Holotype. Madagascar. • 1 major worker; Antananarivo; Ambohimanga; -18.76036, 47.56372; alt. 1294 m; 8 Sep 2015; B.L. Fisher et al. leg.; montane rainforest, ground nest; BLF37302; CASENT0720924 (CASC). Paratypes. • 2w., 1s.; same data as for holotype; CASENT0923279, CASENT0720925 (CASC, MHNG).

Madagascar. –Antananarivo: • 2w., 1s.; Ambohitrangano; -18.72801, 47.57878; alt. 1383 m; 3 Oct 2015; B.L. Fisher et al. leg.; disturbed montane rainforest, under root mat, litter on rock; BLF37326 (CASC). • 1w., 1s.; Station Forestière Angavokely; -18.92207, 47.74157; alt. 1460 m; 9 Mar 2013; B.L. Fisher et al. leg.; rainforest, under root mat, litter on rock; BLF31333 (CASC). • 1w., 1s.; Tsimbazaza; -18.928, 47.527; alt. 1300 m; 16 Dec 2006; B.L. Fisher et al. leg.; park/garden, ex rotten log; BLF16240 (CASC).

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 moderately deep; sides of the head with dense, very long, erect pilosity; medial part of frons with thick, moderately sparse, and irregular rugae, interspaces predominantly smooth and sometimes with indistinct rugulae; lateral sides of head, area posterolateral from eyes and occipital lobes with relatively dense to dense, thick, and irregular rugae; interspaces predominantly smooth or indistinctly rugulate; scape, when laid back, exceeding the midlength of head by two-fifths of its length; inner hypostomal teeth distinct, large, closely spaced, and triangular, with rounded apex directed inward; outer hypostomal teeth lobe-like, slightly 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; lateral sides of pronotum, mesonotum, katepisternum, anepisternum, and propodeum rugofoveolate, pronotal dorsum with thick to thin, irregular to transverse rugae with smooth to indistinctly rugofoveolate interspaces; katepisternum with smooth notch; body dark brown. Minor workers. Head with sparse foveolae; frons with foveolae reduced medially with moderately thick, longitudinal, and interrupted rugae; area posterolateral from eyes with reduced sculpture, predominantly smooth; scape, when laid back, surpassing the posterior head margin by two-fifths of its length; promesonotum moderately high and short; promesonotal groove absent; propodeal spines minute and triangular; propodeum predominantly smooth, its dorsum with sparse, short, and transverse rugae and indistinct foveolae; mesonotum, anepisternum, katepisternum, and propodeum predominantly smooth, with sparse and indistinct foveolae on anterolateral and anterior sides; body brown.

Major workers. Measurements (N = 4): HL: 0.95–1.03 (0.98); HW: 0.93–1.02 (0.96); SL: 0.61–0.63 (0.62); EL: 0.13–0.13 (0.13); WL: 0.84–0.92 (0.88); PSL: 0.14–0.16 (0.15); MTL: 0.55–0.59 (0.57); PNW: 0.45–0.5 (0.47); PTW: 0.14–0.16 (0.15); PPW: 0.3–0.35 (0.31); CI: 97.5–104.4 (101.5); SI: 62.0–67.0 (64.6); PSLI: 13.2–15.9 (15.0); PPI: 41.1–49.8 (47.2); PNI: 46.2–50.9 (48.8); MTI: 55.6–61.6 (58.9).

