Structure and function of the musculoskeletal ovipositor system of an ichneumonid wasp

Abstract Background Modifications of the ovipositor appear to have played a prominent role in defining the host range of parasitoid hymenopterans, highlighting an important contributing factor in shaping their oviposition strategies, life histories and diversification. Despite many comparative studies on the structure of the hymenopteran terebra, little is known about functional aspects of the musculoskeletal ovipositor system. Therefore, we examined all inherent cuticular elements and muscles of the ovipositor of the ichneumonid wasp Venturia canescens (Gravenhorst, 1829), investigated the mechanics of the ovipositor system and determined its mode of function. Results We found that the movements of the ichneumonid ovipositor, which consists of the female T9 (9th abdominal tergum), two pairs of valvifers and three pairs of valvulae, are actuated by a set of six paired muscles. The posterior and the anterior 2nd valvifer-2nd valvula muscles flex and extend the terebra from its resting towards an active probing position and back. The dorsal T9-2nd valvifer muscle is modified in V. canescens and forms distinct bundles that, together with the antagonistically acting ventral T9-2nd valvifer muscle, change the relative position of the 2nd valvifer to the female T9. Thereby, they indirectly tilt the 1st valvifer because it is linked with both of them via intervalvifer and tergo-valvifer articulation, respectively. The 1st valvifer acts as a lever arm that transfers movements to the 1st valvula. The posterior T9-2nd valvifer muscle and the small 1st-valvifer-genital membrane muscle stabilize the system during oviposition. Conclusions From our examination of the elements of the musculoskeletal ovipositor system of ichneumonids, we discussed leverages and muscle forces and developed a functional model of the underlying working mechanisms adding to our understanding of a key feature that has largely determined the evolutionary success of the megadiverse Ichneumonidae with more than 24,000 hitherto described species.