Rhopalomastix johorensis Wheeler W.M., 1929

Wang, Wendy Y., Yong, Gordon W. J. & Jaitrong, Weeyawat, 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 (3), pp. 301-340 : 316-322

Rhopalomastix johorensis Wheeler W.M., 1929 stat. n.

( Figs. 24–39 )

Rhopalomastix rothneyi subsp. johorensis Wheeler, W.M., 1929: 96

Rhopalomastix janeti Donisthorpe, 1936: 55 (w.q.m). syn. n.

Types. Rhopalomastix rothneyi subsp. johorensis . Three syntype workers (on 1 pin) from Singapore (Overbeck) (MCZ, MCZ_ENT00023066, examined). One worker (top on pin) is selected as the lectotype. Rhopalomastix janeti . Holotype and paratype workers (BMNH, USNM) from Thailand, Bangkok (A. Manjikul). Paratype workers (USNM, USNM_ENT00529551) were examined. New synonym.

Diagnosis. Workers. 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 and gentle median depression. In profile, clypeus projected slightly forward from dorsal margin of head, almost entirely flat or straight; dorsum and lateral surfaces of head with fine and dense striations; 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; angle of propodeal junction in profile obtuse and rounded, posterior propodeum face steeply sloped. 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, tonal patterns identical.

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. Dorsal view, anterior pronotal margin angulate at median point, posterior propodeal margin weakly convex; posterior face of propodeum rounded and smooth. Anterior half of head striated; posterior half mostly smooth and shining. Entire dorsum of mesosoma finely striated and shiny; lateral surface of mesosoma striate-reticulate, interspaces smooth and shining. Overall darker blackish brown as compared to worker, tonal patterns identical.

Male. Relatively large size (non-types: HL 0.44, HW 0.4–0.42). Head in full face view broad ovate; clypeus in profile weakly convex and almost flat anteriorly, rugulose and weakly shining; petiole subtriangular, apex blunt; dorsal outline of postpetiole almost entirely flat, cinctus between postpetiole and gaster indistinct. Anterior half of head dorsum rugulose-reticulate, interspaces punctured and weakly shining, rest of head substriate-reticulate and shiny, with scattered punctures; body mostly superficially substriate-reticulate with smooth and shining interspaces. Body colour generally dark grey-brown, head blackish; joints of legs and antennae paler in tone.

Worker measurements. Three 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.

Eighteen 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.

Queen measurements. Three non-types (n = 3, 1 alate, 2 dealate): TL 2.53–3.1; HL 0.54–0.56; HW 0.51– 0.53; SL 0.2–0.22; ML 0.8–0.88; MsW 0.42–0.46; EL 0.16–0.18; EW 0.12–0.13; CI 94–96; REL 31–35; SI 38–43.

Male measurements. Two non-types (n = 2): TL 2.4-2.53; HL 0.46-0.48; HW 0.44; SL 0.08; ML 0.88-0.92; MsW 0.52; EL 0.25-0.26; EW 0.2-0.21; CI 92–96; REL 57–59; SI 18.

Redescription of Worker (lectotype, paralectotypes and non-types). Workers monomorphic with broad size variation. Head in full face view sub-rectangular, clearly longer than broad (only slightly in smaller worker), posterolateral corners smooth and 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 absolute profile, clypeus projecting forward slightly from dorsal margin of head, flat or weakly convex for most of its length. Antennal scape short and thin, extending slightly beyond midlength of head when lain backwards. Mesosoma in profile box-shaped, dorsal outline weakly convex, propodeal junction curved and obtuse, posterior face of propodeum shallowly concave and steeply sloped; mesopleuron demarcated from lateral face of pronotum and propodeum by distinct sutures. In dorsal view, mesosoma subrectangular, anterior pronotal margin broadly convex, sometimes slightly angulate at median point in large worker, humeral corners obtuse and rounded; lateral sides almost parallel and straight, not much narrower posteriorly than anteriorly, posterior margin of propodeum weakly concave. In profile, petiole inclined posteriorly, anterior margin weakly concave, slightly higher than broad, petiolar node bell-shaped, apex of node convex and blunt, about as high as post-petiole; dorsal margin of postpetiole in profile weakly convex. In profile, 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 post-petiole oblong or elliptical, both broader than long; post-petiole larger and wider than petiole.

