Pore structures in insect olfactory sensilla: A review of data and concepts

Abstract

An historical overview is given of the most important discoveries and hypotheses regarding stimulus transport in insect olfaction. The great structural similarity between the pore tubules of olfactory single-walled wall-pore sensilla and the epicuticular filaments of non-olfactory cuticle may reflect not only a similar composition but also a similar transport mechanism. The new “wick concept” of pore tubules comprises 2 developmental periods. First, during ontogeny of the sensillum, pore tubules may be involved in the secretion of the material of the outermost epicuticular layers. The pore tubules may function like the wick in an oil lamp, taking up lipoid molecules from the sensillum lymph for outward transport. During the second period, after the sensillum has been completed, the pore-tubule wick may work as a dispenser of odorant molecules in an inward direction. The large surface of pore tubules as compared with the cross section of the outer pores could facilitate the binding of odorant molecules by the odorant-binding proteins in the sensillum lymph. In double-walled wall-pore sensilla, on the other hand, pore tubules are not involved in stimulus transport. In this class of olfactory sensilla, the dendrites are protected by a palisade of cuticular fingers, and openings between these fingers, the spoke channels, are the stimulus transport pathways. The fundamentally different topology of sensillar wall pores hints at a separate phylogenetic origin of the two categories of insect olfactory sensilla.