Rhopalomastix impithuksai Wang & Jaitrong, 2021

Wang, Wendy Y., Yong, Gordon W. J. & Jaitrong, Weeyawat, 2021, Revision of the elusive ant genus Rhopalomastix (Hymenoptera, Formicidae, Myrmicinae) in Thailand based on morphology and DNA barcodes, with descriptions of three new species, European Journal of Taxonomy 739 (1), pp. 117-157 : 123-128

Rhopalomastix impithuksai Wang & Jaitrong sp. nov.

urn:lsid:zoobank.org:act:DC6C24A1-9813-4DED-8B83-150BBCCFA291

Figs 2a , 3 (worker), 4 (male), 16a

Diagnosis

Worker

Monomorphic with broad size variation (HL 0.4–0.55; HW 0.44–0.52). Head in full-face view subquadrate, lateral margins broadly convex and nearly parallel ( Fig. 3a ). In profile, short dorsal clypeal face nearly flat or weakly convex and downward sloping, differentiated from steep and mostly flat anterior clypeal face by indistinct rounded edge ( Fig. 3c ). Dorsal outline of mesosoma very weakly and broadly convex, almost entirely flat ( Fig. 3d ). In dorsal view, anterior margin of pronotal disc (thereinafter defined as an area excluding the pronotal collar) broadly convex, humeral corners rounded and indistinctly angulate especially in small worker, lateral margins of propodeum broadly convex with rounded posterolateral corners, posterior propodeal margin indistinct and weakly concave ( Fig. 3b ). Mandible with upper half of outer face finely striate, lower half mostly smooth and shining ( Fig. 2a ). Median clypeal face carinulate, interspaces punctate and weakly shiny; supraclypeal area with darkpigmented carinae ( Fig. 3a ). Narrow vertexal area adjacent to and along posterior margin of head without standing hairs, hairs on gena sparser posteriorly, posterolateral face with sparse or no hairs. Anterior rising face of petiole with sparse and few decumbent and standing hairs, mostly close to apex. Vertexal area of head dorsum anterior to median concavity of posterior margin darker blackish-brown compared to rest of head ( Fig. 3a ).

Male

Moderately large size. Head in full-face view subcircular, distinctly wider than long with broadly convex posterior margin ( Fig. 4a ); in lateral view, head dorsum almost flat, ventral margin strongly convex without distinct posteroventral angle ( Fig. 4b ); dorsal clypeal face rounding into weakly convex anterior clypeal face at indistinct rounded edge ( Fig. 4b ). Median ocellus suboval and similar to lateral ocelli in size and shape ( Fig. 4a, c ). Frontal lobes shallowly raised and indistinctly differentiated from supraclypeal area; posterior apex of supraclypeal area emarginate and continuous with narrow median furrow between frontal lobes ( Fig. 4a ). In lateral view, anterodorsal face of mesoscutum distinctly higher than anterior face of pronotum ( Fig. 4e ). In posterior view, posterior propodeal face emarginate, rounding into and undifferentiated from lateral propodeal face.

Etymology

The species is named after Dr Viroch Impithuksa, whose assistance was vital for the discovery of the former.

Material examined

Holotype THAILAND • worker; Northeast Thailand, Sakhon Nakhon Province, Phu Phan Ratchaniwet Palace ; 17 Jun. 2018; W. Jaitrong leg.; nest in bark of mango tree; colony code: WJT170618-5; THNHM-I-21751 .

Paratypes THAILAND • 7 workers; same collection data as for holotype; THNHM-I-21752 to THNHM-I-21758 • 6 workers, 1 ♂; same collection data as for holotype; GenBank: MW267045 to MW267048 (4 workers barcoded); ZRC_ENT00007608 .

Description

Worker

Measurements. Paratype workers (n = 5): EL 0.07–0.1; EW 0.05–0.06; HL 0.47–0.55; HW 0.44–0.52; ML 0.54–0.70; PronW 0.28–0.36; PtH 0.16–0.20; PtL 0.17–0.22; SL 0.18–0.20; TL 1.84–2.30; CI 94– 96; PtHI 84–100; REL 16–21; SI 38–41.

