Polyrhachis gravis

Polyrhachis gravis is a characteristic species of the dry, mulga and spinifex clad country of the central Australia. It has been collected in the Pilbara region in Western Australia and across the Northern Territory to western Queensland (Kohout, 2013).

Identification
A member of the Polyrhachis gravis species-group. Kohout (2013) - In spite of being widely distributed, P. gravis is relatively morphologically uniform with only minor variations from the types detected in some specimens from Western Australia and Queensland. These are manifest mostly in the direction of the body sculpturation, the apical width of the propodeal dorsum and the length of the dorsal petiolar spines that can vary even in specimens of a single nest series (e.g. specimens from Kunoth Pdk, nr Alice Springs).

Key to Polyrhachis gravis species-group workers

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Castes
Sexuals and immature stages unknown.

Nomenclature

 *  gravis. Polyrhachis (Campomyrma) gravis Clark, 1930c: 15, fig. 1 (w.) AUSTRALIA.

Worker
Worker.-Length, 7·5-9 mm.

Black. Mandibles, apical segments of the antennae, legs and four posterior coxae reddish brown, anterior coxae black. In a few examples the tibiae are darker than the femora.

Shining. Head very finely striate-rugose longitudinally. Clypeus slightly rugose behind, punctate in front. Mandibles very finely and densely striate longitudinally. Pronotum longitudinally arched striate-rugose, diverging outward behind, almost transverse in front. Mesonotum and epinotum longitudinally striate-rugose, the striae following the contour of the segments. Sides of the thorax longitudinally striate, much stronger than on the dorsum, declivity transversely striate. N ode transversely striate in front and behind. Gaster finely and microscopically striate-punctate, with a longitudinally arched direction. Anterior coxae finely transversely rugose.

Hair yellow, erect, very short and sparse throughout, except on the apex of the gaster.

Head slightly longer than broad, the occipital border appearing strongly convex, but really composed of three straight portions, the base, or centre, short, the portions from the base to the angles three times longer than the base, sides convex. Frontal carinae parallel, or very feebly diverging behind. Clypeus broad and convex, not carinate, the anterior border broadly produced, straight, feebly crenulate. Eyes large and convex, placed at the posterior angles. Scapes extending beyond the occipital border by more than half their length; first segment of the funiculus slightly longer than the second, the others subequal to the apical. Mandibles armed with six large, sharp teeth. Thorax one and one-half times longer than broad. Pronotum almost twice as broad as long, convex and marginate in front and sides, the posterior border almost straight, the anterior angles bluntly produced. Mesonotum broader than long, one and a-half times broader in front than behind, the sides marginate. Epinotum one-third longer than broad, fully twice as broad in front as behind, the sides strongly marginate, produced behind as short, sharp teeth, directed upward, their length equal to their distance apart at the base, The declivity abrupt, concave. as long as the dorsum. Node thick, broader than long, furnished with four sharp, slender spines, the middle pair slightly longer than the lateral pair, longer than their distance apart, parallel, the points of the lateral pair level with the base of those in the middle. First segment of the gaster strongly margined in front, and anterior two-thirds of the sides. Legs long and slender.

Kohout (2013) - (holotype cited first, paratype second): TL c. 10.58, 8.77, 8:12-10.58; HL 2.43, 2.25, 2.18-2.48; HW 2.17, 1.93, 1.93-220; CI 89, 86, 86-92; SL 274, 2.53, 2.49-2.81; SI 126, 131, 125-131; PW 1.72, 1.56, 1.56-1.72; MTL 3.28, 3.09, 3.09-3.38 (1 +1 +14 measured).

Type Material


Kohout (2013) - Based on the original description, it appears that both available specimens of P. gravis were regarded by Clark as equals, i.e. syntypes. However, one of the specimens is furnished with a red tag which reads: T-6238, Type. When Dr Robert W. Taylor examined both specimens, he evidently considered this specimen to be the holotype and added a new red tag reading ‘HOLOTYPE, T-6238, Polyrhachis gravis Clark’. I am following Taylor’s decision and accept this specimen as the holotype of P. gravis, with the second specimen labelled with ‘PARATYPE, T-9088’ on a blue tag, considered a paratype.