Polyrhachis gerstaeckeri Forel
Creators: name: Rigato, Fabrizio
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Hymenoptera
Formicidae
Polyrhachis
Polyrhachis gerstaeckeri

description: Polyrhachis gerstaeckeri Forel
(Figures 6 a–c)
Polyrhachis gerstaeckeri Forel, 1886: 197. Holotype worker, TANZANIA: Zanzibar (M. Hildebrandt) (MNHU) [examined]. Polyrhachis cubaensis var. striolatorugosa Mayr, 1893: 195. Holotype worker, TANZANIA: Zanzibar (F. Stuhlmann) [not seen, not at NHMW]. Syn. n.
Diagnosis. A species in the viscosa -group with this distinctive character combination: 1) propodeal dorsum and declivity separated by a thin, medially arched ridge; 2) petiole quadrispinose (the mid-pair of spines longer); 3) all mesosomal segments distinctly transverse in dorsal view; 4) sculpture finely reticulate-punctate with an ill-defined longitudinal rugulation on mesosomal dorsum and cephalic vertex.
Holotype worker. HL 1.60, HW 1.43, CI 89, SL 1.70, SI 119, FW 0.45, FI 31, PW 1.47, WL 2.08, HTL 1.72. (N.B.: gaster missing from the holotype)
Clypeus faintly carinate, its anterior margin evenly convex. Head in full face view oval, wider behind than in front; sides and posterior margin weakly convex. Eyes placed close to posterior corners and moderately protruding. Scapes moderately long. Mesosoma stout, in profile pronotum anteriorly and propodeum slightly convex, mesonotum almost flat. In dorsal view pronotum, mesonotum and propodeum distinctly wider than long. Pronotal teeth mostly anteriorly directed, well developed and strong. Pronotum and mesonotum transversely weakly convex, propodeum more arched. 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, medially more raised and almost toothlike. Propodeal teeth small, upturned and blunt. Petiole with four spines: dorsal pair almost parallel, upward directed and moderately bent backward, distinctly longer than the lateral pair. In frontal (or posterior) view the space between the dorsal pair very weakly concave, almost straight.
Mandibles mostly finely shagreened. The whole body finely reticulate-punctate with superimposed fine and moderately developed rugulosity especially on cephalic and mesosomal dorsum. Head mostly reticulate-rugulose; mesosomal dorsum with longitudinal rugulosity. Propodeal declivity superficially finely reticulate.
Standing hairs almost absent, except for those fringing the anterior clypeal margin. Pubescence on the body very short and sparse.
Colour black throughout.
A recently collected worker from Zanzibar (figs. 6a-c), the type locality of gerstaeckeri, has the following data:
HL 1.50, HW 1.38, CI 92, SL 1.66, SI 120, FW 0.42, FI 30, PW 1.37, WL 1.93, HTL 1.63.
Very similar to gerstaeckeri holotype, except that the margin between propodeal dorsum and declivity is more evenly convex and less medially pronounced.
Also, it bears the following features, which cannot be seen in the gasterless holotype:
First gastral tergite finely reticulate-punctate and faintly shining, widely and slightly concave anteriorly. Scattered standing hairs occurring on gastral tergites IV-V and all sternites. Gastral pubescence very short, sparse and inconspicuous (distance between adjacent hairlets at least twice the pubescence length).
Also, I assign to gerstaeckeri 3 workers from coastal continental Tanzania and 1 from coastal Kenya: HL 1.45– 1.58, HW 1.31–1.47, CI 87–95, SL 1.60–1.76, SI 118–123, FW 0.38–0.43, FI 29–31, PW 1.15–1.36, WL 1.85– 2.08, HTL 1.52–1.72. (n=4)
Comment. Bolton (1973) thought P. gerstaeckeri Forel was a synonym of P. cubaensis Mayr (see above); yet, for the reasons reported under cubaensis, and after the examination of the gerstaeckeri type, I consider gerstaeckeri as a distinct species and senior synonym of striolatorugosa Mayr. Both gerstaeckeri and striolatorugosa types come from Zanzibar and the original description of striolatorugosa (whose type has not been found) well matches the type of gerstaeckeri.
However, P. gerstaeckeri does not correspond to the description of P. cubaensis provided by Bolton (1973). Polyrhachis gerstaeckeri differs for its transverse propodeal dorsum, less developed median prominence of the propodeal ridge between the propodeal teeth (Bolton also reported propodeal spines instead of teeth for “ cubaensis ”), a weaker longitudinal rugosity, and absence of standing hairs from the dorsum of the body (some hairs are mentioned as occurring on head and gastral dorsum of the specimen described by Bolton, 1973).
I examined the specimens Bolton used to describe cubaensis Mayr (whose type he did not see) and I think they correspond to P. cubaensis subsp. wilmsi Forel. Forel’s short description of wilmsi points out some important diagnostic features. Hence, I remove the subsp. wilmsi from the synonymy with cubaensis and raise it to species rank (see Comment under wilmsi).
Polyrhachis gerstaeckeri shares several features with P. wilmsi, but the latter has a better defined margination between the propodeal dorsum and the declivity, with a stronger median prominence, and a more conspicuous rugulation on the head.
The main differences between P. gerstaeckeri and P. wilmsi can be summarized as follows:
Polyrhachis gerstaeckeri Polyrhachis wilmsi
Size smaller, HL ≤ 1.60, HW <1.50. Size larger, HL ≥ 1.70, HW> 1.50
Frons narrower, FI ca. 30 Frons wider, FI ca. 35
Frons and vertex devoid of standing hairs. Frons and vertex bearing a few pairs of standing hairs. Propodeal dorsum and declivity separated by a more or less Propodeal dorsum and declivity separated by a strongly arched ridge, medially at most hardly forming a toothlike arched ridge, medially forming a distinct toothlike prominence. Propodeal dorsum bearing upturned small teeth prominence. Propodeal dorsum bearing stout upturned spines at its posterior corners. at its posterior corners.
Propodeal dorsum about 1.5 times as wide as long Propodeal dorsum about as wide as long
Gastral pubescence more sparse. Each element is at least Gastral pubescence denser. Each element is at most about as about twice as distant as its own length from the closest one distant as its own length from the closest one
Material examined. TANZANIA: Unguga Region, Jozani Forest Res. Zanzibar, 19 m, 6.26755 S 39.41111 E, 30.x–4.xi.2007, CEPF-TZ-10.4-F25 ground water forest, hand collected (P. Hawkes, M. Bhoke, U. Richard) (1 w, AFRC: AntWeb code CASENT0235679); Tanga, Maxamba, 29.v.1990, 72 A, on Citrus (Löhr) (3 w, BMNH). KENYA: Coastal province, Arabuko Sokoke forest, 50 m ca., 39°55’45.1” E 3°19’16.1” S, vi.2009, hand coll. (F. Hita Garcia & G. Fischer) (1 w, HLMD).
description: Published as part of Rigato, Fabrizio, 2016, The ant genus Polyrhachis F. Smith in sub-Saharan Africa, with descriptions of ten new species. (Hymenoptera: Formicidae) in Zootaxa 4088 (1) on pages 22-24, DOI: 10.11646/zootaxa.4088.1.1, http://zenodo.org/record/261854