The first wasp‐deceiving Calyptratae fly: <scp> <i>Brevialata deceptrix</i> </scp> Dios &amp; Gudin, <b>gen. et sp. nov.</b> (Diptera: Tachinidae), a new parasitoid of <scp> <i>Agelaia vicina</i> </scp> (de Saussure, 1854) (Hymenoptera: Vespidae) and

Abstract Flies of the family Tachinidae are parasitoids of a wide range of arthropods, and some tachinid lineages specialised and coevolved with distinct hosts. A few of these tachinids use social Hymenoptera as hosts. However, they are usually attacked by their hosts and present different strategies to avoid their aggressiveness. Here, we describe the first parasitoid fly that enters a wasp nest without being attacked, Brevialata deceptrix Dios &amp; Gudin, gen. et sp. nov. , reared from nests of Agelaia vicina (de Saussure, 1854) in Cajuru, São Paulo, Brazil. Additionally, it is the first tachinid fly with reduced wings, presenting a peculiar morphology, with reduced chaetotaxy, stout legs, and tarsi with digitiform extensions. Males of B. deceptrix Dios &amp; Gudin, gen. et sp. nov. are fully described and illustrated, including detailed images of tarsal structures. We present a discussion on B. deceptrix Dios &amp; Gudin, gen. et sp. nov. remarkable morphology, systematic positioning and curious life habits.