Polyrhachis schenckii

Kohout (2013): Polyrhachis schenkii is rather common in suitable localities in open eucalypt forest and savannah woodland. It is a ground nesting species with nest entrances usually hidden under a stone, piece of wood or a tuft of grass.

Identification
A member of the schenkii species-group in the Polyrhachis subgenus Hagiomyrma. Kohout (2013): Polyrhachis schenkii is easily recognised by its small size, light-red or reddish brown colour and the strongly posteriorly converging lateral margins of the promesonotal dorsum. It is one of the most widely distributed Hagiomyrma species, ranging from Papua New Guinea south across the Torres Strait to northern Australia. Polyrhachis schenkii forms a number of variable populations. The pronotal humeri are very narrowly rounded or bluntly angular in Australian specimens, while they are distinctly angular in most specimens from Papua New Guinea. The antennal scapes of specimens from the Northern Territory are somewhat longer than those of other populations (SI 151-159 in specimens from the Northern Territory versus 138-149 in Queensland and PNG specimens). Also, the posterior face of the petiolar node is rather evenly convex in specimens from New Guinea and Cape York Peninsula, while in specimens from north Queensland and the Northern Territory the posterior face is uneven, with the base somewhat broader and swollen in lateral view. However, when specimens across the whole distribution are compared, no other significant differences are evident and I consider them to represent a single, albeit variable, species.

Key to Polyrhachis Hagiomyrma group species

Distribution
Kohout (1988) - Papua New Guinea across the Torres Strait islands to Australia, where it occurs on Cape York Peninsula, and in the Northern Territory.

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

Castes
Males and immature stages in ANIC spirit collection.

Nomenclature

 *  schenkii. Polyrhachis schenkii Forel, 1886f: 198 (w.) NEW GUINEA. Wheeler, G.C. & Wheeler, J. 1968: 221 (l.). Combination in P. (Hedomyrma): Santschi, 1920a: 569; in P. (Hagiomyrma): Emery, 1925b: 185.

Worker
Kohout (2013) - (syntype cited first): TL c. 5.2, 4.99-6.50; HL 1.45, 1.31-1.56; HW 1.15, 0.97-1.20; CI 79, 74-79; SL 1.55, 1.43-1.78; SI 135, 135-159; PW 1.00, 0.81-1.06; MW 0.40- 0.47; PMI 210-250; MTL 1.65, 1.53-2.00 (1+36 measured).

Anterior clypeal margin with emarginate, irregularly denticulate, median flange, flanked by acute angles. Clypeus with distinct median carina; sinuate in profile. Frontal triangle poorly indicated. Frontal carinae sinuate with weakly raised margins; central area rather flat with weakly impressed frontal furrow. Sides of head in front of eyes converging towards mandibular bases in almost straight line; behind eyes, sides rounding into convex occipital margin. Eyes relatively large, convex, in full face view clearly exceeding lateral cephalic outline. Ocelli lacking. Pronotal humeri narrowly rounded or bluntly angular (distinctly angular in some PNG specimens). Lateral margins of promesonotal dorsum with very narrowly raised margins, strongly converging posteriorly. Metanotal groove weakly impressed. Propodeal dorsum with lateral margins terminating in horizontal, divergent, acute spines. Petiole with posterior face sloping in even, moderately convex line; dorsum armed with horizontal, widely divergent, acute spines. Anterior face of first gastral segment higher than full height of petiole, evenly rounding onto dorsum.

Mandibles very finely, longitudinally striate with piliferous pits. Head, mesosoma and petiole distinctly reticulate-punctate; spines smooth and polished. Gaster shagreened.

Mandibular masticatory and outer borders with numerous, yellowish or golden, curved hairs. Anterior clypeal margin with a few longer setae medially and short setae fringing margin laterally. Numerous, mostly short, erect hairs on all body surfaces, including antennal scapes and outline of head in full face view; rather longer, somewhat posteriorly directed hairs on gaster. Rather diluted, closely appressed, golden pubescence over most body surfaces; pubescence usually more abundant, somewhat longer and mostly pale golden or silvery on dorsum of gaster, but almost lacking in some specimens.

Yellowish- to rusty-red, with distinctly darker patches variously on vertex of head, dorsum of mesosoma and petiole. Mandibles with teeth dark reddish-brown; spines light reddish-brown. Anterior clypeal margin, frontal carinae and lateral margins of mesosoma narrowly lined with dark brown or black. Appendages, including coxae, light to medium reddish-brown; tarsi a shade darker. Gaster mostly dark, reddish-brown.

Queen
Kohout (2013) - TL c. 6.85-7.16; HL 1.56-1.59; HW 1.18-1.22; CI 74-78; SL 1.56-1.59; SI 128-135; PW 1.40-1.47; MTL 1.81-1.84 (6 measured).

Apart from sexual characters, very similar to worker except: pronotal humeri narrowly rounded or bluntly angular in some specimens; mesoscutum as wide as long, lateral margins converging anteriorly into moderately rounded anterior margin; median line weakly indicated; parapsides flat; mesoscutum in profile widely rounded onto flat dorsum. Mesoscutellum flat, only marginally raised above dorsal plane of mesosoma. Propodeum armed with pair of horizontal, acute spines; dorsum rounding medially into virtually vertical declivity. Petiole with posterior face weakly convex; spines rather short, widely divergent. Sculpturation, pilosity, pubescence and colour identical to worker.