Before the wasp-waist: comparative anatomy and phylogenetic implications of the skeleto-musculature of the thoraco-abdominal boundary region in basalHymenoptera (Insecta)

The skeleto-musculature of the metathorax and first abdominal segment was s tudied in representatives from all 'symphytan' families. Forty-three inform ative characters were coded and scored. The distribution of character state s are discussed with reference to recent cladistic treatments of the Hymeno ptera. Previously unreported autapomorphies for the Hymenoptera are the sep aration of the metathoracic trochantins from the metepisterna and metacoxae , the position of the metafurca anteriorly on the discrimenal lamella of th e metathorax and the presence of second abdominal sternum (S2)-metacoxal mu scles. The absence of metapleuro-S2 muscles is an autapomorphy for the non- xyelid Hymenoptera. Putative autapomorphies of the Tenthredinoidea are: (1) the presence of transverse metanotal muscles, (2) the subdivision of the s econd phragmo-third phragmal muscles, part of which arises from the metalat erophragmal lobes, (3) the posterior thoracic spiracle occlusor muscles ari sing from the mesepistema, (4) the absence of trochantins and metanoto-troc hantinal muscles and (5) the presence of elongate lateral metafurcal arms. Having the paracoxal sulci extending along the anterior margins of the mete pisterna and the anterior metafurcal arms reduced are synapomorphies for al l tenthredinoid families excluding Blasticotomidae. The presence of transve rsely extended cenchri with hooks on their entire surface is a putative syn apomorphy for Diprionidae + Cimbicidae + Argidae + Pergidae. The clade Cimb icidae + Argidae + Pergidae is supported by the absence of metanotometabasa lar muscles, the fusion of the first abdominal tergite (T1) with the metepi mera and the absence of posterior metapleuro-metafurcal muscles. Autapomorp hies of the Cimbicidae are the absence of the metalaterophragmal lobes and the metalaterophragmal-metafurcal muscles. Having the mesoscutello-metanota l muscle inserting on a projection from the anterior margin of the metanotu m, surrounding the tendon with sclerotised cuticle, is a synapomorphy for t he Argidae and Pergidae. Autapomorphies of the Cephoidea are the absence of cenchri, the presence of distinct articulations between T1 and the metepim era, and having the paracoxal sulci extending subparallel with the metafurc al discrimen. The monophyly of the Siricidae is supported by the absence of the anapleural clefts and the presence of an elongate mesospina projecting posteriorly between the anterior metafurcal arms. The presence of a membra nous pouch ventrally of T1 and of large T1-metafurcal muscles is unique to Xiphydria camelus among the taxa examined. The absence of hind wing tegulae , posterior metapleuro-metafurcal, metanoto-trochantinal and anterior metan oto-metacoxal muscles, and the presence of elongate lateral metafurcal arms are synapomorphies for Xiphydriidae + Orussidae + Apocrita. The Orussidae greatly resembles the Apocrita in the region studied, a synapomorphy for th e two taxa being the presence of metepisternal depressions. An autapomorphy for the Apocrita is the fusion of T1 with the metapleural arms; these stru ctures closely abut in Orussidae. The fusion of T1 with the metepimera was preceded by the reduction of the posterior parts of the metepimera, as obse rved in Anaxyelidae, Xiphydriidae, and Orussidae. This makes the lines of f usion between T1 and the metepimera confluent with the metapleural sulci in the Apocrita. There is no compelling evidence for considering the configur ation of T1 and the metepimera in Cephoidea to be incipient in the formatio n of the propodeum in Apocrita. The close association between the meso- and metathorax and the integration of T1 in the metathorax evolved gradually twice within the basal hymenopter an lineages, culminating in the Apocrita and the Cimbicidae + Argidae + Per gidae clade.