Polyrhachis decumbens

Polyrhachis decumbens builds arboreal nests of silk and vegetation debris between the leaves of trees and shrubs. (Kohout 2006)

Identification
Similar to Polyrhachis nomo from New Guinea. Both share the characteristic pile of short, decumbent hairs covering most of the body. However, the pubescence in P. decumbens is abundant, while it is rather diluted in P. nomo. They differ in other aspects, including their relative size (HL 1.22-1.40 in P. decumbens versus 1.47-1.50 in P. nomo), and in the shape of their heads. In P. nomo the head is wider than long (CI 104-106) with the eyes not reaching the lateral cephalic outline in full face view. In P. decumbens the head is mostly longer than wide (CI 93-100) and the eyes clearly break the lateral cephalic outline. Both species feature rounded lateral pronotal margins, but several P. decumbens specimens from Iron Range have weakly indicated, blunt humeral angles and the pronotal dorsum is distinctly widest across the shoulders. Some of these specimens also feature more-or-less distinct, rudimentary propodeal spines or tubercles, that are completely absent in specimens of other populations of P. decumbens and in P. nomo. The spines of the petiole in P. decumbens are subequal, while in P. nomo the lateral petiolar spines are distinctly longer.

Distribution
Cape York Peninsula, north of the 13° parallel, extending to southern Papua New Guinea.

This taxon was described from Australia.

Abundance
Relatively common in its favored rainforest habitats.

Castes
Males unknown, immature stages (eggs, larvae and pupae) in the spirit collection. (Kohout 2006)

Worker
Dimensions (holotype cited ﬁrst): TL c. 5.14, 4.79-5.49; HL 1.34, 1.22-1.40; HW 1.31, 1.17-1.40; CI 98, 93-100; SL 1.65, 1.50-1.78; SI 126, 121-130; PW 1.00, 0.87-1.03; MTL 1.65, 1.65-1.93 (23 measured).

Clypeus in proﬁle straight; basal margin moderately impressed. Frontal triangle indistinct. Frontal carinae sinuate, margins very weakly raised anteriorly, rather ﬂat posteriorly. Sides of head in front of eyes weakly convex; rounding behind eyes into convex occipital margin. Eyes convex, in full face view clearly breaking lateral cephalic outline. Ocelli lacking. Pronotum in dorsal view with humeri widely rounded; greatest pronotal width at or near mid-length of segment. Mesosomal dorsum in proﬁle evenly convex; promesonotal suture distinct, metanotal groove lacking. Petiole with anterior face almost straight, posterior face weakly convex; dorsum armed with four subequal, acute spines. Subpetiolar process acute anteriorly, rounded posteriorly. Anterior face of ﬁrst gastral segment straight, relatively low, narrowly rounding onto dorsum of segment.

Head, mesosoma and gaster shagreened, with intensity of sculpturation markedly increasing laterally, becoming distinctly wrinkled; sculpturation strongly reticulate-rugose on meso- and metapleurae and sides of petiole.

Several curved and suberect hairs on mandibular masticatory borders with shorter appressed hairs towards mandibular bases. Anterior clypeal margin with 1 long and 2 slightly shorter, anteriorly directed setae medially and several short setae fringing margin laterally. Mostly paired, medium length, erect hairs near anterior and basal margins of clypeus, along frontal carinae and on vertex; tuft of usually 4 erect, undulated, medium length hairs on summit of mesosoma. Gaster with numerous semierect hairs lining posterior margins of apical segments, more numerous on venter. Pubescence consisting of abundant very ﬁne, short, decumbent and semierect hairs everywhere, including appendages, but excluding propodeal declivity that is smooth and shiny.

Colour. Black with mandibles, clypeus, sides of head and tibiae medium to dark reddish-brown, except mandibular masticatory borders and legs, including coxae, distinctly lighter. Dorsum of gaster black, posterior margins of segments diffusely lined with dark reddish-brown.

Queen
Dimensions (queen from nest of holotype cited ﬁrst): TL c. 7.00, 7.26; HL 1.62, 1.72; HW 1.43, 1.59; CI 88, 92; SL 1.96, 2.06; SI 137, 129; PW 1.56, 1.68; MTL 2.46, 2.59 (2 measured). Apart from sexual characters, closely resembling worker except: pronotal humeri rounded; mesoscutum marginally wider than long with lateral margins converging anteriorly, forming relatively narrowly rounded anterior margin; median line bifurcate and weakly raised; parapsides ﬂat anteriorly, slightly raised posteriorly; mesoscutum in proﬁle with widely rounded anterior face and very weakly convex dorsum. Mesoscutellum only weakly convex, marginally elevated above dorsal plane of mesosoma. Metanotal groove distinct. Propodeum armed with distinct denticles; dorsum descending abruptly into virtually vertical declivity. Sculpturation, pilosity and colour virtually as in worker.

Male
Unknown.

Type Material
Holotype (QMT99334), 3 paratype workers and paratype queen (from holotype nest) and 2 paratype workers in ; 4 paratype workers (2 from holotype nest) in ; 2 paratype workers (from holotype nest) each in, , and.

Type Locality Information
HOLOTYPE: QUEENSLAND, Cape York Pen., Lockerbie Scrub, 10°46’S, 142°29’E, 19-23.iii.1987, ex nest between leaves, R.J. Kohout acc. 87.67 (worker). PARATYPES: data (and nest) as for holotype (15 workers, 1 dealate queen); data as for holotype, except RJK acc. 87.66 (4 workers).