Polyrhachis lachesis

Polyrhachis lachesis inhabits open eucalypt forests and savannah woodlands, seemingly preferring bare ground without a grass cover as their nesting sites. It is not a very common species which occurs from Lakefield on Cape York Peninsula, south to about Gladstone.

Identification
A member of the schenkii species-group in the Polyrhachis subgenus Hagiomyrma. Kohout (2013) - Polyrhachis lachesis is very similar to Polyrhachis paxilla, with most distinguishing characters given under the latter species. It is also relatively close to Polyrhachis schenkii and it seems that Forel originally considered P. lachesis to be a subspecies of the latter (the original labels on syntype specimens read ‘P. Schenki For. r. Lachesis For.’). However, they are are easily separated by differences in their size (HL 1.31-1.56 in P. schenkii versus 1.84-1.96 in P. lachesis), 210-250 in P. schenkii versus 160-185 in P. lachesis) and the outline of the pronotal humeri that, in P. lachesis, are produced into dilated, dorsally shallowly concave, rounded prominences, while they are narrowly rounded or subangular in P. schenkii.

Key to Polyrhachis Hagiomyrma group species

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Castes
Male and immature stages unknown.

Nomenclature

 *  lachesis. Polyrhachis lachesis Forel, in Emery, 1897d: 582 (footnote) (w.) AUSTRALIA. Combination in P. (Hagiomyrma): Emery, 1925b: 185.

Worker
Kohout (2013) - (syntypes cited first): TL c. 7.56-7.96, 7.06-8.21; HL 1.87-1.90, 1.84-1.96; HW 1.43-1.47, 1.40-1.53; CI 76-77, 76- 79; SL 2.18-2.21, 2.14-2.34; SI 150-152, 148-155; PW 1.25-1.34, 1.22-1.34; MW 0.78, 0.69-0.81; PMI 160-172, 175-185; MTL 2.53-2.56, 2.50-2.74 (2+10 measured).

Anterior clypeal margin with acutely denticulate median flange, laterally flanked by widely obtuse teeth. Clypeus with distinct median carina; sinuate in profile, posteriorly rounding into moderately impressed basal margin. Frontal triangle indistinct. Frontal carinae sinuate with narrowly raised margins. Sides of head in front of eyes weakly convex towards mandibular bases; behind eyes, sides rounding into convex occipital margin. Eyes moderately convex, in full face view only reaching (in some modern specimens) or exceeding (in syntypes) lateral cephalic outline. Ocelli lacking. Pronotal humeri produced into dilated, dorsally shallowly concave, rounded prominences with distinctly raised margins; lateral margins behind humeri emarginate or notched, subparallel and rounding into laterally deeply impressed promesonotal suture. Mesonotum with lateral margins very narrowly raised, posteriorly converging into poorly indicated metanotal groove; propodeal margins terminating in slender, subparallel spines, obliquely elevated at bases and sinuate in side view. Petiole armed with pair of slender, divergent, acute spines with tips weakly curved outwards. Anterior face of first gastral tergite straight at base, widely rounding onto dorsum.

Mandibles longitudinally striate with piliferous pits. Clypeus closely punctate; vertex, sides of head and dorsum of mesosoma reticulate-punctate with sculpture forming weakly impressed vermiculate pattern, somewhat more distinct on vertex of head. Sides of mesosoma and posterior face of petiole finely reticulate. Anterior face of petiole, spines and gaster very smooth and highly polished.

Mandibles with curved, golden hairs at masticatory borders and shorter hairs along outer margins. Anterior clypeal margin with several setae medially and fringe of shorter setae laterally. Dorsa of head, mesosoma, and petiole with numerous, short, bristle-like hairs, many hairs fringing lateral and dorsal outline of head; appendages, including antennal scapes, with very short, erect hairs. Gaster with abundant, posteriorly inclined, short golden hairs; hairs marginally longer around apex and on venter of gaster. Very sporadic, short, appressed, golden pubescence in various densities over most body surfaces; pubescence denser and silvery on propodeal declivity and venter of gaster.

Generally light to medium reddish-brown with vertex and mesosomal dorsum a shade darker; mandibular teeth, anterior margin of clypeus, frontal carinae, lateral margins of mesosoma, including spines, and mesopleural margins very narrowly lined with black.

Queen
Kohout (2013) - TL c. 8.37; HL 1.81; HW 1.37; CI 76; SL 1.93; SI 141; PW 1.62; MTL 2.28 (1 measured).

Apart from sexual characters very similar to worker, except: pronotal humeri widely rounded with margins somewhat weakly, irregularly, notched. Mesoscutum marginally longer than wide, rather low anteriorly and flat in lateral view; median line distinct; parapsides flat. Mesoscutellum flat, marginally elevated above dorsal plane of mesosoma. Propodeal spines short, subparallel; petiolar spines shorter than in worker, obliquely elevated, divergent. Sculpturation distinctly coarser than in worker, notably on dorsa of mesoscutum and mesoscutellum, which are rather deeply rugose, without any regularity in pattern. Pilosity, pubescence and colour virtually identical to worker.