Polyrhachis semiobscura appears to be lignicolous in its nesting habit. The only known colony was found nesting in two adjacent galls attached to a twig of a small sapling in the open forest at Iron Range Nat. Park. At Varirata Range in Papua New Guinea numerous workers of P. semiobscura were collected from the rims of the pitchers of a species of Nepenthes. Despite the large numbers of workers and their rather busy activity around the rims, none were found trapped in the liquid at the bottom of the pitchers.
Polyrhachis semiobscura is one of only four Hagiomyrma species originally described from specimens from outside the Australian mainland, the others being Polyrhachis denticulata, Polyrhachis metella and Polyrhachis schenckii.
A member of the penelope species-group in the Polyrhachis subgenus Hagiomyrma. In P. semiobscura, Australian specimens have distinctly less dilated pronotal humeri and more abundant dorsal pubescence, notably on mesonotum and propodeum, compared to their New Guinean counterparts. However, considering their uniformity in other characters, such as the longitudinal striation of the head, the position of the propodeal spiracles on short, laterally projecting tubercles and the rather remarkable differences in the size of the workers (HL 1.31-1.84) from both populations, Kohout (2013) is hesitant to consider them separate species.
Keys including this Species
The known distribution of P. semiobscura ranges from New Ireland in the Bismarck Archipelago to Papua New Guinea and south across the Torres Strait islands as far as Iron Range on Cape York Peninsula.
Distribution based on Regional Taxon Lists
Distribution based on AntMaps
Distribution based on AntWeb specimens
Check data from AntWeb
Male unknown. Immature stages present in the QM collection.
The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.
- semiobscura. Polyrhachis (Hagiomyrma) semiobscura Donisthorpe, 1944c: 65 (w.) NEW GUINEA (Bismarck Archipelago).
Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.
Kohout (2013) - (holotype cited first): TL c. 7.00, 5.04-7.62; HL 1.71, 1.31- 1.84; HW 1.51, 1.09-1.64; CI 88, 83-89; SL 1.91, 1.53-1.96; SI 126, 119-140; PW 1.28, 0.94-1.37; MW 0.68, 0.59-0.78; PMI 188, 151-188; MTL 2.12, 1.68-2.37 (1+9 measured).
Anterior clypeal margin with denticulate, median flange. Clypeus with distinct median carina; straight, or very weakly sinuate in profile with shallowly impressed basal margin. Frontal triangle poorly indicated. Frontal carinae sinuate with weakly raised margins. Sides of head in front of eyes weakly convex towards mandibular bases; behind eyes, sides rounding into convex occipital border. Eyes moderately convex, in full face view not or only marginally exceeding lateral cephalic outline. Ocelli lacking. Pronotum with humeri dilated (distinctly less dilated in Australian specimens), rounded, with lateral margins somewhat emarginate behind, subparallel or weakly converging into well impressed promesonotal suture. Mesonotum with lateral magins converging into medially indistinct, flat, metanotal groove. Propodeal spiracles situated on moderately projecting tubercles. Propodeal margins terminating in more-or-less horizontal, subparallel, acute spines with tips weakly curved outwards. Petiole with divergent, slender spines, weakly elevated in lateral view. Anterior face of first gastral segment higher than full height of petiole, widely rounding on dorsum.
Mandibles finely, longitudinally striate with piliferous pits. Head, mesosoma and petiole reticulate-punctate; vertex with more-or-less regular, rather distinct, longitudinal striae, extending anteriorly between eyes and frontal carinae and along sides of head; sculpturation rather smooth with distinctly polished appearance. Dorsum of mesosoma with reticulae more irregular and less distinct; spines finely sculpured along entire length. Gaster finely shagreened.
Mandibular masticatory borders with several curved, golden hairs and a few short, erect hairs along outer borders towards bases. Anterior clypeal margin medially with only a few, anteriorly directed setae. All body surfaces with rather short, mostly erect hairs, barely as long as half greatest diameter of eyes. Only a few, very short, bristle-like hairs lining leading edge of antennal scapes. Hairs distinctly longer and more posteriorly directed around apex and on venter of gaster. Closely appressed, mostly silvery or pale golden pubescence in various densities on head, mesosoma and petiole; pubescence distinctly more abundant and more golden on mesonotal and propodeal dorsa in Australian specimens. Gaster with somewhat longer, golden pubescence on dorsum, silvery on venter.
Black; mandibular teeth dark reddish-brown.
Kohout (2013) - TL c. 8.06; HL 1.72; HW 1.43; CI 83; SL 1.81; SI 126; PW 1.62; MTL 2.12 (1 measured).
Apart from sexual characters very similar to worker, except: dorsum of mesoscutum wider than long with widely rounding anterior margin in dorsal view; flat in lateral view; median line bifurcate anteriorly; parapsides weakly raised. Mesoscutellum flat, not elevated above dorsal plane of mesosoma. Spines distinctly shorter than in worker. Sculpturation, pilosity, pubescence and colour similar to worker.
- Holotype, worker, Kavieng, New Ireland, Papua New Guinea, L.E. Cheesman, The Natural History Museum.
- Donisthorpe, H. 1944c. A new subgenus and three new species of Polyrhachis F. Smith (Hym., Formicidae). Entomol. Mon. Mag. 80: 64-66 (page 65, worker described)
- Kohout, R.J. 2013. Revision of Polyrhachis (Hagiomyrma) Wheeler, 1911 (Insecta: Hymenoptera: Formicidae: Formicinae). Memoirs of the Queensland Museum, Nature 56, 487-577.