The structure and function of the ovipositor within Hymenoptera have been cited as factors leading to the evolutionary success of the order. Parasitic Hymenoptera display a variety of morphological adaptations of the ovipositor that are linked not only to the type of host attacked, but how the host is attacked and where the parasitoid egg is deposited. Some Cynipoidea possess a morphological feature of the ovipositor unique within Hymenoptera: the ovipositor clip. This structure is postulated to prevent the escape of the parasitoid host during oviposition by locking the tip of the ovipositor to the host cuticle. When the ovipositor clip was described following laboratory observations, it was suggested that parasitoids had to gain some experience using the clip to avoid being injured or killed during oviposition. A survey of Cynipoidea using light microscopy and scanning electron microscopy revealed that some Figitinae and Eucoilinae possess the clip. Mapping of the presence/absence of the ovipositor clip onto two competing cynipoid phylogenies indicates this structure evolved in parallel in Figitinae and Eucoilinae. Upon linking host records to these species, a pattern emerges that suggests all species within these subfamilies that possess the clip attack semi-concealed Diptera hosts. Species that are known to attack fully concealed or fully exposed hosts all lack the ovipositor clip, suggesting an evolutionary cost is associated with possession of the clip. It is concluded here that the ovipositor clip is at least partially responsible for the successful colonization by the Figitidae of this extremely diverse group of dipteran hosts.