Polyrhachis semiobscura

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 schenkii.

Identification
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.

Distribution
This taxon was described from New Guinea (Bismarck Archipelago).

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.

Nomenclature

 *  semiobscura. Polyrhachis (Hagiomyrma) semiobscura Donisthorpe, 1944c: 65 (w.) NEW GUINEA (Bismarck Archipelago).

Additional References

 * Kohout, R.J. 2013. Revision of Polyrhachis (Hagiomyrma) Wheeler, 1911 (Insecta: Hymenoptera: Formicidae: Formicinae). Memoirs of the Queensland Museum, Nature 56, 487-577.