Dorsal and lateral surfaces of head striated but shining; clypeal surface also striated, mandible mostly smooth and shining, with weak basal striations; ventral surface of head striate-reticulate and shining. Lateral surfaces of mesosoma substriate-reticulate and shiny, sculpture coarser closer to dorsal surface, interspaces punctate and weakly shining. Petiole and postpetiole superficially reticulate, interspaces mostly smooth and shining, apices also weakly striated. Gaster largely superficially reticulate with smooth and shining interspaces, scattered punctures.

Head and antenna covered with a mix of short standing and suberect hairs; sparse longer standing hairs present mainly on the clypeus, mandible and antennal scape. Mesosoma with abundant but scattered short standing hairs and sparse longer erect hairs around dorsal margin. Dorsal surface of petiole, postpetiole each with abundant short standing hairs of nearly uniform length, with a pair of long erect hairs. Dorsal and ventral surfaces of gaster covered with abundant and dense erect, suberect and decumbent short hairs, much denser and longer hairs relative to head and mesosoma, interspersed by 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.

Description. Queen. Relatively large size. Head in full face view subrectangular, not much more rectangular than that of large worker; posterolateral corners smooth and rounded, posterior margin nearly straight with a weak median depression. Posterior margin of eye located below midline of head in full face view. In profile, clypeus projected slightly forward from dorsal margin of head, very weakly convex and almost straight. Scape short, barely surpassing posterior margin of eye when lain back on head. In profile, mesosoma subcylindrical, more elongate than worker, dorsal outline broadly and weakly convex, propodeal junction rounded and smooth, posterior face of propodeum weakly convex and smooth; sutures and sulci on lateral surface of mesosoma distinct. In dorsal view, mesosoma sub-cylindrical, anterior margin of pronotum strongly convex humeral corners angulate though rounded, mesoscutum almost as wide as long, sides of propodeum roundly convex, posterior propodeal margin indistinct and weakly convex. In profile view, petiole subtrapezoidal, slightly inclined posteriorly, anterior margin weakly concave; apex of petiolar node smooth and rounded, slightly higher than postpetiole. In dorsal view, petiole suboval, less than twice as wide as long; postpetiole globular, broader and larger than petiole.

Anterior half of head striated and shining, posterior half of head mostly smooth and shining with scattered punctures, feeble striations on and around the vertex; ventral surface of head mostly smooth and shining. Median section of clypeus striate-reticulate, interspaces punctured and weakly shining. Mandible mostly smooth and shining with very feeble striations. Lateral surface of mesosoma weakly striate-reticulate, interspaces smooth and shining; entire dorsum finely striated and shiny. Lateral and dorsal surfaces of petiole and postpetiole superficially reticulate, interspaces smooth and shining. Gaster superficially reticulate, interspaces smooth and shining with scattered punctures.

Dorsal and ventral surfaces of head with numerous scattered short standing and decumbent hairs, with sparse longer erect hairs; antennal scape with many short suberect hairs and sparse longer erect hairs. Dorsum of mesosoma with sparse short standing hairs, and sparser long erect hairs on anterodorsal and dorsolateral margins. Petiole and postpetiole pilose, with abundant short standing hairs, and sparse long erect hairs. Gaster very pilose, covered with abundant appressed and decumbent hairs, hairs on ventral surface denser than on dorsum.

Overall uniformly dark-brown; legs lighter brown in tone, 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. Relatively large size. Head in full face view broad ovate, distinctly narrower posteriorly than anteriorly, posterior margin strongly convex; eye below transverse midline of head, outline of eye roundly convex and exceeding lateral margin of head. In profile, head broad-ovate and lobular, lateral corners smooth and rounded, ventral and posterior margins forming a continuous curved outline; clypeus projected strongly from dorsal margin of head, anterior outline weakly convex and almost flat. In profile, mesosoma subcylindrical and relatively elongate, dorsal outline broadly and weakly convex, propodeal junction rounded and smooth, posterior face of propodeum steep and almost vertical. In dorsal view, mesosoma obovate, narrower posteriorly, notauli absent. In profile view, petiole inclined posteriorly, longer than high, short and subtriangular with blunt and rounded apex, posterior slope weakly convex, forming a continuous curved outline with apex; anteroventral extension of subpetiolar process reduced to a short and shallow translucent lamella. Dorsal outline of postpetiole almost entirely flat, differentiated only a little from gaster by an indistinct cinctus.

Anterior half of head dorsum rugulose-reticulate, interspaces punctured and weakly shining, rest of head substriate-reticulate and shiny, with scattered punctures. Dorsum of mesosoma mostly sub-striated and shiny; lateral surface of mesosoma and dorsum of propodeum superficially striate-reticulate with smooth and shining interspaces. Petiole, postpetiole and gaster superficially reticulate, interspaces smooth and shining.

