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 Latreil le (Hymentoptera : Scelionidae) and related genera is described in det ail 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 b ody 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 per mit the Scelio-type system greater extendability. Telescopic extension of multiple sections of greatly elongated intersegmental membrane, op erated by changes in hydrostatic pressure, allows the entire oviposito r system, including the terminal metasomal tergite (T7 + T8), to be ex serted from the body cavity during oviposition, thus extending the ran ge 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 th eir associated musculature. Similar to other scelionids, orientation o f the ovipositor in Scelio is controlled by contraction of muscles con necting the proximal head of the ovipositor with the fused terminal me tasomal tergite (T7 + T8). A model for the mechanics of extension and retraction of the Scelio-type ovipositor system is proposed, and is su pported by anatomical evidence, behavioural observations and direct ma nipulation of the system. The evolution of this system in relation to the exploitation of a particular host group, the eggs of Orthoptera, a nd its significance in resolving phylogenetic problems within the Scel ionidae, are also discussed.