Monomorium rabirium

This species is know to occur in grassland and shrub habitats.

Identification
Bolton (1987) - One of the smallest members of the salomonis-group and one of the most reduced, in terms of pilosity and sculpture. M. rabirium is characterized by the absence of dorsal pilosity, very faint sculpture, anterior shifting of the eyes, light colour and small size. The position of the eyes in this species is reminiscent of the setuliferum-group, but the basal tooth of the mandible is not markedly reduced in size, the scapes are longer than is seen in Monomorium setuliferum and its allies, and the head in rabirium is narrower. As the eyes in Monomorium esharre, a close relative, are slightly in front of the mid length it seems reasonable to assume that these two species are convergent upon the condition seen in the setuliferum-group as regards the eyes, whilst showing other diagnostic characters referring them to the salomonis-group.

Mandibular sculpture in rabirium is much fainter than in other salomonis-group members, and for this reason the species runs out twice in the key, once with the members of the salomonis-group and setigerum-group where the mandibles are usually conspicuously sculptured, and once elsewhere among species where they are smooth.

The closest relatives of rabirium are Monomorium zulu and Monomorium osiridis (mediocre-complex) which share the lack of pilosity and anteriorly shifted eyes. Also these species appear to have a palp formula of 1,2 (based in each case on an in situ count), lower than the usual PF 2,2 seen elsewhere in the group. M. osiridis, from Kenya, separates from rabirium by having the head conspicuously sculptured. For differentiation of rabirium and zulu see under the latter.

Distribution based on Regional Taxon Lists
Afrotropical Region: Botswana.

Nomenclature

 *  rabirium. Monomorium rabirium Bolton, 1987: 358 (w.) BOTSWANA.

Worker
Bolton (1987) - Holotype. TL 1.7, HL 0.44, HW 0.33, C1 75, SL 0.32, SI 97, PW 0.22, AL 0.44.

Anterior margin of median portion of clypeus approximately transverse, indented medially. With the head in full-face view the eyes conspicuously in front of the midlength of the sides. Antennal scapes when laid straight back from their insertions not reaching the occipital margin. Maximum diameter of eye 0.27 x HW and with 7 ommatidia in the longest row. Sides of head shallowly convex, weakly converging posteriorly behind the level of the eyes. Occipital margin broadly and extremely shallowly concave. Promesonotal dorsum feebly convex in profile, sloping shallowly behind to the very weakly impressed metanotal groove. Dorsum of head without standing hairs behind the level of the frontal lobes. Alitrunk, petiole and postpetiole without hairs. First gastral tergite without standing hairs, even the apical transverse row of hairs which is almost universal in the salomon is-group is absent here. (Apical transverse rows of standing hairs are present on the second and third tergites.) Mandibular sculpture very feeble. Dorsum of head sculptured only with the last faint vestiges of superficial reticular patterning, almost entirely effaced. Pronotal dorsum faintly reticulate to feebly shagreenate, the mesonotum more obviously shagreenate and the propodeal dorsum finely reticulate-shagreenate. First gastral tergite faintly superficially shagreenate to smooth. Colour pale brownish yellow.

Paratypes. TL 1.7, HL 0.45-0.48, HW 0.34-0.36, CI 75-77, SL 0.32-0.34, SI 92-97, PW 0.22-0.23, AL 0.46-0.48 (6 measured). Maximum diameter of eye 0.26-0.28 x HW and with 7-8 ommatidia in the longest row. Otherwise as holotype.

Type Material
Bolton (1987) - Holotype worker, Botswana: Okavango Delta, Maxwee, grassland, sample no. 26, 10.x.1975 (A. Russell-Smith). Paratypes. 7 workers with same data as holotype (BMNH; ).