THE PHYLOGENY OF LOWER HYMENOPTERA (INSECTA), WITH A SUMMARY OF THE EARLY EVOLUTIONARY HISTORY OF THE ORDER

A cladistic analysis of the lower Hymenoptera, including all the 'symp hytan' families and the apocritan families Stephanidae, Megalyridae, T rigonalyidae, Ibaliidae, Vespidae and Gasteruptiidae, has been underta ken. A total of 98 characters were scored for 21 taxa. Twenty equally parsimonious minimum-length trees were obtained. The phylogenetic stat us of the Xyelidae is uncertain: they might be monophyletic, or the Xy elinae might be the sistergroup of the rest of the Hymenoptera. The no n-xyelid Hymenoptera are probably monophyletic; the phylogeny Tenthred inoidea + (Megalodontoidea + (Cephidae + (Anaxyelidae + (Siricidae + ( Xiphydriidae + (Orussidae + Apocrita)))))) is proposed for this clade. The Blasticotomidae are probably the sistergroup of all other Tenthre dinoidea, but tenthredinoid phylogeny is otherwise uncertain. Substant ial homoplasy occurs within the 'sircoid' families, making the relativ e positions of the Anaxyelidae and Siricidae uncertain. The Stephanida e might be the sistergroup of the rest of the Apocrita; the phylogeny of the remaining apocritan taxa included is insufficiently elucidated. The phylogeny proposed here supports the hypothesis that the appearan ce of parasitism in the Hymenoptera took place in the common ancestor of Orussidae + Apocrita, the host of which was probably wood boring in sect larvae. The exact larval mode of feeding of the ancestral hymenop teran cannot be determined due to the diversity of lifestyles in the b asal lineages of the order.