoralis; separation of metapleuron from propodeum indistinct; propodeal junction not strongly angulate, showing rather round transition from dorsum to declivity, declivity laterally only weakly marginate. Petiolar node extensively smooth and shiny. Head and petiole largely dark orange-brown; mandible, antenna and gaster slightly darker brown; mesosoma disc dark reddish brown.

Male. Body relatively smaller than that of worker, body sculpture not consistently strong. Mandible falcate, apically bluntly pointed. Furrow separating metapleuron from propodeum shallow; petiolar node in anterior view with sharply or bluntly pointed apex, almost entirely smooth to superficially sculptured and shiny. Maximum diameter of lateral ocellus equal to or slightly longer than minimum distance between lateral ocellus and eye. Body almost entirely light yellowish brown with darker greyish blotches present on head and mesosomal dorsum. Basal disc of abdominal sternite IX subpentagonal with angular posterolateral corners; anterodorsal margin of valviceps weakly produced; dorsolateral carina of valviceps absent; ventral margin of valviceps undulate with ca. 33 denticles.

Worker measurements. Holotype (CASENT0901334): EL 0.55; EW 0.45; HL 2.80; HW 2.30; IFLW 0.71; MDL 1.71; PTH 1.15; PTL 0.70; SL 2.86; WL 4.09; CI 82; MDI 59; PTHI 164; SI 119. Non-types (N = 6; values of PTH, PTL, SL, WL, PTHI, SI obtained from 5 out of 6 specimens – previous syntype of O. tuberculatus excluded from measurement due to extensive damage): EL 0.46–0.60; EW 0.29–0.40; HL 2.68–3.31; HW 2.15–2.73; IFLW 0.65–0.81; MDL 1.57–1.78; PTH 1.05–1.25; PTL 0.64–0.80; SL 2.80–3.08; WL 3.99–4.33; CI 80–82; MDI 54–64; PTHI 156–164; SI 127–130.

Male measurements. 3 non-types (N = 3): EL 0.95–0.96; EW 0.55–0.58; FWL 7.20–7.60; HL 1.30–1.36; HW 1.68–1.75; OL 0.27–0.28; PTH 0.88–0.96; PTL 0.70–0.75; SL 0.25 ; WL 3.51–3.65; CI 128–129; PTHI 123–128; SI 14–15.

(based on holotype, Singapore and Philippine specimens). Relatively large compared to male (HL 2.70–2.80; WL 3.95–4.00). Head in full face view (Figs 6A, 7A) longer than broad, with posterior margin weakly and broadly concave; occipital carina well-developed and dark pigmented; median furrow deep and rather broad, not much darker than rest of head dorsum; bottom of furrow with weak median carina that is stronger in its anterior portion; area along each side of furrow slightly swollen; vertex posteriorly with pair of low protuberances, each located at same distance from median furrow and occipital carina; temporal prominence low; extraocular furrow shallow; ocular ridge distinctly elevated, narrow, slightly widened toward median furrow; frons somewhat distinctly differentiated from vertex especially laterally; frontal carinae very short, diverging slightly posterad, fading out into ordinary carinae on frons; distance between anterior margin of ocular ridge and anterior margin of eye subequal to major axis of eye. Mandible rather slender; masticatory margin distinctly dentate with 11–12 denticles; dentition may not be equal between left and right mandibles; subapical tooth ca. 1.5–2 times as long as broad, tapering with somewhat acute apex (but not sharply pointed); apex sometimes worn and truncate. Palp formula 4, 4. Mesosoma in lateral view (Figs 6D, 7D) relatively slender compared to rest of body, constricted at mesonotum; pronotum including its anteromedian lobe (neck) rather long, in lateral view with gently sloping anterior face that is continuous to dorsal face, in dorsal view with roundly convex lateral margins; mesopleuron with carinate anteroventral ridge, which in dorsal view appears as a weak protuberance, (mesopleuron) demarcated by a distinct continuous sulcus dorsally from mesonotum and posteriorly from metapleuron (sulcus not margined with distinct carina); metapleuron delineated from propodeum by indistinct shallow furrow; dorsal outline of propodeum in lateral view flat to very shallowly concave; propodeal dorsum separated from declivity by weak carina with junction rounded and not strongly angulate; propodeal declivity in posterior view only weakly margined laterally. Petiolar node (Figs 6E, 7E) conical with pointed apical spine, in lateral view anterior slope almost entirely straight or weakly convex, posterior slope weakly convex and slightly steeper than anterior slope; apical spine relatively short compared to petiolar height, in frontal view gradually tapering with broad base and blunt apex, entire spine usually upright and not strongly directed posterad; subpetiolar process subtriangular with rounded apex, almost as long as high. Gastral tergite I large, in dorsal view as long as tergites II–IV combined, in lateral view with anterior slope short and vertical.

