Holotype. Madagascar. • 1 major worker; Antananarivo; 3 km 41°NE Andranomay, 11.5 km 147°SSE Anjozorobe; -18.47333, 47.96; alt. 1300 m; 5 Dec 2000; Fisher et al. leg.; montane rainforest, ex dead twig above ground; BLF02436; CASENT0427789 (CASC). Paratypes. • 4w., 1s.; same data as for holotype; CASENT0427788, CASENT0427791, CASENT0427785 (CASC, MHNG).

Moderately large species. Major workers. Head in full-face view oval, not widening posteriorly, with anterior and posterior sides convex, in lateral view sub-oval, ventral and dorsal faces convex, occipital cleft very shallow; sides of the head with dense, moderately long, suberect pilosity; medial frons with moderately dense, thin, longitudinal to irregular and interrupted rugae, rugae in posteromedial part slightly directed outward, interspaces shiny with sparse and distinct rugofoveolae; occipital lobes and area posterolateral from eyes never smooth; scape, when laid back, exceeding the midlength of head by two-fifths of its length; inner hypostomal teeth distinct, large, closely spaced, triangular, with rounded apex directed upward; outer hypostomal teeth lobe-like, wider than inner hypostomal teeth and approximately the same height, apex directed outward; inner and outer hypostomal teeth closely spaced and not connected by concavity; mesosoma with thick and sparse foveolae; lateral sides of pronotum and propodeum with smooth notches; body yellowish brown. Minor workers. Head foveolate; anteromedial part of frons with smooth notch; area posterolateral from eyes predominantly smooth; scape, when laid back, surpassing the posterior head margin by one-fifth of its length; promesonotum moderately high and short; promesonotal groove absent; propodeal spines minute and triangular; mesosoma with thick foveolae; body yellowish brown.

Major workers. Measurements (N = 2): HL: 0.96, 1.04; HW: 0.92, 1.01; SL: 0.64, 0.63; EL: 0.12, 0.13; WL: 0.94, 1.0; PSL: 0.17, 0.18; MTL: 0.61, 0.62; PNW: 0.38, 0.44; PTW: 0.13, 0.14; PPW: 0.23, 0.25; CI: 103.5, 103.0; SI: 69.7, 62.4; PSLI: 17.7, 17.4; PPI: 57.8, 53.6; PNI: 41.0, 43.4; MTI: 66.0, 61.2.

Head. In full-face view oval, not widening posteriorly, with anterior and posterior sides convex (Fig. 59B); occipital cleft very shallow. In lateral view sub-oval; ventral and dorsal faces convex; inner hypostomal teeth visible. Sides of the head with dense, moderately long, suberect pilosity; whole head with dense, long, decumbent to erect pilosity. Medial part of frons with moderately dense, thin, longitudinal to irregular and interrupted rugae, rugae in posteromedial part slightly directed outward, interspaces shiny with sparse and distinct rugofoveolae; anterolateral sides with longitudinal, thin, and relatively dense rugae; posterolateral sides with irregular, thin, and relatively dense rugae; interspaces shiny with relatively dense and distinct rugofoveolae. Occipital lobes with sparse, thick, and irregular rugae; interspaces rugofoveolate. Gena with relatively sparse, thick, and longitudinal rugae and rugofoveolate interspaces. Area posterolateral from eyes shiny, with dense rugofoveolae. Centre of clypeus smooth and shiny, lateral sides with indistinct rugulae; median notch present, moderately wide, and shallow; median longitudinal carina absent; lateral longitudinal carinae absent. Scape, when laid back, exceeding the midlength of head by two-fifths of its length; pilosity subdecumbent to erect (Fig. 59B, D). Inner hypostomal teeth distinct, large, closely spaced, triangular, with rounded apex directed upward; outer hypostomal teeth lobe-like, wider than inner hypostomal teeth and approximately the same height, apex directed outward; inner and outer hypostomal teeth closely spaced and not connected by concavity (Fig. 64Q). Mesosoma. In lateral view, promesonotum short, angular, and moderately low, posterior mesonotum moderately steep, mesonotal process indistinct, tubercle-like; promesonotal groove absent; metanotal groove absent; propodeal spines moderately long, moderately narrow, with acute apex; humeral area laterally weakly produced (Fig. 59D). Surface shiny with thick and sparse foveolae; lateral sides of pronotum and propodeum with smooth notches. Pilosity sparse, long, and erect (Fig. 59D, F). Petiole. Shiny with fine and sparse foveolae; node smooth, low, triangular, with rounded and thin apex, in rear view node dorsoventrally slightly concave; pilosity moderately sparse and erect (Fig. 59D, F). Postpetiole. Shiny and smooth; in dorsal view oval, lateral margins medially with two dentate projections; pilosity long, moderately sparse, and erect (Fig. 59D, F). Gaster. Shiny and smooth; pilosity moderately dense, long, and erect (Fig. 59D, F). Colour. Yellowish brown; mandibles and gaster slightly darker; legs yellowish (Fig. 59D, F).

