Polyrhachis yarrabahensis

This is an arboreal nester, building polydomous nests of silk and vegetation debris upon the leaves of various lowland rainforest trees, shrubs and vines (Kohout 1999; Robson & Kohout 2005, 2007; Kohout, 2012).

Identification
Kohout (2012) - P. yarrabahensis is similar to Polyrhachis lombokensis. In addition to trivial differences in the degree of the descending angle of the propodeal declivity, the convexity of the eyes and the outline of pronotal humeri (listed by Kohout & Taylor 1990), the most obvious characters separating them are the intensity of sculpturation, their colour and the development of the propodeal teeth. In P. yarrabahensis the sculpturation, notably of the head, is distinctly reticulate-punctate, with the intensity increasing in front of the eyes and towards the sides of the head. The colour of the head is distinctly darker, almost black in some specimens, contrasting with the mostly medium to dark reddish-brown colour of the rest of the body. In contrast, the sculpturation of the head in P. lombokensis is very fine, rather polished and the body, including the head, is more-or-less uniformly medium reddish-brown. The propodeal teeth in P. yarrabahensis are rather poorly developed and more-or-less angular, while they are distinctly upturned and acute in P. lombokensis. The differences in the queens of the species are even more distinct. The sculpturation, notably of the head, in P. lombokensis is very fine and highly polished, while in the queen of P. yarabahensis it is markedly more distinct than in the worker, consisting of small punctures superimposed upon rather irregular reticulation. In P. yarrabahensis the mesoscutum in dorsal view is about as long as wide, with the lateral margins strongly converging anteriorly, forming a rather narrowly rounded anterior margin. In profile, the mesoscutellum is convex and distinctly elevated above dorsal plane of mesosoma. In contrast, the mesoscutum in P. lombokensis is distinctly wider than long, with a rather widely rounded anterior margin. The mesoscutellum is only weakly raised above dorsal plane of mesosoma, and is virtually flat before rounding into the propodeal groove. In addition, the mesosomal dorsum of P. lombokensis has several medium length, erect hairs that are completely absent in P. yarrabahensis.

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

The known distribution of P. yarrabahensis extends from New Guinea south to northern Queensland, where it appears to be most abundant within the Wet Tropics region.

Castes
Males and immature stages (eggs, larvae and pupae) in QM spirit collection. (Kohout 2012)

Nomenclature

 *  yarrabahensis. Polyrhachis (Myrmatopa) lombokensis var. yarrabahensis Forel, 1915b: 115 (w.q.) AUSTRALIA. Raised to species: Kohout & Taylor, 1990: 520. Junior synonym of lombokensis: Kohout, 2000: 205. Revived from synonymy: Kohout, 2012: 47.

Worker
Kohout (2012) - TL c. 5.44-6.80; HL 1.43-1.68; HW 1.15-1.40; CI 78-83; SL 1.78-2.12; SI 147-159; PW 0.69-0.90; MTL 1.81-2.25 (30 measured).

Anterior clypeal margin with wide median truncation flanked by blunt denticles. Clypeus with poorly defined, blunt, median carina; clypeus in profile straight for most of its length, abruptly rounding into moderately impressed basal margin. Frontal triangle distinct. Frontal carinae sinuate with only moderately raised margins; central area relatively wide; frontal furrow distinct anteriorly, poorly indicated posteriorly. Sides of head in front of eyes converging towards mandibular bases in virtually straight line; behind eyes sides rounding into evenly convex occipital margin. Eyes convex, in full face view clearly breaking lateral cephalic outline. Ocelli lacking. Pronotal humeri obtusely angular, almost rounded in some specimens; lateral pronotal margins rather blunt posteriorly; promesonotal suture strongly impressed. Mesonotum with lateral margins strongly converging posteriorly; metanotal groove distinct. Propodeum with lateral margins blunt, terminating posteriorly in poorly developed, right-angled teeth. Petiole armed with a pair of relatively short, dorsally and weakly posteriorly directed, diverging spines. Anterior face of first gastral segment distinctly higher than full height of petiole, widely rounding onto dorsum.

Mandibles very finely longitudinally striate with numerous piliferous pits. Head distinctly reticulate-punctate, sculpturation more intense towards mandibular bases and sides of head. Dorsum of mesosoma and petiole more finely reticulate-punctate with pronotal sculpture consisting of irregular, mostly longitudinal and laterally curving fine striae. Sides of mesosoma finely, mostly obliquely, reticulate. Antennal scapes finely, closely punctate. Fore coxae and gaster very finely shagreened, highly polished.

Mandibles with a few curved, golden hairs near masticatory borders and numerous short, appressed hairs towards mandibular bases. Anterior clypeal margin with fringe of short setae. Several medium length, paired hairs near anterior and basal clypeal margins and along frontal carinae. A few somewhat longer hairs on fore coxae and single shorter hairs on venter of trochanters and femora. Gaster with several longer hairs lining dorsoposterior margins of apical segments, hairs more numerous on gastral venter.

Colour. Generally medium to dark reddish-brown; head mostly dark reddish-brown to black, with mandibles and anterior portion of clypeus distinctly lighter. Mandibular teeth and anterior clypeal margin narrowly lined with dark brown. Dorsum of mesosoma often blotched dark reddish-brown, almost black in some specimens, with pronotal collar, sutures and sides of mesosoma distinctly lighter. Antennae and legs, including coxae, mostly light to medium reddish-brown. Petiole and gaster reddish-brown; base and venter of gaster a shade lighter; margins of segments lined with dark brown.

Queen
Kohout (2012) - TL c. 8.01-8.87; HL 1.90-1.96; HW 1.47-1.57; CI 77-83; SL 2.28-2.34; SI 149-156; PW 1.62-1.75; MTL 2.43-2.56 (12 measured). Apart from sexual characters, closely resembling worker except: distinctly larger; pronotal humeri bluntly angular; mesoscutum as long as wide, with lateral margins converging anteriorly, forming rather narrowly rounded anterior margin; median line distinct; parapsides flat; anterior margin in side view widely rounding onto flat dorsum. Mesoscutellum convex, elevated above dorsal plane of mesoscutum. Propodeum immarginate; dorsum posteriorly armed with short, upturned teeth. Petiolar spines virtually identical to those in worker. Sculpturation and pilosity as in worker. Generally darker coloured than worker with head and dorsum of mesosoma very dark reddish-brown or black; mandibles, pronotal collar and sides of pronotum, mesopleuron, petiole and appendages a shade lighter, reddish-brown. Colour of gaster as in worker.