Because of problems of rarity, sampling bias, and general lack of informative characters, immature stages of parasitic Hymenoptera are seldom used to resolve phylogenetic relationships. However, the ant-parasitic Eucharitidae are an exception. The adults and immature stages of Schizaspidia diacammae n. sp. (Eucharitidae: Eucharitinae: Eucharitini) are described from collections of cocooned pupae of its ant host, Diacamma scalpratum (Smith) (Formicidae: Ponerinae: Ponerini), in Thailand. Additional collections of planidia and pupae of Schizaspidia nasua (Walker) from Odontomachus rixosus (Smith) (Ponerini) and related Eucharitini suggest a high degree of conservatism in larval morphology across a monophyletic group of ant parasitoids that attack Ponerinae, Ectatomminae, and Myrmeciinae (PEM Clade). Pupae are especially informative, having peculiar bladder-like processes projecting from the ocelli, dorsal mesoscutellum, and laterally from the abdominal tergites. There are also a set of unusual sclerotized bars found laterally on the basal metasomal tergite. Such processes are unknown elsewhere in Hymenoptera. These same pupae offer further insights into the development of the scutellar spines. Based on a molecular analysis of relationships within the PEM Clade, we propose that scutellar spines have evolved multiple times within the group and across Eucharitidae.