Figs. 25, 69.
Etymology.- Greek "xouthos" (yellowish-brown) + "soma" (neut. "body")
Material examined.- Holotype: [[worker]], Prov. Toliara, Tsimanampetsotsa, 6.7 km 130 SE Efoetse, 24°06'S, 43°46E 25 m, 18-22.iii.2002 B.L. Fisher et al. BLF 6160/sifted litter, spiny forest/thicket/CASENT 0020507 6160(17) (CAS). Paratypes: Prov. Toliara (all specimens with same collection data as holotype):
3 [[worker]] (ANIC); 2 [[worker]], 1 [[queen]] (BMNH); 2 [[worker]] (CAS); 2 [[worker]], 1 [[queen]] (MCZ).
Other material examined: Prov. Toliara: Foret Tsinjoriaka ["Tsinjoriaky"], 6.2 km 84 E Tsifota 6-10.iii.2002 Fisher et al. (19 [[worker]], 1 [[queen]]); Ifaly, 17.ix.1993 W. E. Steiner & R. Andriamasimanana (2 [[worker]]) (MCZ); Mahafaly ["Mahafely"] Plateau 6.2 km 74 ENE Itampolo, 21-25ii.2002, Fisher et al. (18 [[worker]]); Madarano ["Manderano"], 10.iv.2002, Frontier Project MG030 (1 [[worker]]); Ranobe, Frontier Project 5-28.i.2003, MGFO54 (2 [[worker]]); Ranobe, Frontier Project 25-28.iv.2003, MGFO64 (2 [[worker]]); Res. Berenty, 10.ii.1993 P. S. Ward (1 [[worker]]) (MCZ); Tsimanampetsotsa, 6.7 km 130 SE Efoetse, 18-22.iii.2002 B.L. Fisher (15 [[worker]], 6 [[queen]]); Tsimanampetsotsa, Bemanateza, 23.0 km 131 SE Beheloka, 22-26.iii.2002 B.L. Fisher (18 [[worker]], 2 [[queen]]).
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 moderate, eye width 1-1.5x greatest width of antennal scape; (in full-face view) eyes set below midpoint of head capsule; (viewed in profile) eyes 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 12; antennal club three-segmented. Clypeal carinae always well-defined; anteromedian clypeal margin emarginate, clypeal carinae terminating in blunt angles; 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 formula 2,2. Mandibular teeth three, plus minute, basal denticle or angle; mandibles with sub-parallel inner and outer margins, smooth (except for piliferous pits); masticatory margin of mandibles approximately vertical or weakly 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 anteriad, convexity reduced posteriad; 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 very sparse or absent. Metanotal groove strongly impressed, with distinct transverse costulae. Propodeum shining and smooth, with a few weak 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 nearer metanotal groove than declivitous face of propodeum. Vestibule of propodeal spiracle absent or not visible. Propodeal lobes present as rounded flanges.
Petiole and postpetiole: Petiolar spiracle lateral and situated within anterior sector of petiolar node; petiolar node, in profile, cuneate, vertex tapered; appearance of node shining and smooth throughout; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 4:3 and1:1; anteroventral petiolar process absent or vestigial; ventral petiolar lobe absent; height ratio of petiole to postpetiole between 4:3 and 1:1; height-length ratio of postpetiole between 4:3 and 1:1; 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 foreparts tawny-yellow to orange (postpetiole sometimes darker), antenna brown, gaster chocolate. Worker caste monomorphic: HML 1.14 HL 0.44 HE 0.35 CeI 80 SL 0.33 SI 94 PW 0.24
Other worker measurements: HML 1.13-1.32 HL 0.43-0.5 HW 0.35-0.42 CeI 79-84 SL 0.32-0.39 SI 90-93 PW 0.23-0.28 (n=20).
Queen description.- Head: Head square; vertex always planar; frons shining and longitudinally striolate, with some smooth areas; pilosity of frons a mixture of well-spaced, distinctly longer erect and semi-erect setae interspersed with shorter setae or setulae, which are decumbent or appressed, longer setae thickest on vertex. Eye ovoid, narrowed posteriad; (in full-face view) eyes set at about midpoint of head capsule; (viewed in profile) eyes set around midline of head capsule.
Mesosoma: Anterior mesoscutum smoothly rounded, thereafter more-or-less flattened; pronotum, mesoscutum and mesopleuron shining and mainly smooth, vestigial striolae, if present, confined to anterior katepisternum; length-width ratio of mesoscutum and scutellum combined about 2:1; axillae separated by width of at least one axilla to narrowly separated (i.e., less than width of one axilla); standing pronotal/mesoscutal setae consisting of well-spaced, incurved, erect and semierect setae only; appressed pronotal, mescoscutal and mesopleural setulae very sparse or absent. Propodeum shining and smooth, with a few weak striolae on metapleuron; propodeum always smoothly rounded; propodeal dorsum convex; standing propodeal setae consisting of up to a dozen or more longer erect and shorter sub-erect setae; appressed propodeal setulae very sparse or absent; propodeal spiracle nearer metanotal groove than declivitous face of propodeum; propodeal lobes present as well-developed, rounded flanges.
Wing: Wing not seen (queen dealated).
Petiole and postpetiole: Petiolar spiracle lateral and situated within anterior sector of petiolar node; node (viewed in profile) cuneate, vertex tapered; appearance of node shining and weakly striolate posteriad; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 4:3 and 1:1; anteroventral petiolar process absent or vestigial; height ratio of petiole to postpetiole between 4:3 and 1:1; height-length ratio of postpetiole between 3:2 and 1:1; postpetiole shining and weakly striolate posteriad; 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 foreparts dark yellowish-brown, gaster and antennal scape brown. Brachypterous alates not seen. Ergatoid or worker-female intercastes not seen.
Queen measurements: HML 2.27-2.43 HL 0.63-0.66 HW 0.58-0.61 CeI 91-95 SL 0.49-0.52 SI 83-86 PW 0.50-0.54 (n=9).
Remarks.- Monomorium xuthosoma strongly resembles M. termitobium form "binatu" but can be distinguished from that form by its larger propodeal spiracle, the slightly different shape of the propodeum, its higher postpetiole and the pilosity of the promesonotum (at least four prominent pairs of erect setae present, including the infrahumeral pair). The queen is similar in form to that of "binatu", but the frons is markedly longitudinally striolate. The male is unknown. This unobtrusive member of the M. rhopalocerum complex is confined to Toliara province, where almost all specimens seen have come from the southwestern corner. One slightly aberrant worker (MCZ) was taken from Berenty Reserve, in the south-east. The species appears to be confined to spiny forest, where individuals have been taken from sifted litter and members of a colony were found in a rotten log.