Dorsum of head with abundant evenly distributed and uniform short standing hairs, lateral surface with sparse standing hairs; ventral surface of head with numerous short appressed hairs. Dorsum of mesosoma, petiole and postpetiole with scattered short standing hairs. Gaster pilose, with numerous standing and decumbent hairs.

Body generally dark grey-brown, head more blackish and darker than rest of body; joints of legs and antennae paler in tone.

Distribution. Singapore, Thailand ( Fig. 76 ).

Habitat. 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).

Remarks. We carefully compared our queen specimens with the holotype queen (head missing) of Rhopalomastix rothneyi from India (QR), as we did for R. javana . We found the Singapore queen specimens (SJHQ) similar to the holotype, except for the following differences: 1) in SJHQ, pronotal length in dorsal view is more than half of propodeal length; in QR, pronotal length is less than half of propodeal length; 2) in SJHQ, posterior face of propodeum in profile view sloped at a slight angle, propodeal junction correspondingly obtuse; in QR posterior face of propodeum nearly entirely vertical, propodeal junction almost right-angled, though rounded; 3) SJHQ generally larger than QR (MsW 0.44–0.46 in SJHQ, MsW 0.42 in QR).

We compared workers and males of the Sri Lankan specimens (non-types of R. rothneyi ) with the Singaporean specimens of R. johorensis , as we did for R. javana . The worker from Singapore (SJHW) differs from the Sri Lankan worker (SLW) by three main characters: 1) SJHW is of a generally larger size—SJHW PronW> 0.3, SLW PronW ± 0.3; 2) for SJHW, dorsal margin of mesosoma in profile view distinctly broadly convex with dorsum of propodeum sloped downwards, in SLW, dorsum of mesosoma is nearly entirely straight with propodeal dorsum flatly level for most of its length; 3) slope of the propodeal declivity in SJHW is gentler with an obtuse propodeal junction; in SLW, the slope of the propodeal declivity is steep with its propodeal junction almost right-angled. Our observations mostly concur with Wheeler’s (1929), except his observation of smaller eyes; large Singaporean workers in this study can have 17–29 ommatidia, eye size relative to head very variable among all workers (REL 16–20).

The male from Singapore (SJHM) differs from the Sri Lankan male (SLM) in four main character traits. Firstly, SGM’s head is generally larger than SLM’s head—SJHM HL 0.46–0.48, HW 0.44; SLM HL 0.44, HW 0.4–0.42. Secondly, the mesoscutal width of SJHM is larger than that of SLM—SJHM MsW 0.52, SLM MsW 0.48-0.5. Thirdly, for SJHM the dorsum of mesosoma is mostly sub-striated and shiny; in SLM, dorsum of mesosoma is mostly smooth and shiny with scattered punctures. Lastly, for SJHM the petiole is more subtriangular, apex forming a continuous curve with the posterior margin; in SLM the petiole is more subtrapezoidal, apical and posterior petiolar margins joined at a distinct angle.

Given the character differences between the Singapore populations of R. johorensis and R. rothneyi , especially between the holotype queen of R. rothneyi and SJHQ, we thereby choose to raise the subspecies johorensis to species status.

Workers of R.johorensis are also quite similar to the workers of R. striata , but can be distinguished from the latter based on characters as explained in “Remarks” under the latter description.

Variation. Out of 6 examined colonies of R. johorensis from Singapore, 1 colony (ZRC_HYM_0000289) stood apart from the others in terms of minor differences in morphology and DNA. For COI (313 bp), barcodes obtained from this colony diverged from other specimens identified as R. johorensis at 1.9 % uncorrected p - distance clustering threshold, when only Singapore specimens were considered. An objective clustering threshold range of 3–4% is usually taken to best reflect actual species delimitation for insects ( Hebert et al. 2003); thus, based on this criterion alone, the colony remains within the percentage threshold for species and can still be considered R. johorensis . As for morphology, worker size variation seemed more constrained in this colonyworkers from the colony mostly resemble larger workers from other colonies, with more ommatidia per eye and darker body colour. No distinctly different small workers were collected. Furthermore, there are morphological traits that differ from large workers in other colonies. The mesosoma is more elongate (ML 0.64–0.74, n=10; vs. 0.52–0.62, n=8), the pronotum is wider (PronW 0.36–0.42; vs. PronW 0.28–0.36), and there is more intranidal variation in the number of ommatidia per eye (11–29 vs. 10–17, 11–16). These differences may simply constitute variation among different colonies and/or phenotypic plasticity under different living conditions, e.g. colonies established in different tree species. Therefore, before more convincing molecular and/or morphological evidence is gathered, we tentatively consider this slightly aberrant colony as R. johorensis .

