Abstract: A significant proportion of taxa was described from a single specimen; however, miscellaneous influences are known to alter a phenotype, raising the question of the validity of taxa that have been reported only once in the history. The purpose of the present study was to highlight one of the possible sources of the high number of once-only taxa; the parasitogenic phenotype. I report observations made using noninvasive X-ray microtomography, which provides direct evidence of the presence of mermithid nematodes in the gaster of certain type specimens, demonstrating that two of the three once-only Myrmica taxa were described on the basis of mermithogenic phenotypes. Microtomographic images show that M. symbiotica (Menozzi, 1925) was described on a mermithogenic phenotype; so, I propose junior synonymy with M. scabrinodis Nylander, 1846. The formerly reported mermithid infestation of M. myrmecophila Wasmann, 1910 holotype, and synonymy with M. sulcinodis Nylander, 1846 is confirmed. Though the holotype of Myrmica schenckioides Boer & Noordijk, 2005 proved to be uninfected, the malformed features of the type specimen raise the possibility that it is a teratological case.