Phylogeny and classification of the extant basal lineages of the Hymenoptera (Insecta)

The phylogeny of the basal hymenopteran lineages, including representatives of all 'symphytan' families, is analysed. In total, 236 morphological char acters were scored for 44 exemplars, including six outgroup, two xyelid, 16 tenthredinoid, five pamphilioid, three cephoid, five 'siricoid', one oruss id, and six apocritan taxa. The dataset is analysed with parsimony under eq ual weights and under implied weights. The monophyly of the Hymenoptera is strongly supported but the sistergroup of the Hymenoptera cannot be identif ied with confidence. The relationships of the 'symphytan' lineages are Xyel oidea+(Tenthredinoidea + (Pamphilioidea + (Cephoidea + (Anaxyelidae + (Siri cidae + (Xiphydriidae + (Orussoidea + Apocrita))))))). Many of the relation ships between the superfamilies, especially in the basal branching pattern, are only weakly corroborated. The monophyly of most superfamilies is well supported, and all may be monophyletic except the 'Siricoidea', which is cl early paraphyletic. It is difficult to decide whether the Siricidae or the Anaxyelidae are the closest relatives of Xiphydriidae + (Orussoidea + Apocr ita). The support for the sistergroup relationship between the Orussoidea a nd the Apocrita is substantial, putative synapomorphies being provided by m ost character systems. There is also good evidence in favour of the monophy ly of the Apocrita. The internal phylogeny of the Tenthredinoidea differs c onsiderably from the results of earlier analyses. The Blasticotomidae are t he sistergroup of the Tenthredinoidea s.s. Relationships at the base of the Tenthredinoidea s.s. are weakly supported. It is uncertain whether the Ten thredinidae are monophyletic or comprise a basal paraphyletic grade within the Tenthredinoidea s.s. The Diprionidae may be the sistergroup to the Cimb icidae + (Argidae + Pergidae). Most relationships within the Cimbicidae + ( Argidae + Pergidae) clade are well corroborated, with the exception of the monophyly of the Argidae. It is proposed to elevate the Anaxyelidae and the Xiphydriida both to superfamily status. The family-level classification of the Tenthredinoidea will probably also have to be changed, but this must a wait further clarification of the phylogeny of this superfamily. (C) 2001 T he Linnean Society of London.