The Platygastroidea are a diverse group of mostly small to tiny wasps, the common biology for which is endoparasitism of insect and spider eggs. No analytically-based phylogeny exists for the superfamily, and the current suprageneric classification is flawed in part because of its reliance on homoplasious and pleisiomorphic morphological characters. To determine platygastroid relationships as a basis for investigating host and ovipositor evolution, phylogenies of > 70 in-group species (representing 55 genera) were reconstructed by parsimony and Bayesian methods using three molecular markers; the mitochondrial cytochrome oxidase I and the nuclear genes 28S and 18S rRNA . The results strongly support the monophyly of the superfamily and one of the two families, Platygastridae, but the Scelionidae are most likely polyphyletic. However, within the Scelionidae, there is a well supported 'main scelionid clade' that contains the majority of genera assigned to the family. At the subfamilial level, both putative subfamilies of Platygastridae, the Platygastrinae, and Sceliotrachelinae, are likely to be polyphyletic. Within the Scelionidae, both the Teleasinae and Telenominae are monophyletic, but the Scelioninae is clearly not so. The current tribal classification for the Scelionidae is in need of major reassessment because no tribes, with the exception of the Scelionini s.s ., were found to be monophyletic. Further illustrating the problems associated with the current classification is the nonmonophyly of a number of genera, namely Opisthacantha Caloteleia , Telenomus , Trimorus , Teleas and Idris. Analysis of ovipositor evolution in the superfamily revealed that the Ceratobaeus -type ovipositor system is ancestral; however, this trait was lost prior to the evolution of the main scelionid clade, for which the Scelio -type ovipositor system is ancestral and defines a mostly paraphyletic assemblage. Ancestral state analysis indicates that the Ceratobaeus -type ovipositor was subsequently re-evolved in the main scelionid clade, representing a possible contradiction of Dollo's law. Previously, the tribal placement has been used to predict the host associations of genera for which host data were unavailable. However, the fact that most tribes are not monophyletic throws into doubt any such speculation based on the current classification.

Members of the tribe Baeini are unique among the Scelonidae as endoparasitoids of the eggs of spiders, whereas all other scelionids parasitise insect eggs. The monophyly of the Baeini has previously been assumed based on this host association and several morphological characteristics, none of which are unequivocal. Here we present a phylogeny for the group using sequence data from two genes and 32 ingroup taxa, mostly from Australia. Combined analysis of the nuclear 28S rRNA gene and the mitochondrial CO1 gene showed that the tribe is not monophyletic, because two genera (Neobaeus Austin and Mirobaeoides Dodd) are resolved in a clade separate from the rest of the Baeini. The two most speciose genera, Idris Foerster and Ceratobaeus Ashmead, are not resolved as monophyletic, indicating that a metasomal horn, which acts as a recess for the internally retracted ovipositor, has evolved multiple times within the tribe. Mapping of wing reduction states that occur in the Baeini revealed multiple wing reduction events, possibly associated with adaptations for searching for host egg masses in crytobiotic habitats, and/or penetrating the silk egg sacs of spiders. The molecular phylogeny generated in this study differs significantly from previous morphological phylogenies, and has resolved relationships that have otherwise been intractable in morphological studies because of homoplasy associated with reduction in body size.