Polyrhachis robusta

Identification
A member of the Polyrhachis continua species group in the subgenus Myrma

Kohout (2013) - Polyrhachis robusta bears a close similarity to Polyrhachis pulleni, with differences between them discussed above under the latter species. Polyrhachis robusta also somewhat resembles Polyrhachis continua with which it is sympatric at both localities it has been collected. However, they are easily separated with P. continua being distinctly more slender and lacking the hairs on the antennal scapes and petiolar node that are rather abundant in P. robusta. Also, the petiolar spines in P. continua are virtually parallel, while they are distinctly divergent in P. robusta.

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

Castes
Male and immature stages unknown.

Nomenclature

 *  robusta. Polyrhachis robusta Kohout, 2013: 36, figs. 35, 38-39 (w.q.) PAPUA NEW GUINEA.

Worker
Worker. Dimensions (holotype cited first): TL c. 12.90, 12.65-13.00; HL 3.12, 3.03-3.12; HW 2.28, 2.15-2.28; CI 73, 71-74; SL 4.38, 4.28-4.38; SI 192; 185-199; PW 2.15, 2.03-2.18; MTL 4.54, 4.43-4.59 (1+3 measured).

Anterior clypeal margin arcuate, shallowly truncate. Clypeus without distinct median carina, weakly sinuate in profile, with shallow basal margin. Frontal triangle distinct.

Frontal carinae sinuate with strongly raised margins; central area relatively narrow with weakly impressed frontal furrow. Sides of head in front of eyes weakly diverging anteriorly before rounding into mandibular bases; behind eyes, sides produced into blunt carina extending towards rather prominent occipital corners. Eyes convex; in full face view clearly exceeding lateral cephalic outline. Ocelli lacking. Pronotal dorsum with long, slender, anterolaterally directed, acute spines; bases of spines somewhat dorsomedially flattened with lateral edges acute and continuous with distinctly upturned pronotal margins. Promesonotal suture distinctly impressed laterally, rather flat medially; mesonotal dorsum transverse with distinctly raised lateral margins, converging into medially flat metanotal groove. Propodeum with lateral margins raised anteriorly, flat posteriorly and terminating in distinct, short, transverse ridges; propodeal dorsum descending into declivity in weakly curved, medially uninterrupted line. Petiole scale-like with anterior face flat, posterior face weakly convex, strongly converging dorsally; dorsal margin armed with two slender, posteriorly curved, acute spines with inner margins forming widely open ‘U’-shaped petiolar dorsum; lateral margin of petiole below base of each spine with short, distinct, acute tooth. Anterior face of first gastral segment flat at base, rounding anterodorsally onto dorsum.

Mandibles finely, longitudinally striate at bases; striae less regular and with numerous piliferous pits towards masticatory borders. Clypeus and vertex along occipital margin and towards occipital corners reticulate-punctate; sculpturation obliquely striate on sides and longitudinally striate from vertex towards central area and between frontal carinae and eyes. Pronotal dorsum and bases of spines finely reticulate-punctate with sculpture more regularly striate towards promesonotal suture; mesonotum rather regularly longitudinally striate, with striae continuous medially onto basal portion of propodeum; striae much finer and somewhat medially ‘U’-shaped posteriorly. Propodeal declivity and petiole very finely reticulate-punctate, spines towards tips almost smooth. Gaster shagreened.

Mandibles along outer margin and towards masticatory borders with numerous erect and suberect, relatively long, golden hairs with somewhat reddish tint. A few, rather long setae arising medially from anterior clypeal margin with shorter setae fringing margin laterally. Numerous, semierect or erect, medium length or relatively long hairs of same colour rather abundant on clypeus and rest of head; numerous hairs fringing outline of head in full face view; antennal scapes with numerous, relatively long hairs along leading edge and somewhat fewer hairs along inferior edge. Mesosoma and legs with numerous, medium length hairs; petiole with several hairs around base and a few hairs along lateral margins and spines. Gaster with numerous, posteriorly directed, relatively long hairs, some as long as greatest diameter of eyes. Closely appressed, rather diluted, silvery pubescence over most body surfaces, more dense and distinctly reddish on gastral dorsum.

Colour. Black; mandibular teeth and apical funicular segments reddish brown. Legs black or very dark reddish brown with trochanters and apical tarsal segments medium reddish brown.

Queen
Dimensions: TL c. 13.51, HL 3.18; HW 2.34; CI 73; SL 4.18; SI 179; PW 2.72; MTL 4.33 (1 measured).

Queen larger than worker with usual characters identifying full sexuality, including three ocelli, complete thoracic structure and wings. Pronotal spines shorter. Mesoscutum only marginally wider than long; lateral margins strongly converging anteriorly into narrowly rounded anterior margin; median line distinct; parapsides virtually flat; mesoscutum in profile with widely rounded anterior face and virtually flat dorsum. Mesoscutellum only weakly convex, not elevated above dorsal plane of mesosoma. Propodeum almost flat in outline, with lateral margins weakly raised anteriorly and terminating in short ridges that extend medially as a blunt, transverse border, partly dividing propodeal dorsum from declivity. Petiole with dorsal spines distinctly shorter. Sculpturation, pilosity, pubescence and colour very similar to worker.

Type Material
Holotype worker: Papua New Guinea, N. Distr. Managalase Area, 2500-3000’, viii.1965, R. Pullen. Paratypes: data as for holotype (2 workers); Northern Prov., Owen Stanley Ra., nr Mamba Pltn, c. 7 km WNW of Kokoda, 08°51’S, 147°41’E, 500 m, 31.viii-1.ix.1984, stray on felled trees (R.J. Kohout acc. 84.403) (worker); ditto, ex rotten log (RJK acc. 84.391) (♀). Holotype worker in ; 1 paratype worker and paratype queen in ; 1 paratype worker each in and.

Etymology
The name alludes to its rather robust body in comparison with the closely similar Polyrhachis continua.

References based on Global Ant Biodiversity Informatics

 * Kohout R. J. 2013. A review of the Polyrhachis continua species-group of the subgenus Myrma Billberg (Hymenoptera: Formicidae: Formicinae) with keys and descriptions of new species. Australian Entomologist 40(1): 13-46.