Head in full face view densely striate in most parts; frontal lobes and frons with strong striae that are longitudinal to diverging, interspaces mostly smooth and shining; anterolateral area of antennal fossa smooth and shiny; vertex with much finer striae that are often indistinct; vertex lobe, temple and lower gena with faint striation, partly micropunctate, slightly shiny; median disc of clypeus largely sculptured superficially and shiny. Dorsum and venter of mandible finely and superficially sculptured and rather shiny, lateral face smooth and shiny. Pronotum in dorsal view densely micropunctate to microreticulate with weak lustre, in lateral view irregularly striate in its anterior and posterior portions; anteromedian lobe (neck) with coarse strigae; mesonotum in dorsal view coarsely strigate with interspaces weakly microsculptured; mesopleuron densely sculptured and matte in its anterodorsal portion (upper 1/5), finely punctate but rather shiny in remaining portions, with anteriormost and posteriormost parts striate; metapleuron with sculpture similar to that of mesonotum, but with interspaces distinctly punctate. Propodeal dorsum with strigae similar to those on mesonotum; propodeal declivity with strong transverse striae that are more widely separated from each other than on propodeal dorsum. Anterior face of petiolar node mostly smooth and shiny, basally superficially sculptured with weak strigae, lateral face largely smooth and shining with median section bearing short striae, posterior face smooth and shining. Gaster mostly smooth and shining.

Head largely covered with sparse short pubescent hairs; frons posteriorly with a pair of long erect setae; mandible with scattered appressed pubescence, masticatory margin ventrally lined with multiple long yellowish setae. Dorsum of mesosoma with sparse appressed or decumbent pubescent hairs, pronotal disc with a few sparse long erect hairs. Anterior face of petiolar node with short appressed or decumbent pubescent hairs, relatively more pilose than mesosoma; posterior face of node without hairs. Entire gaster with sparse fine appressed pubescence and scattered long erect setae. Legs including coxae largely with fine but dense pubescence; ventral faces of coxae with a few sparse long erect hairs; ventral face of basal segment of protarsus with dense yellowish and stiff bristle-like hairs.

Head and petiole dark orange-brown; mandible, antenna and gaster slightly darker brown, mesosoma generally darker reddish brown; legs including coxae lighter brown and more yellowish; tarsus darker brown than rest of leg.

