The platygastroid wasp Echthrodesis lamorali has been of considerable interest since its description in 1968, primarily because of its highly modified, densely pilose, wingless body, its distribution and unusual biology. The species is endemic to the Cape Peninsula, South Africa, where it is an endoparasitoid of eggs of the marine spiders Desis formidabilis (Desidae) and Amaurobioides africanus (Anyphaenidae) in the intertidal region. Although a highly aberrant monospecific genus, the phylogenetic relationships of Echthrodesis are confused, in part due to convergence in body form across numerous unrelated platygastroid genera. We used sequence data from the nuclear 28S rRNA and 18S rDNA genes, and the mitochondrial cytochrome oxidase 1 (CO1) gene, to determine the phylogenetic affinities of E. lamorali. We present a revised taxonomic description for the genus and species, as well as new morphological information on the structure of its mouthparts and ovipositor system. Phylogenetic analyses of molecular data place E. lamorali within one of two independent clades of platygastroid wasps that use spider eggs as hosts. Echthrodesis is sister to a group of three genera: Neobaeus (New Zealand; host unconfirmed); Mirobaeoides (Australia; spider eggs); and Embidobia (near cosmopolitan; embiid eggs). Details on the biology, behaviour and morphological adaptations of E. lamorali are provided.