Heterick, B. E., 2006:
Figs. 20, 65-66.
Etymology.- Latin "nigricans" ("blackish").
Material examined.- Holotype: [[worker]], Prov. Toliara, Res. Cap Sainte Marie, 12.3 km 262° W Marovato 25°34'90"S, 45°10'10"E 200 m, 11-15.ii.2002 B. Fisher et al. BLF 5500/ sifted litter, spiny forest/thicket/ CASENT 0020200 5500(LO) (CAS). Paratypes: Prov. Toliara (one worker collection code 5500, one worker collection code 5502, eleven males collection code 5504, otherwise data as for the holotype): 12[[male]] (ANIC); 13[[male]] + 1 [[worker]] (BMNH); 12[[male]] (CAS); 13[[male]] + 1[[male]] + 1[[worker]] (MCZ).
Other material examined: Prov. Toliara: 18 km NNW Betroka, 29.xi.-4.xii.1994 MA. Ivie & DA. Pollock (18 [[worker]]) (MCZ); Res. Beza Mahafaly ["Mahafely"], 18.xi.1984 R.L. Brooks (14 [[worker]]) (MCZ); Cap Sainte Marie, 14.9 km 261 W Marovato 13-19.ii.2002 Fisher et al. (14 [[worker]]); Foret Mahavelo, Isantoria Riv. 5.2 km 44 NE Ifotaka 28.i -1.ii.2002 B.L. Fisher (1 [[male]]); Foret Mite, 20.7 km 29 WNW Tongobory 27.ii-3.iii.2002 Fisher et al. (1 [[worker]]); Mahafaly Plateau, 6.2 km 74 ENE Itampolo 25.ii.2002 Fisher at al. (2); P.N. Isalo, Ambovo Springs, 29.3 km 4 N Ranohira 9-14.ii.2003 Fisher et al. (1 [[queen]]); Reserve Berenty, 10.xii.1992 B.L. Fisher (2 [[worker]]) Res. Berenty, Foret Malaza, 8.6 km 314 NW Amboasary 6.ii.2002 Fisher et al. (9[[male]]).
Worker description.- Head: Head rectangular; vertex planar or weakly concave; frons shining and smooth except for piliferous pits; pilosity of frons a mixture of well-spaced, distinctly longer erect and semi-erect setae interspersed with shorter decumbent setae or setulae. Eye large, eye width 1.5x greater than greatest width of antennal scape; (in full-face view) eyes set at about midpoint of head capsule; (viewed in profile) eyes set anteriad of midline of head capsule, or, set around midline of head capsule; eye elliptical, curvature of inner eye margin may be more pronounced than that of its outer margin. Antennal segments 11; antennal club three-segmented. Clypeal carinae weakly to strongly defined; anteromedian clypeal margin straight; paraclypeal setae moderately long and fine, curved; posteromedian clypeal margin extending slightly beyond level of posterior margin of antennal fossae. Anterior tentorial pits situated nearer antennal fossae than mandibular insertions. Frontal lobes sinuate, divergent posteriad. Psammophore absent. Palp formula2,2. Mandibular teeth three, plus minute, basal denticle or angle; mandibles linear-triangular and smooth (except for piliferous pits); masticatory margin of mandibles strongly oblique; basal tooth a small to minute denticle or angle, much smaller than t3 (four teeth present).
Mesosoma: Promesonotum shining and mainly smooth, vestigial striolae, if present, confined to lower anterior mesopleuron; (viewed in profile) promesonotum broadly convex; promesonotal setae seven to twelve; standing promesonotal setae a mixture of well-spaced, distinctly longer, erect and semi-erect setae which are curved distally and often paired, interspersed with much shorter, incurved, decumbent setae; appressed promesonotal setulae few, mainly on dorsum of promesonotum. Metanotal groove weakly impressed, with faint costulae or costulae lacking. Propodeum shining and smooth, with a few distinct striolae on metapleuron; propodeal dorsum convex; propodeum always smoothly rounded; standing propodeal setae consisting of one prominent pair anteriad, with other shorter setae very sparse or absent; appressed propodeal setulae very sparse or absent; propodeal spiracle equidistant from metanotal groove and declivitous face of propodeum. Vestibule of propodeal spiracle absent or not visible. Propodeal lobes present as vestigial flanges or small strips of cuticle only.
Petiole and postpetiole: Petiolar spiracle lateral and situated within anterior sector of petiolar node; node (viewed in profile) conical, vertex tapered to conical, vertex rounded; appearance of node shining and smooth throughout; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) about 1:1; anteroventral petiolar process absent or vestigial; ventral petiolar lobe present; height ratio of petiole to postpetiole between 4:3 and 1:1; height-length ratio of postpetiole about 4:3; postpetiole shining and smooth; postpetiolar sternite without anterior lip or carina, or this structure vestigial.
Gaster: Pilosity of first gastral tergite consisting of well-spaced, erect and semi-erect setae interspersed with a few appressed setulae.
General characters: Color dark reddish-brown to almost black. Worker caste monomorphic.
Holotype measurements: HML 1.08 HL 0.42 HW 0.37 CeI 87 SL 0.30 SI 82 PW 0.25.
Other worker measurements: HML 1.00-1.12 HL 0.39-0.44 HW 0.34-0.38 CeI 84-90 SL 0.29-0.33 SI 82-89 PW 0.22-0.25 (n=19).
Male description.- Head: (In full-face view) head width -mesosoma width ratio between 4:3 and 1:1; frons finely micropunctate. Compound eyes protuberant and elliptical; margin of compound eye clearly separated from posterior margin of clypeus. Ocelli not turreted. Ratio of length of first funicular segment of antenna to second funicular segment between 2:3 and 1:2. Maximum number of mandibular teeth and denticles three.
Mesosoma: Mesoscutum broadly convex; pronotum and mesoscutum shining and microreticulate throughout; parapsidal furrows vestigial or absent; notauli absent; axillae widely separated (i.e., by width of at least one axilla), axilla fused with scutellum.
Wing: Wing veins predominantly depigmented, with distal segments reduced to vestigial lines; vein m -cu absent; vein cu -a absent.
Petiole and postpetiole: Petiolar spiracle lateral and situated slightly anteriad of petiolar node; node (viewed in profile) conical, vertex tapered; appearance of node shining and microreticulate; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) about 4:3; anteroventral petiolar process absent or vestigial; height ratio of petiole to postpetiole about 3:4; height -length ratio of postpetiole between 2:1 and 3:2; postpetiole shining and microreticulate.
Gaster: Pilosity of first gastral tergite consisting of a mixture of incurved, semi-erect setae and slightly shorter decumbent setae.
General characters: Color chocolate.
Male measurements: HML 1.73-2.07 HL 0.45-0.55 HW 0.53-0.65 CeI 102-118 SL 0.12-0.18 SI 21-33 PW 0.53-0.73 (n=20).
Remarks.- This smallish species, the only Malagasy Monomorium apart from Monomorium exiguum that has an 11-segmented antenna, appears to be confined to Toliara Province, where it is not uncommon in spiny forest. Several workers and males have also been collected in gallery and tropical dry forests. The profile of the worker mesosoma of Monomorium nigricans is not unlike that of the African Monomorium bequaerti Forel (Fig. 92 in Bolton 1987), but the postpetiole is more rounded and the color is much darker than in bequaerti . The relationship of this species to other small African and Malagasy Monomorium is uncertain, but the appearance of the worker and the male suggests it may belong to the M. exiguum complex. Workers have been collected in sifted litter or from beating low vegetation, or as ground foragers, and ground nests and nests in dead twigs have also produced worker specimens. Males have been taken in malaise traps.