Morphology, ultrastructure, and probable functions of the sense organs on the ovipositor stylets of the hymenoptran parasitoid, Venturia canescens (Gravenhorst)

Abstract

To help understand the mechanism of host discrimination during oviposition of a parasitoid wasp, Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae), the ultrastructure and morphology of the sense organs on the dorsal and ventral stylets of the ovipositor were examined using scanning and transmission electron microscopy. Ovipositor stylets were scanned for sense organs, because only unsheathed ovipositors are used for probing, stabbing, and host discrimination. Five types of sensilla and secretary pores were identified. Based on their pattern of innervation, the surface-dome and pitted-dome sensilla were thought to have a gustatory function and, thus, are the best candidates for host-discrimination. The arrangements of dendrites of type I and type II campaniform sensilla suggested that these acted as pressure- and mechano-receptors, respectively. Coeloconic sensilla might act as thermo-hygroreceptors, whilst secretary pores might provide lubrication during the process of stabbing for oviposition. Surface-dome sensilla were the most numerous type found followed by secretary pores, pitted-dome sensilla, and coeloconic sensilla. Almost all the sensilla and secretary pores were more numerous on the dorsal than on the ventral stylets, and the maximum number of these sensilla and secretary pores occurred on the distal end of the ovipositor stylets and decreased toward the proximal end. The surface-dome sensilla and secretary pores occurred in alternate rows while the pitted-dome sensilla were scattered all over the surface of the ovipositor stylets, especially on the sides of the stylets.