Head. In full-face view oval, not widening posteriorly, with anterior and posterior sides convex (Fig. 20B). In lateral view sub-oval; ventral and dorsal faces convex; inner hypostomal teeth visible. Sides of the head with dense, very long, erect pilosity; whole head with dense, long, decumbent to erect pilosity. Medial part of frons with thick, moderately sparse, irregular rugae, interspaces predominantly smooth and sometimes with indistinct rugulae; lateral sides of head, area posterolateral from eyes, and occipital lobes with relatively dense to dense, thick, and irregular rugae; interspaces predominantly smooth or indistinctly rugulate. Gena with relatively dense, thick, and longitudinal rugae and smooth interspaces. Centre of clypeus smooth and shiny, lateral sides with indistinct rugulae; median notch present, moderately wide, and shallow; median longitudinal carina present; lateral longitudinal carinae absent. Scape, when laid back, exceeding the midlength of head by two-fifths of its length; pilosity subdecumbent to erect (Fig. 20B, D). Inner hypostomal teeth distinct, large, closely spaced, triangular, with rounded apex directed inward; outer hypostomal teeth lobe-like, slightly 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. 63B). Mesosoma. In lateral view, promesonotum short, angular, and moderately low, posterior mesonotum moderately steep, mesonotal process distinct, tubercle-like; promesonotal groove absent; metanotal groove absent; propodeal spines moderately short, with wide base and acute apex; humeral area laterally weakly produced (Fig. 20D). Surface shiny; lateral sides of pronotum, mesonotum, katepisternum, anepisternum, and propodeum rugofoveolate, pronotal dorsum with thick to thin, irregular to transverse rugae with smooth to indistinctly rugofoveolate interspaces; katepisternum with smooth notch. Pilosity moderately dense, long, and erect (Fig. 20D, F). Petiole. Shiny with fine foveolae; node smooth to finely foveolate, low, triangular, with rounded and thin apex, in rear view node dorsoventrally straight to slightly convex; pilosity moderately sparse and erect (Fig. 20D, F). Postpetiole. Shiny and foveolate; dorsum with reduced sculpture and smooth notch; in dorsal view oval, lateral margins medially with two dentate projections; pilosity long, moderately sparse, and erect (Fig. 20D, F). Gaster. Shiny and smooth; pilosity dense, long, and erect (Fig. 20D, F). Colour. Dark brown; legs and antennae yellow (Fig. 20D, F).

Minor workers. Measurements (N = 7): HL: 0.55–0.6 (0.56); HW: 0.46–0.51 (0.47); SL: 0.58–0.62 (0.6); EL: 0.09–0.11 (0.1); WL: 0.67–0.76 (0.7); PSL: 0.08–0.1 (0.09); MTL: 0.44–0.48 (0.46); PNW: 0.32–0.38 (0.33); PTW: 0.09–0.1 (0.1); PPW: 0.13–0.14 (0.14); CI: 116.0–123.7 (119.4); SI: 119.5–130.0 (126.6); PSLI: 14.4–17.8 (15.7); PPI: 64.7–72.5 (69.0); PNI: 67.9–73.9 (70.0); MTI: 93.6–101.7 (96.6).

Head. Cephalic margin indistinctly concave or straight (Fig. 20A). Pilosity relatively sparse, long, decumbent to suberect. Sculpture shiny with sparse foveolae; frons with foveolae reduced medially with moderately thick, longitudinal, and interrupted rugae; area posterolateral from eyes with reduced sculpture, predominantly smooth; antennal sockets with few thick, curved outward rugae and indistinctly foveolate interspaces. Clypeus with median longitudinal carina absent; two lateral longitudinal carinae absent. Scape, when laid back, surpassing the posterior head margin by two-fifths of its length; pilosity dense, subdecumbent to erect (Fig. 20A, C). Mesosoma. In lateral view, promesonotum moderately high and short, arched; promesonotal groove absent; metanotal groove distinct; propodeal spines minute and triangular (Fig. 20C). Sculpture shiny; propodeum predominantly smooth, its dorsum with sparse, short, transverse rugae and indistinct foveolae; mesonotum, anepisternum, katepisternum, and propodeum predominantly smooth, with sparse and indistinct foveolae on anterolateral and anterior sides. Pilosity moderately dense, long, and erect (Fig. 20C, E). Gaster. With sparse, erect pilosity (Fig. 20C, E). Colour. Brown, legs slightly brighter (Fig. 20C, E).

From the type locality.

The species was collected between 1294–1460 m in elevation, in rainforest, montane rainforest and park. Nests were located under rootmat on rock, in the soil, and in rotten logs.

Pheidole ambohimanga 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 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 ambohimanga sp. nov. is sympatric with P. vadum sp. nov. but morphologically is most similar to P. analavelona sp. nov. It can be easily separated from P. vadum sp. nov. based on thicker and exclusively irregular rugae on frons, deeper occipital cleft in majors, and presence of additional rugae on head in minors. Majors of P. ambohimanga sp. nov. can be distinguished from P. analavelona sp. nov. based on smooth to very indistinctly rugoreticulate interspaces on head, while minor workers have head sculpture foveolae strongly reduced to absent on frons, predominantly smooth area posterolateral from eyes, and mesonotum, anepisternum, katepisternum, and propodeum predominantly smooth, with sparse and indistinct foveolae on anterolateral and anterior sides. Minors of P. ambohimanga sp. nov. can be easily separated from other members of the sikorae group based on short and moderately high promesonotum and additional, moderately thick to thick, longitudinal, and interrupted rugae on head.