Monomorphic workers with broad intranidal size variation. Head in full-face view subquadrate, slightly longer than wide, in large worker head wider posteriorly, in small worker width of head mostly uniform throughout entire length; posterior margin broadly and weakly convex with a shallow median concavity, posterolateral corners roundly convex, lateral margins of head weakly convex and nearly parallel; median furrow short, distinctly much wider and more shallow in posterior half; anterior clypeal margin with broadly convex median section ( Fig. 3a ). Eye with 12–17 ommatidia. Supraclypeal area with darkpigmented longitudinal carinae, posterior apex carinate and acute, distinctly inserted between frontal lobes and raised above deep anterior end of median furrow. In lateral view, clypeus projecting strongly from dorsal margin of head, short dorsal clypeal face nearly flat or weakly convex and downward sloping, differentiated from steep and mostly flat or weakly convex anterior clypeal face by indistinct rounded edge ( Fig. 3c ). Mesosoma in lateral view box-shaped or subcylindrical (generally in smaller workers), dorsal outline very weakly and broadly convex, almost entirely flat, propodeal junction roundly obtuse, propodeal declivity steep and shallowly concave; propodeal dorsum differentiated from posterior declivitous face by weak marginate edge ( Fig. 3d ). In dorsal view, mesosoma subrectangular, anterior margin of pronotal disc broadly convex, humeral corners rounded and indistinctly angulate especially in smaller workers, lateral margins of propodeum broadly convex, transitioning to posterior propodeal margin at bluntly rounded posterolateral corners; posterolateral corners weakly projected posteriorly and shallowly differentiated from inner poster propodeal face; posterior propodeal margin indistinct and weakly concave ( Fig. 3b ). Posterior face of propodeum weakly marginate, rounding into lateral propodeal face at indistinct angulate edge. In lateral view, petiole subtriangular, inclined posteriorly, slightly longer than high or sometimes as long as high, anterior face steep and weakly concave; apex roundly convex, apical dorsum rounding into weakly convex posterior face at indistinct rounded edge, posterior face steeper and shorter than anterior face ( Fig. 3d ). Petiole higher and longer than postpetiole, dorsal margin of postpetiole weakly convex. In dorsal view, petiole suboval, wider than long; postpetiole globular, less than twice as wide as petiole.

Dorsum of head largely finely substriate and shiny with scattered punctures, lateral face largely superficially substriate-reticulate with scattered punctures and interspaces smooth and shining. Ventral face of posterior half of head mostly superficially reticulate with interspaces smooth and shining. Mandible in full-face view with upper half of outer face finely striate, lower half mostly smooth and shining, area immediately adjacent to masticatory margin punctate ( Fig. 2a ). Median clypeal face carinulate, interspaces punctate and weakly shiny ( Fig. 3a ). Lateral face of mesosoma largely substriate- reticulate with interspaces smooth and shining ( Fig. 3d ); dorsum of mesosoma finely striate and shiny ( Fig. 3b ); posterior face of propodeum superficially and weakly reticulate with interspaces smooth and shining. Lateral faces of petiole and postpetiole superficially substriate-reticulate with interspaces smooth and shining ( Fig. 3d ), apicodorsal faces superficially reticulate with interspaces smooth and shining. Gaster superficially reticulate with interspaces smooth and shining, sculpture on first gastral tergite stronger than rest of gaster.

Dorsum of head largely with fairly dense though well-spaced short erect and suberect hairs interspersed with sparse long erect hairs, narrow vertexal area adjacent to and along posterior margin of head without standing hairs, gena with short erect and decumbent hairs that are sparser posteriorly, posterolateral face with sparse or no hairs. Ventral face of head anteriorly with fairly dense short standing and decumbent hairs, sparser standing hairs posteriorly. Mesosoma dorsum with sparse, scattered short standing hairs mostly closer to dorsolateral margin, paired long erect hairs sparsely present along dorsolateral margin, with one pair each flanking the anterior margin of pronotal disc, pronotum, mesonotum and propodeum, sometimes two pairs flanking propodeum. Anterior rising face of petiole mostly hairless with few decumbent and standing hairs close to apex, posterior declivity with dense short standing hairs; apex of petiolar node with few short erect hairs and 1–2 pairs of long erect hairs. Postpetiole with fairly dense short erect and suberect hairs, and one pair of long erect hairs. Gaster pilose with dense erect and suberect hairs, interspersed with sparse long erect hairs.

