THE FINE-STRUCTURE OF THE MULTIPOROUS SENSILLA ON THE ANTENNA OF THE DIVING BEETLE GRAPHODERUS-OCCIDENTALIS HORN (COLEOPTERA, DYTISCIDAE)

The antenna of the diving beetle Graphoderus occidentalis Horn has a cluster of multiporous (MP) sensilla on the anteromedial aspect of each flagellomere. These sensilla are modified basiconic pegs that are sunk in longitudinal sockets and flattened to a platelike surface externally. On the terminal flagellomere, approximately 15 multiporous pitted sensilla (MPp) have a pitted external surface and each is innervated by one or two morphologically similar neurons. In addition, eight multiporous grooved sensilla (MPg) have longitudinal surface grooves with pores opening at the bottom of each groove; each sensillum is innervated by two neurons that differ in electron density. In both sensillar types the pores open into channels that traverse the sensory cuticle. The branched dendritic terminations in the underlying chamber are bathed in an electron-dense sensillar liquor that extends into the channels leading to the surface pores. The neuronal perikarya in both types, and the proximal portion of the dendrites in the MPg, are unsheathed and exposed to the hemolymph. The putative function of these sensilla as odor detectors is discussed.