Anatomy and mechanics of the telescopic ovipositor system of Scelio latreille (Hymenoptera: Scelionidae) and related genera

The telescopic ovipositor system possessed by the genus Scelio Latreille (Hymentoptera : Scelionidae) and related genera is described in detail for the first time, and found to be anatomically and mechanically unique amongst the parasitic Hymenoptera. Its basic plan is similar to the Ceratobaeus-type ovipositor system described previously for other scelionids, in that the ovipositor is invaginated entirely into the body cavity when at rest and attached to the terminal metasomal segment only by a lightly sclerotized collapsible membranous tube. There are, however, significant anatomical and mechanical modifications that permit the Scelio-type system greater extendability. Telescopic extension of multiple sections of greatly elongated intersegmental membrane, operated by changes in hydrostatic pressure, allows the entire ovipositor system, including the terminal metasomal tergite (T7 + T8), to be exserted from the body cavity during oviposition, thus extending the range of the ovipositor by as much as 3.5 times its length. These changes are accompanied by the incoporation of the lateral apodemes into the wall of the most distal segment of telescopic tube, and the loss of their associated musculature. Similar to other scelionids, orientation of the ovipositor in Scelio is controlled by contraction of muscles connecting the proximal head of the ovipositor with the fused terminal metasomal tergite (T7 + T8). A model for the mechanics of extension and retraction of the Scelio-type ovipositor system is proposed, and is supported by anatomical evidence, behavioural observations and direct manipulation of the system. The evolution of this system in relation to the exploitation of a particular host group, the eggs of Orthoptera, and its significance in resolving phylogenetic problems within the Scelionidae, are also discussed.