Barcodes of most specimens of R. johorensis identified from Thailand clustered with Singapore specimens between 0–0.6% ( Fig. 1 ), except for specimens from East Thailand which diverged from all other R. johorensis specimens by 4.2% ( Fig. 1 ). The East Thailand workers (EJHW) are morphologically identical to the types and other populations of R. johorensis examined in this study, except for one minor character: the sculpture on the lateral surface of the head is visibly weaker than on the dorsum of the head, and the interspaces are more smooth and shining. Given the absence of stronger morphological differences, and insufficient DNA support for cryptic species, we currently consider EJHW as R. johorensis .

Non-type material examined. SINGAPORE: 7 workers, 1 queen, Mandai Road (1.41385°N, 103.80481°E), nest behind bark of living tree, 29 Nov 2016, G. W. Yong and B. Ho leg., ZRC _HYM_0000509 ( ZRC) ; 12 workers, 1 queen, Hougang Ave 3 (1.34769°N, 103.88838°E), mango tree, 12 Dec 2016, G. W. Yong and S.X. Chui leg., ZRC _HYM_0000511 ( ZRC) ; 12 workers, 1 queen, Pulau Tekukor (1.23081°N, 103.83774°E), 15 Jan 2017, G.W. Yong leg., ZRC _HYM_0000578 ( ZRC) ; 12 workers, Mandai Road (1.41318°N, 103.79405°E), nest behind bark of Aquilaria malaccensis , 21 Nov 2016, G. W. Yong and S.X. Chui leg., ZRC _HYM_0000291; 6 workers, 1 male, Mandai Road (1.41318°N, 103.79405°E), nest behind bark of Macaranga gigantea , 21 Nov 2016, G. W. Yong and S.X. Chui leg., ZRC _HYM_0000292 ( ZRC) ; 12 workers, Pulau Tekukor (1.23081°N, 103.83774°E), mango tree, 9 Dec 2016, G. W. Yong and Y.G. Tan leg., ZRC _HYM_0000510 ( ZRC) ; 12 workers, Mandai Road (1.41333°N, 103.79839°E), 13 Jan 2017, G.W. Yong leg., ZRC _HYM_0000577 ( ZRC) ; 12 workers, 1 male, Mandai Road (1.41333°N, 103.79839°E), 18 Nov 2016, nest behind bark of Campnosperma auriculatum, G.W. Yong and S.X. Chui leg., ZRC _HYM_0000289 ( ZRC) ; 12 workers, 2 queens, Bukit Timah Nature Reserve (1.35127°N, 103.78161°E), nest in bark of Artocarpus integer (Thunb.) Merr. /cempedak, 1 Nov 2017, W. Wang leg., colony no.WW-SG17-022, ZRC _HYM_0001794 ( ZRC) . THAILAND: Central Thailand, Pathum Thani Prov., Khlong Luang Dist. , Kholng 5, 3 m a.s.l. (14.18083°N, 100.71000°E), mango tree, 3 Mar 2018, W. Jaitrong leg., colony no.WJT 030318 -1 ( THNHM, ZRC) ; same locality, date and collector, colony no. WJT 130218 -1 ( THNHM, ZRC) ; Central Thailand, Pathum Thani Prov., Khlong Luang Dist. , Kholng 3, 10 m a.s.l. (14.17528°N, 100.66333°E), mango tree, 9 May 2018, W. Jaitrong leg., colony no. WJT 090518 -1 ( THNHM) ; same locality, date and collector, colony no.WJT 090518 -2 ( THNHM) ; Central Thailand, Ang Thong Prov., Chaiyo Dist. , Lakfa Subdist. , 14 m a.s.l. (14.68278°N, 100.46444°E), 7 Apr 2018, W. Jaitrong leg., colony no. WJT 070418 -2 ( THNHM, ZRC) ; same locality, date and collector, colony no.WJT 070418 -1 ( THNHM) ; same locality, date and collector, colony no.WJT 070418 -3 ( THNHM, ZRC) ; same locality, date and collector, colony no.WJT 070418 -4 ( THNHM, ZRC) ; same locality, date and collector, WJT 070418 -5 ( THNHM, ZRC) ; Central Thailand, Saraburi Prov., Phu Kae Botanical Garden , 89 m a.s.l. (14.67056°N, 100.88500°E), 18 Mar 2018, W. Jaitrong leg., colony code. WJT 180318 -1 ( THNHM, ZRC) ; Central Thailand, Saraburi Prov., Kang Koi Dist. , Ban Chaom , nr. Ched Kod waterfall (14.47111°N, 101.16111°E), 8 Jan 2018, W. Jaitrong leg., colony no.WJT 080118 -3 ( THNHM) ; Central Thailand, Nakhon Nayok Prov., Banna Dist. , from mango tree, 11 May 2018, W, Jaitrong leg., colony no.WJT 110518 -1 ( THNHM) ; E. Thailand, Chanthaburi Prov., Tha Mai Dist. , Burapecholathid Road , 5 m a.s.l. (12.57083°N, 101.91611°E), 17 Mar 2018, W. Jaitrong leg., colony no.WJT 170318 -1 ( THNHM, ZRC) ; E. Thailand, Chanthaburi Prov., Tha Mai Dist. , Khlong Khud Subdist. , 18 m a.s.l. (12.57167°N, 101.93111°E), 17 Mar 2018, W. Jaitrong leg., colony no.WJT 170318 -2 ( THNHM, ZRC) ; same locality, date and collector, colony no.WJT 170318 -3 ( THNHM, ZRC) ; S. Thailand, Trang Prov., Nayong Dist. , Ban Nakhaw Sia , 28 m a.s.l. (7.50833°N, 99.71639°E), under bark of Azadirachta excelsa (Jack) Jacobs , 24 Jan 2018, W. Jaitrong leg., colony no.WJT 240118 -2 ( THNHM, ZRC) ; same locality, date and collector, colony no.WJT 240118 -3 ( THNHM) ; same locality, date and collector, colony no.WJT 240118 -4 ( THNHM, ZRC) .

