Madagascar and its surrounding islands are among the world’s greatest biodiversity hotspots, harboring predominantly endemic and threatened communities meriting special attention from biodiversity scientists. Building on the considerable efforts in recent years to inventory the Malagasy ant fauna, the myrmicine genus Nesomyrmex is reviewed and (1) subdivided into four major groups based on salient morphological features corroborated by numeric morphology: angulatus-, hafahafa-, madecassus- and sikorai-groups, and (2) the hafahafa species-group endemic to Madagascar is revised. Diversity within hafahafa species-group was assessed via hypothesis-free nest-centroid-clustering combined with gap statistic to assess the number of clusters and to determine the most probable boundaries between them. This combination of methods provides a highly automatized, objective species delineation protocol based on continuous morphometric data. Delimitations of clusters recognized by these exploratory analyses were tested via confirmatory Linear Discriminant Analysis. These results suggest the existence of four morphologically distinct species, Nesomyrmex capricornis sp. n., N. hafahafa sp. n., N. medusus sp. n. and N. spinosus sp. n.; all are described and an identification key for their worker castes using morphometric data is provided. Two members of the newly outlined hafahafa species-group, N. hafahafa sp. n., N. medusus sp. n., are distributed along the southeastern coast Madagascar and occupy rather large ranges, but two other species, N. capricornis sp. n. and N. spinosus sp. n., are only known to occur in small and isolated forest, highlighting the importance of small forest patches for conserving arthropod diversity.
