Polyrhachis viscosa

Nests directly into sandy soil, usually in open localities. Foragers occur mostly on the ground but also ascend low bushes, trees and grass stems. A savannah and arid-zone species, it is interesting to note its occurrence on the coastal plains of Ghana as well as inland in the savannah proper. Alate females were recorded from Sudan in August. (Bolton 1973)

Identification
Rigato (2016) - A viscosa-group species with a uniquely (among all African Polyrhachis) apically enlarged scape and proximally flattened 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: Burkina Faso, Democratic Republic of Congo, Eritrea, Ethiopia, Ghana, Kenya, Malawi, Nigeria, Somalia, South Africa, Sudan, Uganda, United Republic of Tanzania, Zimbabwe.

Nomenclature

 *  viscosa. Polyrhachis viscosus Smith, F. 1858b: 71, pl. 4, fig. 41 (w.) SOUTH AFRICA. Combination in P. (Myrma): Santschi, 1914b: 143. Senior synonym of antinorii: Dalla Torre, 1893: 271; of imatongica, spretula: Bolton, 1973b: 330. See also: Arnold, 1924: 749.
 * antinorii. Polyrhachis antinorii Emery, 1877b: 365, fig. (w.) ETHIOPIA. Junior synonym of viscosa: Dalla Torre, 1893: 271.
 * spretula. Polyrhachis (Myrma) viscosa var. spretula Santschi, 1923e: 294 (w.q.) DEMOCRATIC REPUBLIC OF CONGO. Junior synonym of viscosa: Bolton, 1973b: 330.
 * imatongica. Polyrhachis (Myrma) cubaensis subsp. imatongica Weber, 1943c: 388, pl. 16, fig. 22 (w.) SUDAN. Junior synonym of viscosa: Bolton, 1973b: 330.

Worker
Bolton (1973) - TL 5.9-7.6, HL 1.56-1.96, HW 1.29-1.63, CI 80-86, SL 1.52-2.07, SI 119-135, PW 1.00-1.37, MTL 1.48-2.04. (30 measured.)

Clypeus usually with the anterior margin narrowly notched medially. Apex of scape greatly swollen in dorsal view, three or more times the width just distal of the basal neck and forming a hood which hides the base of the first funicular segment in dorsal view. First segment of funiculus dorsoventrally flattened basally. Eyes flat to weakly convex. Alitrunk marginate throughout its length. Promesonotal suture distinct; metanotal groove absent from or only very faintly present on the dorsal alitrunk, its location usually marked only by a weak indentation of the lateral margination or by a break in the sculpturation. Pronotal spines long and acute; propodeal teeth short, upcurved, connected by a transverse ridge running across the posterior margin of the propodeum, the ridge raised into a blunt tubercle medially. The size of this median propodeal tubercle is variable, being almost absent in small individuals, but occasionally as large as the propodeal teeth. Node of petiole with the lateral pair of spines long and directed upwards and backwards. Between them is a pair of shorter, acute spines of variable length. Anterior face of the first gastral segment concave medially.

Dorsum of alitrunk and gaster without erect hairs; all surfaces of body with a very sparse short, greyish pubescence, which may be absent.

Clypeus with a fine, longitudinal striation, loosely overlaid by fine shallow punctures. Head and alitrunk loosely and finely reticulate-rugose with the interspaces reticulate-punctate. Gaster finely and densely reticulate-punctate.

Rigato (2016) - (n=16): HL 1.58–1.87, HW 1.30–1.59, CI 79–86, SL 1.70–2.15, SI 125–142, FW 0.40–0.50, FI 30–35, PW 1.11–1.47, WL 1.97–2.50, HTL 1.66–2.20.

Queen
Bolton (1973) - As worker, with the usual differences associated with the caste. Pronotal spines and propodeal teeth reduced, but often the median tubercle on the posterior propodeal margin is enlarged or double.