Sister-group relationships of the Platygastroidea and Chalcidoidea (Hymenoptera) - an alternate hypothesis to Rasnitsyn (1988)

Evidence for the hypothesis that Platygastroidea and (Chalcidoidea + Mymarommatoidea) are sister groups is reviewed and an alternate hypothesis of relationships is proposed for Platygastroidea. Platygastroidea is postulated to form a monophyletic group with Pelecinidae, Proctotrupoidea and Vanhorniidae based on common possession of an annular pronotum and a mesopleuroal-mesotrochanteral muscle. Hypotheses of character-state transformation are illustrated diagrammatically to explain distribution of the mesotrochanteral depressor muscle and relative structure of the pronotum, mesothoracic spriacle, prepectus, and pronotalmesepisternal attachment throughout the nonaculeate Apocrita. The states of 16 characters are defined in one table and another table summarises the distribution of the states in Orussidae and Apocrita. Retention of an independent prepectus and a mesotergal-mesotrochanteral depressor muscle indicated exact relationships. Chalcidoidea + Mymarommatoidea are indicated as possibly being more closely related to Diapriidae or Ceraphronoidea based on other characters. It is postulated that structure of the mesocoxal articulation of taxa assigned to the Evaniomorpha may represent a retained symplesiomorphy rather than a synapomorphy. Mesocoxal articulatory structure is illustrated by scanning electron photomicrographs.