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The ant genus Odontomachus Latreille is reviewed for Sumatra, the sixth largest island in the world and located in western Indonesia. Previously three species were recorded from the island: O. simillimus F. Smith, O. rixosus F. Smith, and O. latidens Mayr. We add two species to the fauna, O. procerus Emery stat. nov and Odontomachus minangkabau sp. nov. The new species belongs to O. rixosus species group, and it is morphologically most similar to O. rixosus and O. pararixosus Terayama & Ito. However, it can be separated from the latter two by its large body (HL 3.13-3.55 mm, WL 4.15-4.65 mm), the masticatory margin with 11-14 denticles, and dark-colored body. Odontomachus latidens subsp. sumatranus Emery is newly synonymized with O. procerus. The castes and sexes of the known species are also described, including the first descriptions of the male for O. latidens, O. procerus, and O. rixosus. A key to the Sumatran species based on the worker caste is provided, and the bionomics of each species is summarized.

Many cryptic species have been discovered in various taxonomic groups based on molecular phylogenetic analyses and mating experiments. Some sympatric cryptic species share equivalent resources, which contradicts the competitive exclusion principle. Two major theories have been proposed to explain the apparent lack of competitive exclusion, i.e. niche-based coexistence and neutral model, but a conclusive explanation is lacking. Here, we report the co-occurrence of cryptic spider wasp species appearing to be ecologically equivalent. Molecular phylogenetic analyses and mating experiments revealed that three phylogenetically closely related species are found sympatrically in Japan. These species share the same resources for larval food, and two of the species have the same niche for nesting sites, indicating a lack of competitive exclusion. This evidence may suggest that ecologically equivalent species can co-occur stably if their shared resources are sufficiently abundant that they cannot be over-exploited.

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