Stimulation by chemicals involves the senses of smell (olfaction) and taste (gustation). Olfaction implies the ability to detect compounds in the gaseous state. Taste, in humans, refers to detection of compounds in solution, or in a liquid state, by receptors in the oral cavity. Insects have comparable receptors on many parts of the body, often using them for purposes unrelated to feeding, and they also have the ability to detect chemicals on dry surfaces as well as in solution. For these reasons, it is usual to refer to ‘contact chemoreception’ in insects rather than ‘taste’. The distinction between olfaction and contact chemoreception is usually clear, although olfactory receptors can respond to substances in solution and contact chemoreceptors respond to high concentrations of some odors. Processing within the central nervous system is, however, quite different. The axons from all olfactory receptors terminate in the antennal lobes in the brain (section 20.4.2) whereas axons from contact chemoreceptors terminate within the ganglion of the body segment on which the receptors occur.

Reviews: Frazier, 1992 – chemoreceptor physiology; Morita & Shiraishi, 1985 – chemoreceptor physiology; Payne, Birch & Kennedy, 1986 – olfaction

OLFACTION

Structure of receptors

Olfactory sensilla comprise cuticular and cellular components comparable, in a general sense, to those of cuticular mechanoreceptors and contact chemoreceptors (Fig. 24.1, and compare Figs. 23.1, 24.8).

Review: Zacharuk, 1985

Cuticular components The cuticle of olfactory receptors is characterized by the presence of numerous small pores which permit the entry of chemicals (Figs. 24.1, 24.2).