Head, dorsum of mesosoma and gaster generally dark brown with rest of body more yellowish brown, small worker lighter brown relative to large worker. Vertexal area of head dorsum anterior to median concavity of posterior margin darker blackish-brown compared to rest of head ( Fig. 3a ), tips of antennae and legs pale yellowish-brown.

Male

Measurements. One paratype male, antennal funiculi damaged with missing segments (n = 1): TL 2.93; HL 0.46; HW 0.48; SL 0.06; PtH 0.20; PtL 0.28; ML 0.90; MsW 0.50; EL 0.24; EW 0.19; CI 104; PtHI 71; REL 50; SI 13.

Moderately large size, similar to R. johorensis male. Head in full-face view subcircular, distinctly wider than long, posterior margin broadly convex ( Fig. 4a ). Eye very large (20–25 ommatidia in the longest axis); posteriormost point of eye slightly exceeding transverse midline of head, outline of eye bulging distinctly from lateral margin of head ( Fig. 4a ). Median ocellus suboval, wider than long, similar to lateral ocelli in size and shape; area of head dorsum anterior to median ocellus weakly depressed, continuous with weak median furrow separating frontal lobes; frontal lobes shallowly raised and indistinctly differentiated from supraclypeal area ( Fig. 4a, c ). Posterior apex of supraclypeal area emarginate, continuous with narrow median furrow between frontal lobes ( Fig. 4a ). In lateral view, head broad-ovate with almost flat dorsum, dorsal face rounding into posterior face at indistinct edge, posterior margin rounding into strongly convex ventral margin forming a continuous curve with no distinct posteroventral angle; clypeus strongly projecting from dorsum of head, dorsal clypeal face gently sloped downwards and nearly flat, differentiated from weakly convex anterior clypeal face by indistinct rounded edge ( Fig. 4b ).

In lateral view, mesosoma sub-oblong, dorsal margin weakly and broadly convex, propodeal junction roundly obtuse; anterodorsal face of mesoscutum distinctly higher than anterior face of pronotum; propodeal dorsum weakly convex and gently downward sloping, rounding into steep and weakly convex posterior declivity at indistinct rounded posterior edge ( Fig. 4e ). In posterior view, posterior propodeal face emarginate, rounding into and undifferentiated from lateral propodeal face. In dorsal view, mesosoma obovate, notauli absent, parapsidal line indistinct ( Fig. 4d ). In lateral view, petiole inclined posteriorly, subtriangular, longer than high, anterior margin long, steep and weakly concave; apex indistinct from posterior face, forming a continuous roundly convex posteroapical face; anteroventral extension of subpetiolar process shallow and subtriangular with rounded apex ( Fig. 4e ). Dorsal outline of postpetiole weakly convex, differentiated from gaster by a weak but distinct cinctus ( Fig. 4e ).

Area between eye and antennal insertion coarsely rugulose-reticulate with interspaces punctured and weakly shiny ( Fig. 4a–b ); rest of head substriate-reticulate with dense punctures, and interspaces smooth and shining; area enclosed by ocelli with slightly coarser sculpture; median area immediately anterior to median ocellus and median furrow largely smooth and shining ( Fig. 4a, c ); clypeus coarsely rugulosereticulate with interspaces punctate and weakly shining ( Fig. 4a–b ). Lateral face of mesosoma largely superficially substriate-reticulate with interspaces smooth and shining( Fig.4e ). In dorsal view, mesoscutum largely superficially substriate-reticulate with interspaces smooth and shining, mesoscutal disc smooth and shining, propodeum superficially reticulate with interspaces smooth and shining ( Fig. 4d ). Lateral face of petiole weakly substriate-reticulate with interspaces smooth and shining ( Fig. 4e ), petiolar apex entirely smooth and shining ( Fig. 4d ). Postpetiole weakly substriate-reticulate with interspaces smooth and shining. Gaster mostly superficially reticulate with interspaces smooth and shining.