DNA Barcoding. SINGAPORE: 46 workers from 9 different colonies collected from various localities were successfully barcoded, breakdown as follows (no. of workers per colony in parentheses): ZRC_HYM_0000291(3), ZRC_HYM_0000292(9), ZRC_HYM_0000509(5), ZRC_HYM_0000510(6), ZRC_HYM_0000511(9), ZRC_HYM_0000577(3), ZRC_HYM_0000578(4), ZRC_HYM_0001794(2), ZRC_HYM_0000289(10). THAILAND: 54 workers from 12 colonies were successfully barcoded, breakdown as follows: WJT 240118 -4(5), WJT 240128 -2(5), WJT 130218 -1(5), WJT 030318 -1(5), WJT 170318 -1(4), WJT 170318 -2(4), WJT 170318 -3(5), WJT 180318 -1(2),WJT 070418 -2(5), WJT 070418 -3(5), WJT 070418 -4(4), WJT 070418 -5(5).

Based on the combined cluster dendrogram ( Fig. 1 ), barcode divergences from other species are as follows: R. glabricephala , 17%; R. javana , 13.8%; R. murphyi , 19.2%; R. striata , 12.5%; R.tenebra , 13.8%.

Figures

FIGURE 1. Cluster dendrogram of COI (313 bp) barcodes of Rhopalomastix from Singapore and Thailand. Objective clustering of barcodes was performed based on uncorrected p-distances using the ‘best close match’ criteria; the number at each ‘node’ represents the percentage divergence or pairwise distance threshold at which clusters split or lump together. Each cluster identified as a species is highlighted in one colour per respective species. Sequences of Thai Rhopalomastix are outlined in red.

FIGURE 76. Distribution of R. javana and R. johorensis in Southeast Asia.

FIGURES 24–27. Lectotype worker of R. johorensis stat. n. 24, habitus in profile; 25, head in full-face view; 26, closeup of head in profile view; 27, body in dorsal view.

FIGURES 28–31. Non-type worker of R. johorensis from Singapore, Hougang Ave 3, ZRC_HYM_0000511; 28, habitus in profile; 29, head in full-face view; 30, closeup of head in profile view; 31, body in dorsal view.

FIGURES 32–35. Non-type queen of R. johorensis from Singapore, Pulau Tekukor, ZRC_HYM_0000578; 32, habitus in profile; 33, head in full-face view; 34, body in dorsal view; 35, closeup of mesosoma in dorsal view.

FIGURES 36–39. Non-type male of R. johorensis from Singapore, Mandai Road, ZRC_ENT0000289; 36, habitus in profile; 37, head in full-face view; 38, body in dorsal view; 39, closeup of mesosoma in dorsal view.