Polyrhachis gibbula

The type material was collected from a dead twig in a semideciduous forest.

Identification
Rigato (2016) - A species in the viscosa-group well characterised by its trapezoidal head with bulging eyes, dorsum of head and mesosoma with distinct longitudinal rugulosity and strongly transversely-arched propodeum.

This taxon looks superficially similar to Polyrhachis gerstaeckeri, but gibbula has a distinctive trapezoidal head, bulging eyes, and well developed dorsal longitudinal rugosity.

Distribution based on Regional Taxon Lists
Afrotropical Region: Kenya, United Republic of Tanzania.

Nomenclature

 *  gibbula. Polyrhachis gibbula Rigato, 2016: 24, figs. 7a-c (w.) KENYA.

Worker
Holotype. HL 1.49, HW 1.29, CI 87, SL 1.58, SI 122, FW 0.44, FI 34, PW 1.39, WL 1.9, HTL 1.57.

Clypeus subcarinate, with an evenly convex anterior margin. Head in full face view subtrapezoidal, wider behind than in front, with distinct, rounded posterior corners and weakly convex posterior margin. Eyes placed posteriorly and strongly bulging. Scapes moderately long. Mesosoma stout; in dorsal view all of its segments distinctly wider than long. In profile mesosoma convex at the level of anterior pronotum and postero-dorsal propodeum, and almost flat medially. Pronotal spines relatively well developed and strong, mostly anteriorly directed. Pronotum and mesonotum transversely evenly and weakly convex; propodeal dorsum humped medially and forming a stronger transverse convexity. Mesosoma distinctly marginate along its sides, the marginations not forming flanges or lobes. Promesonotal and metanotal sutures distinct but not impressed. Propodeal dorsum and declivity separated by a distinct margin. Propodeal teeth small, upturned and sharp. Petiole with four spines, the dorsal pair weakly diverging, directed upward and backward and about twice as long as the lateral pair; in frontal (or posterior) view the space between the dorsal spines almost straight with a very shallow convexity in the middle. First gastral tergite anteriorly concave.

Mandibles mostly finely shagreened and with sparse, small piligerous pits. Integument weakly shining, finely reticulate-punctate throughout (including appendages), gaster more superficially so. Dorsum of head and mesosoma with superimposed fine and dense longitudinal rugulation. Mesosoma laterally roughly reticulate-rugose. Propodeal declivity superficially finely reticulate.

Scattered, thin, yellowish and relatively short standing hairs occur on clypeus, head dorsum (3 pairs from the level of antennal insertions to the vertex), and all gastral tergites and sternites. Pubescence inconspicuous on head and mesosoma, minute and very sparse on the gaster.

Colour black, appendages brown; articulations, scapes and coxae darker.

Paratypes. (n=2). HL 1.38–1.54, HW 1.27–1.32, CI 86–92, SL 1.48–1.62, SI 117–123, FW 0.45–0.47, FI 34–37, PW 1.17–1.43, WL 1.72–2.0, HTL 1.53–1.59. The Kenyan specimen is very similar to the holotype. The specimen from Tanzania is smaller and with slightly different indices. It also has shorter, toothlike pronotal spines, shorter lateral pair of petiolar spines, a less humped propodeum with a weakly convex margination between dorsum and declivity and almost lacks propodeal teeth; yet all of the remaining features well match those of the Kenyan specimens.

Type Material
Holotype. KENYA: 20 km WSW Malindi, 20 m, 3°18’ S, 39°58’ E, 16.xii.1990 (P.S. Ward) # 11167, semideciduous forest, ex dead twig. Paratypes. 1 worker with the same data as the holotype (BMNH).

References based on Global Ant Biodiversity Informatics

 * Rigato F. 2016. The ant genus Polyrhachis F. Smith in sub-Saharan Africa, with descriptions of ten new species. (Hymenoptera: Formicidae). Zootaxa 4088: 1-50.