Dorsum of head with dense short standing hairs; lateral and ventral faces with dense short decumbent and subdecumbent hairs ( Fig. 4a–b ). Dorsum of pronotum and mesoscutum with dense short standing hairs; posterolateral margins of propodeum with sparse short erect hairs ( Fig. 4d–e ). Anterior rising face of petiole without hairs, posterodorsal petiolar face including apex with short standing hairs, sparser than those on mesoscutum; dorsum of postpetiole with short erect hairs, slightly more dense than those on petiole. Gaster pilose with dense standing and subdecumbent hairs, ventral gastral hairs slightly less dense than those on gastral dorsum.

Body generally dark brown; head excluding clypeus, mesoscutum and mesoscutellar disc blackish brown; dorsum of head especially vertex, frons and area around eye with blackish undertones; antennal scape and legs uniformly greyish brown, 2 nd funicular segment yellowish brown.

Distribution

Thailand (central, Nakhon Nayok Province) ( Fig. 16a ).

Remarks

The worker of R. impithuksai Wang & Jaitrong sp. nov. is morphologically similar to R. javana , but can be differentiated by the following characters: 1) condition of supraclypeal area – in R. impithuksai Wang & Jaitrong sp. nov. the supraclypeal area is lined with dark-pigmented longitudinal carinae with posterior apex carinate and usually acute, in R. javana the supraclypeal area does not have such carinae, with posterior apex often indistinct; 2) hairs on anterior face of petiole – in R. impithuksai Wang & Jaitrong sp. nov. there are few decumbent and standing hairs only present close to the apex, while in R. javana , there are standing hairs uniformly distributed on the entire anterior petiolar face; 3) upper half of outer face of mandible-finely striate in R. impithuksai Wang & Jaitrong sp. nov., but smooth and shining in R. javana .

Rhopalomastix impithuksai Wang & Jaitrong sp. nov. workers may also appear similar to the morphologically variable R. johorensis , but the two species can be distinguished by the aforementioned points (2) and (3). In addition, for R. impithuksai Wang & Jaitrong sp. nov. the dorsolateral propodeal margins transition to posterior propodeal margin at bluntly rounded posterolateral corners – together with an almost flat propodeal dorsum, these give rise to a roundly obtuse posterior propodeal junction. In contrast, for R. johorensis , the combination of ill-defined posterolateral corners of the propodeum and a gently downward-sloping propodeal dorsum, give rise to an indistinct though broadly-rounded propodeal junction.

The large worker of R. impithuksai Wang & Jaitrong sp. nov. is also morphologically similar to smaller workers of R. robusta Wang & Jaitrong sp. nov., but may be differentiated based on characters described in more detail under Remarks for the latter species.

Figures

Fig. 2. Closeups of external mandibular surface for workers of 5 Thai species of Rhopalomastix Forel, 1900 (ZRC); white arrows indicate distinct striations on the external mandibular face. a. R. impithuksai Wang & Jaitrong sp. nov., paratype (ZRC_ENT00007608). b. R. robusta Wang & Jaitrong sp. nov., paratype (ZRC_ENT00013949). c. R. javana Wheeler, 1929, non-type (ZRC_ENT00000932). d. R. johorensis Wheeler, 1929, non-type (ZRC_ENT00007874). e. R. parva Wang & Jaitrong sp. nov., paratype (ZRC_ENT00013948).

Fig. 3. Rhopalomastix impithuksai Wang & Jaitrong sp. nov., paratype, worker (ZRC_ENT00007608.1). a. Head in full face view. b. Mesosoma in dorsal view. c. Close-up of clypeus in profile. d. Body in lateral view. Note: anterior face of petiolar node near apex dented/slightly deformed.

Fig. 4. Rhopalomastix impithuksai Wang & Jaitrong sp. nov., paratype, ♂ (ZRC_ENT00007608). a. Head in full face view. b. Head in lateral view. c. Head in dorsal view. d. Mesosoma and waist in dorsal view. e. Body in lateral view. f. Genitalia in posterior view.

Fig. 16. Distribution maps of species of Rhopalomastix Forel, 1900 in Thailand. a. New species: R. impithuksai Wang & Jaitrong sp. nov., R. parva Wang & Jaitrong sp. nov., R. robusta Wang & Jaitrong sp. nov. b. R. javana Wheeler, 1929 and R. johorensis Wheeler, 1929.