The nominal taxon Acanthosoma chrysalis Mayer, 1844 is revised, and a lectotype is designated. The species, which was described from Germany from a number of alleged parasites encysted in the peritoneal wall of the stomach of edible frogs, is shown to be based on first instar larvae of blow flies (Calliphoridae). Argued from the shape and configuration of mouthhooks and abdominal cuticular spines, Acanthosoma Mayer, 1844 is shown to be a junior synonym of Onesia Robineau-Desvoidy, 1830, syn.n., and A. chrysalis is shown to be a junior synonym of O. floralis Robineau-Desvoidy, 1830, syn.n. This species is an obligate parasitoid of earthworms, and it is hypothesized that first instar larvae enter the frogs through infected earthworms.