THE PREORAL CAVITY OF LOWER HYMENOPTERA (INSECTA) - COMPARATIVE MORPHOLOGY AND PHYLOGENETIC SIGNIFICANCE

The skeletal and muscular morphology of the preoral cavity, including the labrum, hypopharynx and labium, was examined in the image in repre sentatives of all the 'symphytan' families as well as the apocritan fa milies Stephanidae, Megalyridae and Trigonalyidae. XyeIidae have compl ex modifications for masticating pollen, remarkably similiar to those of primitive Lepidoptera. These modifications, collectively termed the triturating basket complex, include an asymmetrical distal epipharyng eal wall with a microtrichial brush and an enlarged infrabuccal pouch with heavy cuticular armature that interacts with the mandibles during feeding. There were striking structural differences between the two s ubfamilies of Xyelidae in the ligular region; the reduced glossa and c lubshaped paraglossae of Macroxyelinae resembles those of primitive Le pidoptera, while the well developed, flattened glossa and paraglossae in Xyelidae are similiar to those of most other 'Symphyta'. A putative transformation series, leading from a relatively large labrum with un sclerotised distal epipharyngeal wall lying anterior to the mandibles, as seen in XyeIidae and Tenthredinoidea, to a small and heavily scler otised labrum and distal epipharyngeal wall lying posterior to the man dibles, as seen in 'Siricoidea', Orussidae and the Apocrita, was revea led. These modifications may be adaptations to enable the adult of the families pupating in wood to emerge from the pupal chamber. The Anaxy elidae, Orussidae and Apocrita have similiar configurations of the glo ssa and insertions of the ventral premental adductors. This indicates a close affinity of the Anaxyelidae to Orussidae + Apocrita, a hypothe sis that is in conflict with other character systems. The Orussidae an d Stephanidae share a unique condition in the development of a pair of large apodemes attached to the labrum; this renders the groundplan st ate of the labrum in the Apocrita uncertain. Twentyfive characters wer e defined in an attempt to eludicate the 'Symphyta'-Apocrita transitio n. A numerical cladistic analysis of the characters was undertaken, re sulting in 522 minimum length trees. The characters are also discussed with reference to a cladogram which resulted from an analysis of the characters derived from the present study and a survey of characters f rom literature. (C) 1996 The Norwegian Academy of Science and Letters