Abstract Anatomical studies and behavioural observa- tions indicate that representatives of the Orussidae use vibrational sounding to detect suitable oviposition sites. During host location, vibrations generated by tapping the tips of the antennae against the wood are picked up by the fore legs through the basitarsal spurs, transmitted along the basitarsi to thin-walled areas on the tibiae and through haemolymph to the subgenual organs, where they are transduced into nerve impulses. The apical an- tennomeres are distinctly shaped and have the cuticle thickened distally. The fore basitarsi have weakly sclerotised basitarsal lines proximally and membranous basitarsal spurs distally. The external wall of the fore tibiae have thin-walled areas distally on their posterior parts. Internally, large subgenual organs are situated op- posite the thin-walled areas and each organ consists of 300–400 scolopidial units suspended between a lateral cuticular spine, a ventral sheet and a median ridge. The ovipositor is several times the length of the body of the wasp. When at rest, it extends all the way into the pro- thorax, where it is coiled before extending posteriorly to lie between the third valvulae distally. The ovipositor lies in a membranous ovipositor sac attached posteriorly to the proximal parts of the ovipositor apparatus and the posterior margin of sternum 7. In the ovipositor appara- tus, the anterior parts of the second valvifers are dis- placed and expanded anterodorsally, inverting the first valvifers and the base of the ovipositor. When in use, the ovipositor is extended and retracted by median apode- mes situated on the anterior margins of abdominal sterna 3–7. Longitudinal muscles between the apodemes allow the latter 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 sternum 7 serves to guide the ovipositor into the wood. Despite the alter- ations observed in the ovipositor apparatus in the Oruss- idae, the musculature is almost complete and the mode of operation presumably not much different from that of other representatives of the Hymenoptera. The different ways parasitic wasps with very long ovipositors handle and accommodate these and the implications for the evolutionary history of Hymenoptera are discussed. A. Introduction The Orussidae are a small wasp taxon, comprising about 85 described species world-wide (Vilhelmsen unpub- lished data). They are widespread, but uncommon and only rarely collected. Their phylogenetic position within the Hymenoptera has been under debate for the better part of a century (Vilhelmsen 2001). Orussidae share some prominent plesiomorphic traits with the basal, non- apocritan lineages of the Hymenoptera, the ‘Symphyta’: presence of anal cells in the forewing (only partly devel- oped in Orussidae), presence of cenchri and absence of the ‘wasp-waist’ (the formation of a propodeum and metasoma; see Vilhelmsen 2000a). This led some precla- distic workers to place them among the ‘Siricoidea’ (woodwasps) in the almost exclusively phytophagous ‘Symphyta’ (Rohwer 1912; Bischoff 1926; Ross 1937; Benson 1938; Cooper 1953). Others recognised some of the derived traits that Orussidae have in common with the predominantly parasitic Apocrita: presence of a spe- cialised antennal cleaner apparatus on the fore leg, L. Vilhelmsen ( ✉ ) Department of Systematic Zoology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden e-mail: lars.vilhelmsen@ebc.uu.se N. Isidoro · R. Romani Institute of Agricultural Entomology, Perugia University, Borgo XX Giugno, 06121 Perugia, Italy H.H. Basibuyuk Department of Biology, Cumhuriyet University, 58140, Sivas, Turkey D.L.J. Quicke Centre for Population Biology, Unit of Parasitoid Systematics, CABI Bioscience UK Centre (Ascot), Department of Biology, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK D.L.J. Quicke Department of Entomology, The Natural History Museum, London SW7 5BD, UK Zoomorphology (2001) 121:63–84 © Springer-Verlag 2001 ORIGINAL ARTICLE Lars Vilhelmsen · Nunzio Isidoro · Roberto Romani Hasan H. Basibuyuk · Donald L.J. Quicke Host location and oviposition in a basal group of parasitic wasps: the subgenual organ, ovipositor apparatus and associated structures in the Orussidae (Hymenoptera, Insecta) Accepted: 14 March 2001