Morphology of the elongate sensillum placodeum on the antennae of Aphidius smithi (Hymenoptera: Aphidiidae)

Abstract

Elongate placoid sensilla were present on all 20 flagellar segments in five male Aphidius smithi and all but the basal 1 of 17 segments in five females. They are oriented longitudinally, approximately equidistant around the circumference of a segment. Scanning electron microscopy of the internal cuticular surface disclosed lamellae which divide the sensillum into lateral and median channels. Many transverse ridges are present in the ceiling of the median channel. Unlike other species, there is no cuticular floor of the sensillum. Numerous nerve cell bodies in the subcuticular tissue give rise to dendritic processes, a ciliary region, and a region of dendritic branches which lie parallel to each other within the 1-μm-wide median channel underneath the 0.1- to 0.25-μm-thick dome. Each dendritic branch is composed of a neurotubule surrounded by a plasma membrane. The ascending dendrites are flanked by the microvilli of the trichogen cell. A tormogen cell encloses the trichogen cell and extends into the lateral channels of the sensillum. The relationship of minute pores in the dome with the underlying dendritic branches is unclear. The hypothesis is advanced that the sensilla may be involved in host finding through perception of infrared radiation.