The Platygastroidea is a highly diverse group of small to minute parasitoid wasps. Despite the reduced size, the morphology of the maxillolabial complex of scelionids is very diverse and phylogenetically informative. 81 characters are scored for 129 genera (representing 75% of the total number of known extant genera of scelionids), as well as for seven outgroup taxa. All taxa examined are illustrated with images, SEM micrographs and/or line drawings. Phylogenetic trees resulting from analyses conducted in TNT under various settings were not fully resolved, but some relationships were repeatedly retrieved. The Platygastroidea are usually corroborated. Nixoniini, Sparasionini, Plaumannion and Huddlestonium are frequently retrieved as the most basal platygastroid lineages. Psix and Paratelenomus often form a monophyletic group close to Gryonini. The Scelioninae, Teleasinae and Telenominae are not supported as monophyletic. However, some major scelionid clades are frequently retrieved, but these are not recognized in the current classification of Platygastroidea. The evolution of the palpal formula, highly variably in scelionids, and previously used in platygastroid systematics, is explored. The number of maxillary palpomeres in the ground plan of platygatroids is probably five, that of labial palpomeres two or three, given the variation among basal scelionids. Our study provides relevant information for resolving the phylogeny of the Platygastroidea, but additional character sources have to be explored to obtain a robust phylogenetic hypothesis.
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