Microdontine syrphid flies are obligate social parasites of ants. Larvae prey on ant brood whereas adults live outside the nests. Knowledge of their interaction with their host is often scarce, as it is information about their natural enemies. Here we report the first case of parasitism of a species of microdontine fly by a myrmecophilous eurytomid wasp. This is also the first host record forCamponotophilus delvareiGates, a recently described parasitic wasp discovered in Chiapas, Mexico, within

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... the nests of the weaver ant,Camponotussp. aff.textorForel. Eleven pupal cases of a microdontine fly were found within a single nest of this ant, five of them being parasitized. Five adultC. delvareifemales were reared from a puparium and 29 female and 2 male pupae were obtained from another one. The eurytomid is a gregarious, primary ectoparasitoid of larvae and pupae of Microdontinae, its immature stages developing within the protective puparium of the fly. The species is synovigenic. Adult females likely locate and parasitize their hosts within the ant nest. As some species of Microdontinae are considered endangered, their parasitoids are likewise threatened and in need of accurate and urgent surveys in the future.