Morphology of a unique sensillum placodeum on the antennae of Coeloides brunneri (Hymenoptera: Braconidae)

Abstract

A previously undescribed and unique placoid sensillum was discovered on the antennae of Coeloides brunneri. This elongate, dome-shaped plate organ was examined by light microscopy, and transmission and stereoscan electron microscopy. It differs from other plate organs in that it has two cuticular lamellae suspended internally from the dome of the sensillum. These lamellae separate the internal structure of the plate organ into three channels running the full length of the sensillum. Dendritic branches which enter the plate organ through an aperture in the center of the sensillum's cuticular floor fill the upper two-thirds of the median channel. A tormogen cell fills the two lateral channels and the basal third of the median channel. The linear structure and placement of these plate organs suggest that they are highly directional wave guides capable of perceiving infrared radiation in a manner that would enable C. brunneri to locate its host with a very high degree of accuracy.