Male (based on Singapore specimens). Body smaller than worker (HL 1.30–1.36; WL 3.51–3.65), relatively robust compared to worker. Head in full-face view including eyes much broader than long, with roundly convex posterior margin (Fig. 8A); in lateral view dorsal outline convex around antennal insertion, and vertex rather steeply sloping to occipital carina (Fig. 8B); in dorsal view head strongly narrowed posterad (Fig. 8C); occipital carina thin and low, continuing anteroventrad, becoming more indistinct slightly beyond midlength of lateral face of head. Clypeus in full-face view transverse with median disc distinctly raised and flat, broadly produced anterad, with median part of anterior margin shallowly emarginate, posteriorly weakly demarcated from frons. Mandible reduced, elongate-triangular to falcate, distinctly tapering apicad with bluntly pointed and dark-pigmented apex, without teeth on masticatory margin. Eye large (EL 0.95–0.96; EW 0.55–0.58) , with head in full-face view (Fig. 8A), bulging and breaking lateral (outer) margin of head, inner margin weakly emarginate; distance between eye and mandibular base roughly as long as antennomere 2 (pedicel); with head in obliquely posterior view, outer margin broadly emarginate; ocelli large, positioned on raised bases; major axis of median ocellus equal to or slightly shorter than minimum distance between lateral ocelli; maximum diameter of lateral ocellus equal to or slightly longer than minimum distance between lateral ocellus and eye. Antenna 13-merous; antennomere 1 (scape) short, less than 1/3 as long as and slightly wider than flagellomere 1; pedicel slightly shorter than half of scape-length; flagellomeres each long, narrow and subcylindrical. Palp formula 6,4; palpomere 1 of maxillary palp very short, palpomere 6 longest; labial palp with palpomere 3 shortest. Mesosoma elongate, much longer 'much longer than high and than broad (Fig. 8D, E). Pronotum with very short dorsal plane, in lateral view anterior slope rather steep; lateral face large, weakly widened posterad, with posteroventral area distinctly depressed and concave. With mesosoma in dorsal view (Fig. 8E), mesoscutum almost as long as broad, with strongly convex anterior margin; median line only recognised in anterior half of disc, in its anterior-most portion widened anterad and margined with short carinae; notauli present as broad but shallow depressions in anterior half of mesoscutum but almost unrecognisable in its posterior half; parapsidal sulcus distinct, extending from posterolateral corner to midlength of disc; scutoscutellar sulcus distinct and widened laterally; mesoscutellum much narrower (anteroposteriorly) than mesoscutum, broader than long. In lateral view, dorsal outline of mesoscutum largely flat with short anterior slope; dorsum of mesoscutellum strongly convex; mesopleuron divided into “anepisternum” and “katepisternum” by broad and rather deep furrow that widens ventrad, leading into a concave medioventral depression; “katepisternum” with posteroventral carina near mesocoxa; metanotal disc small, short anteroposteriorly (narrow), well-demarcated from mesoscutellum and propodeum by sharp sulci; metapleuron divided into upper and lower areas by shallow furrow; upper metapleuron posteriorly with ill-defined depression; spiracular sclerite distinct; metapleural gland orifice partially margined with thin carina. Propodeum delineated from metapleuron by weak narrow sulcus, in dorsal view (Fig. 8E) slightly longer than broad; propodeal junction in lateral view rounded, posterior declivity laterally margined with weak and discontinuous carinae; transverse carina on propodeal junction obscure; propodeal spiracle located close to metapleuron, with its opening directed posteriorly; propodeal lobe small but distinct. Petiole (abdominal segment II) (Fig. 8F) sessile, about as long as high, basally margined with a pigmented carina; petiolar node tapering apicad with bluntly pointed apex; in lateral view, posterior slope shorter and steeper than anterior slope. Gaster in dorsal view broadest at segment III (abdominal segment V); gastral segment I longest, gradually widened posterad. Legs rather long in proportion to mesosoma; femora longer than tibiae; meso- and metatibiae each with two apicoventral spurs; pretarsal claw with one inner dent; arolium small.

Head extensively weakly and superficially sculptured, generally shiny; clypeus largely densely microsculptured and matte, supraclypeal area with weaker sculpture and more shiny; mandible, antennal scape and pedicel superficially sculptured and shiny; entire flagellum densely microsculptured and matte. Mesosoma extensively weakly sculptured and shiny; “katepisternum” smoother and shiny in its anterior half; mesoscutellum largely smooth and shiny; median section of metapleuron striate, or relatively smoother than other parts of metapleuron and shiny (variable among individuals); scutoscutellar sulcus longitudinally striate and shiny; shallow furrow delineating metapleuron from propodeum with many strong transverse carinae across its length. Propodeum extensively densely microsculptured, with anterior portion of dorsum strigate, dorsolateral area lined with coarse rugae. Petiolar node largely smooth and shiny. Gaster smooth to very superficially sculptured and shiny. Coxae and femora nearly entirely smooth and shiny; tibiae weakly sculptured and less shiny; tarsi densely microsculptured and weakly shining.

