Fig. 7
Mechanisms driving the various ovipositor movements of Lariophagus distinguendus, and the importance of the terebra movements during the oviposition process. a–c Functional model of the mechanisms driving the various ovipositor movements in the resting and the active probing position (only the left side of the paired ovipositor elements are depicted; not to scale). Acting (input) muscle forces are visualized by solid red arrows and resulting (output) movements by solid black arrows. a Mechanism of the tilting movement of the 1st valvifer and the resulting pro- and retraction of the 1st valvulae (lateral view, left is anterior). Only the two pairs of antagonistically working muscles that are responsible for these movements (m-a-2vf-2vv/m-p-2vf-2vv and m-d-T9-2vf/m-v-T9-2vf) are represented in simplified terms. The muscles stabilizing the ovipositor system (m-1vf-gm; m-p-T9-2vf; m-T9-gm) and those supporting the venom and reproductive systems (m-d-2vf-vr; m-v-2vf-vr) are not shown. Contraction of (both parts of) m-d-T9-2vf (Fm-d-T9-2vf) slides the 2nd valvifer posteriorly and the female T9 anteriorly towards each other (arrow 1), thus indirectly causing the 1st valvifer to tilt anteriorly (arrow 2). This is possible because the 1st valvifer is articulated with both the 2nd valvifer and the female T9 via the intervalvifer and tergo-valvifer articulation, respectively. The 1st valvifer thereby functions as a lever arm that transmits the movement to the dorsal ramus of the 1st valvula (arrow 3) and consequently causes a protraction of the 1st valvula (arrow 4). M-p-T9-2vf and m-T9-gm thereby presumably stabilize the system by holding the 2nd valvifer and the female T9 in position and preventing them from rotating around the articulations. Contraction of m-v-T9-2vf (Fm-v-T9-2vf) slides the 2nd valvifer anteriorly and the female T9 posteriorly apart from each other (arrow 5), thus causing the 1st valvifer to tilt posteriorly (arrow 6). This movement is transmitted via the dorsal ramus of the 1st valvula (arrow 7) and consequently causes its retraction (arrow 8). When the terebra is withdrawn from the substrate, a contraction of the m-a-2vf-2vv (Fm-a-2vf-2vv) presumably causes the bulbs to pivot posteriorly around the basal articulation, thus elevating the 2nd valvula and therefore the whole terebra back into its resting position (arrow 9). b, c Mechanisms of the bending and rotational movements of the terebra (b lateral view, left is anterior; c dorsal view; schematic drawing of wasp in lateral view). During oviposition, a contraction of m-a-2vf-2vv cannot elevate the terebra back towards its resting position (as described in other hymenopteran taxa), because the terebra is anchored at the puncture site in the substrate. In this situation, a contraction of one of the paired m-a-2vf-2vv (Fm-a-2vf-2vv) in the active probing position pulls the corresponding bulb and thus one half of the longitudinally split and asymmetrically overlapping 2nd valvula dorsad along its longitudinal axis (arrow 10) because of the orientation of the muscle and the resulting direction of the force vector. Since the halves of the 2nd valvula are fused at the apex, this movement causes the distal part of the terebra (i.e. the part inside the cavity in the substrate) to bend to the left or right: a contraction of the left m-a-2vf-2vv causes the 2nd valvula and thus the whole terebra to bend to the left (arrow 11), a contraction of the right m-a-2vf-2vv causes a bend to the right. In addition, a contraction of one of the m-p-2vf-2vv (Fm-p-2vf-2vv) in the active probing position presumably causes the 2nd valvula and thus the whole terebra to rotate back and forth at the basal articulation along its longitudinal axis to a certain degree: a contraction of the left m-p-2vf-2vv causes the 2nd valvula and thus the whole terebra to rotate anti-clockwise when viewed from the dorsal side (arrow 12), whereas a contraction of the right m-p-2vf-2vv results in a clockwise rotation (cf. Additional file 1). Contractions of the m-a-2vf-2vv might support these rotational movements. The rotation allows the bending movements to take effect in different directions. d Timeline of the oviposition process of an idiobiont ectoparasitoid wasp highlighting the importance of terebra movements during the various stages (stages in parenthesis do not occur in L. distinguendus; stages with * do not occur in all parasitoid lifestyles but are particularly notable in idiobiont ectoparasitoids). Abbreviations: 1vf: 1st valvifer; 1vv: 1st valvula; 2vf: 2nd valvifer; 2vv: 2nd valvula; 3vv: 3rd valvula; ba: Basal articulation; dr1: Dorsal ramus of the 1st valvula; F: Force; iva: Intervalvifer articulation; m-a-2vf-2vv: Anterior 2nd valvifer-2nd valvula muscle; m-d-T9-2vf: Dorsal T9-2nd valvifer muscle; m-p-2vf-2vv: Posterior 2nd valvifer-2nd valvula muscle; m-v-T9-2vf: Ventral T9-2nd valvifer muscle; T9: Female T9 (9th abdominal tergum); tva: Tergo-valvifer articulation; trb: Terebra