Host location and oviposition in a basal group of parasitic wasps: the subgenual organ, ovipositor apparatus and associated structures in the Orussidae (Hymenoptera, Insecta)

Anatomical studies and behavioural observations indicate that representativ es of the Orussidae use vibrational sounding to detect suitable oviposition sites. During host location, vibrations generated by tapping the tips of t he antennae against the wood are picked up by the fore legs through the bas itarsal spurs, transmitted along the basitarsi to thin-walled areas on the tibiae and through haemolymph to the subgenual organs, where they are trans duced into nerve impulses. The apical antennomeres are distinctly shaped an d have the cuticle thickened distally. The fore basitarsi have weakly scler otised basitarsal lines proximally and membranous basitarsal spurs distally . The external wall of the fore tibiae have thin-walled areas distally on t heir posterior parts. Internally, large subgenual organs are situated oppos ite the thin-walled areas and each organ consists of 300-400 scolopidial un its suspended between a lateral cuticular spine, a ventral sheet and a medi an ridge. The ovipositor is several times the length:of the body of the was p. When at rest, it extends all the way into the prothorax, where it is coi led before extending posteriorly to lie between the third valvulae distally . The ovipositor lies in a membranous ovipositor sac attached posteriorly t o the proximal parts of the ovipositor apparatus and the posterior margin o f sternum 7. In the ovipositor apparatus, the anterior parts of the second valvifers are displaced and expanded anterodorsally, inverting the first va lvifers and the base of the ovipositor. When in use, the ovipositor is exte nded and retracted by median apodemes situated on the anterior margins of a bdominal sterna 3-7. Longitudinal muscles between the apodemes allow the la tter to grip the ovipositor in troughs between them. The ovipositor extends from the abdomen at the tip of sternum 7, and an internal trough on sternu m 7 serves to guide the ovipositor into the wood. Despite the alterations o bserved in the ovipositor apparatus in the Orussidae, the musculature is al most complete and the mode of operation presumably not much different from that of other representatives of the Hymenoptera. The different ways parasi tic wasps with very long ovipositors handle and accommodate these and the i mplications for the evolutionary history of Hymenoptera are discussed.