Minor workers. Measurements (N = 4): HL: 0.52–0.59 (0.56); HW: 0.46–0.51 (0.49); SL: 0.55–0.59 (0.56); EL: 0.1–0.11 (0.11); WL: 0.72–0.73 (0.73); PSL: 0.07–0.08 (0.08); MTL: 0.47–0.51 (0.49); PNW: 0.3–0.33 (0.31); PTW: 0.07–0.09 (0.08); PPW: 0.13–0.14 (0.13); CI: 112.7–116.7 (114.1); SI: 111.3–117.9 (114.9); PSLI: 11.3–15.9 (13.6); PPI: 53.0–62.8 (59.3); PNI: 57.6–65.0 (62.2); MTI: 98.8–101.2 (100.5).

Head. Cephalic margin indistinctly convex or straight (Fig. 59A). Pilosity relatively sparse, long, decumbent to suberect. Sculpture foveolate; anteromedial part of frons with smooth notch; area posterolateral from eyes predominantly smooth. Clypeus with median longitudinal carina absent; two lateral longitudinal carinae absent. Scape, when laid back, surpassing the posterior head margin by one-fifth of its length; pilosity dense, subdecumbent to erect (Fig. 59A, C). Mesosoma. In lateral view, promesonotum moderately high and short, arched; promesonotal groove absent; metanotal groove distinct; propodeal spines minute, triangular (Fig. 59C). Sculpture shiny and with thick foveolae. Pilosity very sparse, moderately long, and erect (Fig. 59C, E). Postpetiole. Short, low, and relatively flat; with few short, erect setae (Fig. 59C, E). Gaster. With sparse, erect pilosity (Fig. 59C, E). Colour. Yellowish brown, vertex slightly darker (Fig. 59C, E).

Latin for shallow in reference to very shallow occipital cleft.

The species was collected at 1300 m in elevation, in montane rainforest. Nest was located in a dead twig above ground.

Pheidole vadum sp. nov. is a member of the group of species characterised by major workers with head in full-face view oval and not widening posteriorly and medial part of frons with thick, moderately sparse, irregular rugae or medial frons with moderately dense, thin, longitudinal anteriorly to irregular posteriorly, interrupted rugae and very shallow occipital cleft. Minor workers of this group have a short and moderately high promesonotum and dark body colouration ranging from orange to brown. The group consists of three species: P. vadum sp. nov., P. analavelona sp. nov., and P. ambohimanga sp. nov. Pheidole vadum sp. nov. is known from the vicinity of Antananarivo and is sympatric with P. ambohimanga sp. nov. Its majors can be easily separated from P. ambohimanga sp. nov. based on presence of moderately dense, thin, longitudinal anteriorly to irregular posteriorly, interrupted rugae on medial frons and very shallow occipital cleft; minor workers can be separated based on predominantly foveolate head sculpture lacking additional rugae. Additionally, minors of P. vadum sp. nov. can be confused with workers of other members of the sikorae group, especially with workers of P. sava sp. nov. and P. sparsa sp. nov., both known from the northern part of the island. However, minor workers of P. vadum sp. nov. can be separated based on the combination of the following characters: body yellowish brown, head and mesosoma predominantly foveolate with no additional sculpture, scape, when laid back, surpassing the posterior head margin by one-fifth of its length, promesonotum short and moderately high, and promesonotal groove absent.