All animals detect and react to molecules in the external environment, including pheromones, signature mixtures of other individuals, and the chemical cues that indicate food, shelter, pathogens or predators. Despite the great external diversity in appearance of antennae and noses, animals of all kinds perceive these chemical stimuli in the same way (Hildebrand & Shepherd 1997; Su et al. 2009; Touhara & Vosshall 2009). Chemosensory receptor proteins are exposed to the outside world in the membrane of chemosensory nerve cells, often through a “window” in an otherwise impermeable skin or cuticle (Figure 9.1). The arrival of an odor molecule (odorant) is converted into a signal to the brain by first binding to a chemosensory receptor protein. Odorants can be any kind of molecule, as large as a protein or as small as formic acid.

Whether in air or water, olfaction is the key sense used to detect most chemical cues, rather than taste (gustation) (see Box 9.1 for the differences between these senses). All signature mixtures (the molecules learned highly varied individual or colony odors; Chapters 1 and 6; Sections 9.8 and 9.9), and probably most pheromones, whatever the size of molecules, are detected by olfaction. However, some pheromones in invertebrates are detected by gustation, including some in Drosophila (Section 9.5.2). Some pheromones in both vertebrates and invertebrates may act directly on the brain or other organs (Sections 9.5.2 and 9.7.3.2).