The fine structure of uniporous and nonporous pegs on the distal antennal segment of the diving beetle Graphoderus occidentalis Horn (Coleoptera: Dytiscidae)

Abstract

The terminal segment of the antenna of the predaceous diving beetle Graphoderus occidentalis Horn bears approximately 50 sensilla of nine different types. There are six types of uniporous chemosensilla, one type of nonporous mechanosensitive peg, and two types of multiporous pegs. Two types of uniporous pegs, which occur only at the antennal tip, have features of both contact chemosensilla and thermo- and hygro-sensilla. Three other types that occur only on the terminal segment below the apex also exhibit some features of chemosensilla and thermo- and hygro-sensilla. The sensilla are innervated by one to four neurons and most are ensheathed by two accessory cells. Four types of sensilla exhibit elaborate sheath cell configurations and secretions associated with their sinus that may reflect an adaptation to an aquatic or a dual aquatic–terrestrial habitat. All are atypical of generalized insect sensilla in having portions of the proximal dendrites and axons as well as the entire neuronal perikarya unsheathed and exposed to the hemocoel. The neuronal perikarya of two sensillar types contain virus-like particles.