| Summary: | All members of the ant genus <i>Odontomachus</i> Latreille, 1804 are venomous ants. Four species in this genus have been identified from Thailand: <i>Odontomachus latidens</i> Mayr, 1867; <i>O. monticola</i> Emery, 1892; <i>O. rixosus</i> Smith, 1757; and <i>O. simillimus</i> Smith, 1758. The three latter species are available and have been used for an outline morphometric study. They display similar morphology, which makes their distinction very difficult except for highly qualified individuals. A total of 80 worker specimens were studied, exploring the contour shapes of their head and pronotum as possible taxonomic characters. The size of each body part was estimated determining the contour perimeter, the values for which were largely overlapping between <i>O. rixosus</i> and <i>O. simillimus</i>; most <i>O. monticola</i> specimens exhibited a significantly larger size. In contrast to the size, each contour shape of the head or pronotum established <i>O. rixosus</i> as the most distinct species. An exploratory data analysis disclosed the higher taxonomic signal of the head contour relative to the pronotum one. The scores obtained for validated reclassification were much better for the head (99%) than for the pronotum (82%). This study supports outline morphometrics of the head as a promising approach to contribute to the morphological identification of ant species, at least for monomorphic workers.
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