Polyrhachis omissa

The type labels, collected from numerous locations, are bereft of biological details.

Identification
A stout species in the viscosa-group, with strongly reduced pilosity, opaque integument, propodeal dorsum and declivity separated by a medially protruding transverse ridge, and petiole armed with a lateral pair of spines and a dorsal pair of teeth. Very similar to Polyrhachis viscosa, but with ordinary shaped scape and first funicular joint.

I often found specimens of P. omissa labelled as P. viscosa and mixed with it, but omissa always lacks the distinctive antennal features that separate viscosa from all other African Polyrhachis. Polyrhachis viscosa has a strongly widened apex of the scape and a strongly depressed first funicular joint (see Fig. 22 in Bolton, 1973). This feature is unique to viscosa and seemingly constant. Although I found several gynes assignable to P. omissa collected in Yemen, Collingwood and Agosti (1996) in their survey of Arabian ants mentioned P. viscosa and Polyrhachis lacteipennis only. Polyrhachis lacteipennis superficially recalls viscosa and omissa, but belongs to the non-African subgenus Myrmhopla and has a slender and immarginate mesosoma and other very distinctive features.

The main features separating omissa from viscosa workers can be summarized as follows:

Distribution based on Regional Taxon Lists
Afrotropical Region: Ethiopia, Kenya, Somalia, Yemen.

Nomenclature

 *  omissa. Polyrhachis omissa Rigato, 2016: 34, figs. 11a-c (w.) SOMALIA.

Worker
Holotype. HL 1.62, HW 1.49, CI 92, SL 1.80, SI 121, FW 0.41, FI 28 PW 1.33, WL 2.10, HTL 1.70.

Clypeus ecarinate, its anterior margin evenly convex and medially crenulate. Head widely oval, distinctly wider around the level of the eyes and much narrower at the level of mandibular insertions. Frons relatively narrow, frontal carinae sinuous. Antennae moderately long. Eyes large and flat, placed close to the posterior corners of the head. Mesosoma stout, nearly flat in profile, mesonotum more than twice as wide as long. Promesonotal suture narrow, but well marked, metanotal suture faint, hardly visible. Pronotal teeth well developed and slightly diverging. Propodeal dorsum bearing an upturned small tooth at each posterior corner; propodeal dorsum and declivity separated by a thin ridge strongly medially raised as an antero-posteriorly flattened lobe. Petiolar scale wide, armed with 4 equidistant spines and teeth: a lateral pair of spines and a dorsal pair of sharp teeth; the space between dorsal teeth straight. First gastral tergite anteriorly concave.

Integument matt; ground sculpture finely reticulate-punctate all over the body and more superficially so on appendages. A superimposed, irregular reticulate rugulosity covers most of head and mesosoma in a somewhat areolate pattern.

Standing hairs almost lacking: occurring only at the anterior clypeal margin and on gastral tergites III–V and all sternites. Pubescence very short and sparse, hardly visible on the body.

Colour black throughout.

Paratypes. (n=10). HL 1.56–1.80, HW 1.39–1.64, CI 88–92, SL 1.68–2.00, SI 118–127, FW 0.35–0.44, FI 25–28, PW 1.20–1.56, WL 1.90–2.32, HTL 1.56–1.98. Mostly consistent with the holotype, but with some minute variations. Eyes more or less slightly convex, metanotal suture sometimes completely lacking dorsally, and median lobe of propodeum reduced to a low convexity.

Queen
Paratypes. (n=12). HL 1.76–1.99, HW 1.49–1.72, CI 82–88, SL 1.81–2.13, SI 116–128, FW 0.42–0.50, FI 28–30, ScW 1.73–2.05, MnL 2.13–2.63, WL 2.93–3.50, HTL 1.85–2.21, Anterior wing length 8.1-9.3.

With the usual caste differences from the worker and with weakly convex eyes. Wings moderately infuscated.

Male
Paratype. HL 1.24, HW 1.10, CI 89, SL 1.40, SI 127, ScW 1.53, MnL 2.16, WL 2.87, HTL 1.95. (I confidently assign to this taxon a single male collected together with several gynes in Yemen).

Mandibles narrow, almost unarmed, with a short masticatory margin bearing a blunt apical cleft tooth and 1 or 2 minute blunt denticles. Anterior clypeal margin entire, evenly convex. Head round with relatively small eyes (maximum eye length: 0.47), Ocelli well developed: MOD (mid-ocellus diameter): 0.18; distance between mid ocellus and each lateral one < MOD. Distance between lateral ocelli: 0.46. Length of anterior wing: approx. 7.4. Petiolar scale thick, wide and low; in frontal view the petiolar dorsum is evenly weakly convex and is separated from sides by a weak blunt angle.

Integument subopaque, finely reticulate-punctate; only the head dorsum bears a trace of rugulo-reticulation recalling that of female castes.

Pilosity reduced, mostly as in female castes.

Body and antennae blackish; legs dark brown, mandibles and most articulations slightly paler. Wings as in gyne.

Type Material
Holotype worker. SOMALIA: Balad, 28.ix.1986 (L. Bartolozzi). Paratypes. YEMEN: Sokna (Tihama), m 200, 20.viii.1965 (G. Scortecci) (17 g, 1 m, MSNM). ETHIOPIA: “da Dimé al Bass Narok”, viii–ix.1896 (Bottego) (1 g, ) [misidentified by Emery (1899) as P. viscosa]; Banno, Sagan-Omo, 10.v.1939 (E. Zavattari) (1 w, MSNG); Caschei, Sagan-Omo, 6.vii.1939 (E. Zavattari) (3 g, MSNG). SOMALIA: “M. Umberto I”, iii.1892–93 [?] (E. Ruspoli) (1 g, MSNG); Ganana, iii.1892–93 [?] (E. Ruspoli) (2 g, MSNG); “Boran Galla, Medio Ganale”, vi.1893 (V. Bottego) (1 w, MSNG); “Basso Ganana”, vii– viii.1893 (V. Bottego) (3 w, MSNG); Eil (Nogal), iii–iv.1938 (S. Venzo) (2 g, MSNM); Eil (A. Falzoni) (1 g, MSNM); Gardo, 810m, 21.x.1957 (G. Scortecci) (2 g, MSNM); same data, but 22.x.1957 (2 g, MSNM); Afgoi, v.1972 (L. Masutti) (2 g, MSNM); Afgoi, 2.x.1986 (L. Bartolozzi) (1 w, MSNM). KENYA: Mackinnon (30 km ca. N-NW of Mombasa), ix.1946 (Meneghetti) (1 w, 1 g, MSNM); Malindi, xii.1993 (R. Regalin) (3 w, MSNM); Archer’s Post, Uaso Nyiro river, 2300’, 6.xii.1969 (M.E. Irwin & E.S. Ross) (4 g, ).

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.