A solitary endoparasitic species of Pteromalidae (Hymenoptera: Chalcidoidea) was reared from pupae of an aphidophagous gall midge, Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae), in Japan. The parasitoid was identified as Gastrancistrus fuscicornis Walker, a species that has been recorded from Europe. An unidentified aphid has been proposed as being a host of G. fuscicornis, but the study reported here confirmed that G. fuscicornis is an endoparasitoid of A. aphidimyza. I consider that G. fuscicornis may not be a parasitoid of aphid. I found a nontentorial pit on the head of male and female of G. fuscicornis. According to the definition of the nontentorial pits, the pit found in G. fuscicornis corresponds to the facial pit. This is the first finding of the facial pit in Chalcidoidea. Morphological differences between G. fuscicornis and its allied species are provided.
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