A theoretical investigation of radiation mechanisms of insect chemoreception

Abstract

A theoretical investigation is performed of the hypothesis that certain insect sensory receptors operate by modal radiation mechanisms, i. e. as dielectric waveguide or resonator aerials, at infrared wavelengths, in the process of chemoreception and temperature measure­ment. The basic physical principles, requirements, and limitations of such mechanisms are discussed. The criterion chosen for the detectability of radiation emitted (absorbed) by stimulus molecules is that the increment (decrement) in signal power be greater than that from fluctua­tions in the background power from sky and ground. Thermal equilibrium processes . Mechanisms involving the emission of characteristic wave­lengths of radiation by vibrationally or rotationally excited odorant molecules in thermal equilibrium with their surroundings, or involving the absorption of characteristic wavelengths of background radiation by odorant molecules in the ground state, are analysed. For both emission and absorption mechanisms, required concentrations of odorant molecules are much greater than observed sensitivity thresholds for pheromones and most scents. There are also severe limitations on the discriminatory ability of any singly innervated sensillum. Non-equilibrium processes . Detection of stimulated or spontaneous emission of infrared radiation from low concentrations of odorant molecules released in excited states by a living organism is extremely difficult. Even for wavelengths in the 3 to 4μm atmospheric window the advantages of non-equilibrium processes are cancelled by the effects of collisions and other factors. However, radiation from fires is readily detectable, by day or night, in the 3 to 4μm window. Triggering of acceptors by non-radiative processes, in which specificity is determined principally by infrared spectrum, is theoretically possible and merits further investigation. Thus radiation mechanisms cannot provide a general mechanism of chemoreception, and in particular cannot account for pheromone detection, but might only operate in rare special cases, for which critical experiments are proposed. A theoretical model of operation of the honeybee pit-organs as dielectric resonators has been constructed as an illustrative example for the analysis.