Polyrhachis robsoni

Polyrhachis robsoni builds polydomous nests between leaves on the lower branches of trees and shrubs in lowland and riparian rainforests. Colonies on Thursday Island have been found nesting in a small patch of dry monsoonal rainforest (Kohout 2006).

Identification
Polyrhachis robsoni is characterised by a highly arched mesosoma, toothed or distinctly angular pronotal shoulders and a completely unarmed propodeum (Kohout 2006).

Distribution
Found from Cooktown south to Mission Beach. It also occurs on Thursday Island in Torres Strait (see biology section below).

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Abundance
A common north Queensland species.

Biology
Kohout (2006): The isolated Thursday Island population has relatively small workers (HL 1.40-1.47 versus 1.47-1.59 in mainland specimens). Colonies are also polygynous and queens echibit polymorphism. In two colonies the queens were distinctly both maco- and microgynous, while another colony contained only microgynous queens. Yet another colony included a graded series of numerous intermediates between the ‘normal’ macrogynous and microgynous queens seen in the other colonies. Thirty seven nests of P. robsoni were collected by Robson at the Cairns Botanic Gardens (16°54’S, 145°45’E), of which 18 were polygynous. In 12 nests queens were of both sizes, in 4 they were only macrogynous and in the remaining 2 nests they were only microgynous.

Castes
Microgynes are an isometric reduction of macrogynes (van Zweden et al. 2007).

Males and immature stages (eggs, larvae and pupae) in and  collections. (Kohout 2006)

Nomenclature

 *  robsoni. Polyrhachis robsoni Kohout, 2006b: 103, figs. 3B, F-G (w.q.m.) AUSTRALIA.

Worker
Dimensions (holotype cited first): TL c. 5.85, 5.29-6.50; HL 1.47, 1.40-1.59; HW 1.50, 1.40-1.59; CI 102, 95-102; SL 1.75, 1.68-1.93; SI 117, 115-126; PW 1.18, 1.09-1.34; MTL 2.00, 1.87-2.21 (32 measured).

Clypeus in profile virtually straight, abruptly rounding posteriorly into moderately impressed basal margin. Frontal triangle indistinct. Frontal carinae sinuate, margins moderately raised; central area weakly concave with distinct frontal furrow. Sides of head in front of eyes weakly convex, converging anteriorly towards mandibular bases; behind eyes sides rounding into convex occipital margin. Eyes convex, in full face view breaking lateral cephalic outline. Ocelli lacking, relative positions of lateral ocelli indicated by shallow depressions in cephalic sculpturation. Pronotum in dorsal view widest just behind humeri that are toothed or, at least, angular. Mesosoma in profile strongly convex; promesonotal suture distinct; position of metanotal groove indicated by rather distinct depression; declivity very steep, almost vertical. Petiole with anterior face very weakly convex, posterior face moderately convex; dorsum armed with four spines; dorsal pair closer to each other than to lateral spines; lateral pair distinctly longer, more slender and rather strongly diverging.

Mandibles very finely, mostly longitudinally rugose with numerous, shallow punctures and piliferous pits. Head and gaster finely, dorsum of mesosoma and petiole more coarsely, shagreened. Sides of mesosoma rather strongly sculptured with intensity increasing laterally, meso- and metapleurae rather strongly reticulate-rugose. Anterior and posterior faces of petiole somewhat transversely wrinkled dorsally with sculpturation becoming coarsely rugose on lower parts. Numerous minute, mostly piliferous punctures over all body surfaces.

Mandibular masticatory border with numerous, curved and suberect hairs. Anterior clypeal margin medially with 1 long and a few shorter, anteriorly directed setae, fringe of shorter setae lining margin laterally. Several, mostly paired hairs arising near anterior and basal clypeal margins, along frontal carinae and on vertex. Generally only 2, erect, straight or undulated hairs, shorter than longest diameter of eye, on summit of mesonotum. Gastral dorsum with medium length, more-or-less erect hairs lining posterior margins of apical segments; hairs distinctly more abundant on venter of gaster. Abundant, mostly appressed hairs arising from numerous punctures and pits on most dorsal body surfaces.

Colour. Black; very narrow band along mandibular masticatory borders, condylae and extreme tips of apical funicular segments, reddish-brown. Most of antennae and tarsi black or very dark brown. Trochanters, femora and tibiae generally light to medium reddish-brown with distal ends of femora and tibiae narrowly, and proximal ends of tibiae more widely, dark brown.

Queen
Dimensions: TL c. 6.60-8.82; HL 1.56-1.96; HW 1.47-1.96; CI 94-102; SL 1.78-2.28; SI 112-121; PW 1.43-2.00; MTL 2.03-2.74 (5 measured). Apart from sexual characters, resembling worker except: pronotal humeri narrowly rounded; mesoscutum only marginally wider than long; lateral margins converging anteriorly into rather widely rounded anterior margin; median line distinctly bifurcate posteriorly; parapsides weakly raised posteriorly; mesoscutum in profile with widely rounded anterior face, ﬂat dorsally. Mesoscutellum weakly convex in profile, moderately raised above dorsal plane of mesosoma; metanotal groove distinct. Propodeum armed with pair of short, strongly upturned teeth that are completely lacking in all workers examined. Petiole in profile bi-convex and spines, notably lateral pair, distinctly shorter than in worker. Remaining features, including sculpturation and colour, as in worker.

Type Material
HOLOTYPE: QUEENSLAND, Daintree R. x-ing, 16°15’S, 145°23’E, 9.ix.2001, riparian rf., R.J. Kohout acc. 2001.26 (worker). PARATYPES: data (and nest) as for holotype (61 workers, 4 dealate queens); ditto, 16.vi.1997, S.K. Robson #559 (47 workers, dealate queen). Holotype (QMT99339), most paratype workers, 3 paratype queens (from holotype nest) and paratype workers and paratype queen in ; 2 paratype workers, 1 paratype queen (from holotype nest) and 3 paratype workers in, 2 paratype workers (from holotype nest) in , , , , , , and.



References based on Global Ant Biodiversity Informatics

 * Kohout R.J. 2006. Review of Polyrhachis (Cyrtomyrma) Forel of Australia, Borneo, New Guinea and the Solomon Islands with descriptions of new species. Memoirs of the Queensland Museum 52: 87-146.