Dorsum of head densely covered with short suberect and erect hairs; longer standing hairs present around ocelli; clypeus and lower frons with longer hairs; ventral face of head densely covered with much shorter standing hairs; ventral edge of mandible with some long erect hairs; scape covered with short decumbent hairs, with slightly longer hairs apically; pedicel and flagellum covered with dense and short pubescence. Mesosoma and petiole almost entirely covered with short appressed to suberect or erect hairs, with a few sparse long erect hairs on pronotum; legs mostly covered with dense appressed pubescence. Gastral tergites and sternites with appressed to decumbent short hairs, and very sparse longer hairs that are largely patchily distributed but more dense nearer posterior margin of each tergite; hair covering sternites generally more dense than that on tergites.

Body almost entirely light yellowish brown; darker greyish blotches present on head and mesosomal dorsum; areas where notauli occur present as pale-pigmented bands; coxae and femora uniformly pale yellowish brown, tibiae less pale, tarsus darker brown than rest of leg, though actual colour often obscured by dense yellowish pubescence.

Tegula distinct, suboval, longer than broad. Wings not infuscate, entirely and finely setose. Forewing (Fig. 8I) with costal vein tubular to large and conspicuous pterostigma; costal, basal and subbasal cells closed; marginal cell 1, submarginal cells 1 and 2 closed; medial vein fading off distally into more spectral state towards lateral wing margin after vein 2r-m. Cross-vein 1m-cu present, discal cell 1 and subdiscal cell 1 closed; cubital vein fading off distally into more spectral state after vein 2cu-a. Hindwing (Fig. 8J) with jugal lobe present; basal and subbasal cells closed; basal cell roughly divided into two halves by longitudinal spectral vein; basal one-third of radial vein (on anterior margin of hindwing) lined with a series of short, stiff hamuli; with wing positioned on the same plane as body, hamuli directed apicad (upwards).

Posterior spine of abdominal tergite VIII rather long and slender in lateral view (Fig. 8H). Pygostyle elongate-digitiform with long hairs in its apical three-quarters (Fig. 9A). Basal disc of abdominal sternite IX subpentagonal, a little broader than long; lateral margin almost straight and slightly diverging posterad; posterolateral corner angulate (Fig. 9B). Posterior lobe of sternite IX a little longer than basal disc, slightly tapering posterad or having almost parallel sides with apical margin weakly and broadly convex. Carina of lower basimere (BmC in Fig. 9C) weak and barely recognised. Dorsal margin of telomere weakly concave; telomeral apex weakly produced (Fig. 9C). Distiventral apex of digitus right-angled in lateral view (Fig. 9C). Apex of cuspis rounded in lateral view. Distiventral apex relatively broad compared to rest of valviceps in lateral view (Fig. 9D); dorsolateral carina absent; subapical lamina narrow and long; diagonal sclerotisation forming a short semi-ellipse; anterodorsal margin of valviceps just weakly produced; ventral margin of valviceps undulate with ca. 33 denticles (Fig. 9D).

Asia – Borneo (Sarawak), Indonesia (Maluku, Sulawesi (Celebes)), Philippines, Singapore; Oceania – Palau, Papua New Guinea, Solomon Islands.

Intertidal littoral areas; workers of this species have been observed foraging in and around or emerging from exposed nest entrances in coral rubble (Brown 1976) or sheer vertical bare limestone rockfaces (Olsen 2009) along coastlines during low tides. Males were collected from mangrove forest using malaise traps, on an offshore island in the southern part of Singapore.

The workers of O. malignus from Singapore (OMSG-w) and the Philippines (OMPH-w) are similar to that of O. litoralis (OL-w), but may be distinguished from the latter by the characters listed in ‘Remarks’ under O. litoralis. Minor but non-negligible morphological differences between the O. malignus holotype (OMH), synonymised forms (e.g., O. retrolatior (OR-w)), OMSG-w and OMPH-w are discussed in the ‘Results’ section.