[Functional properties of chemoreceptor systems]

It is proposed that the majority of organisms with developed sensory chemoreception possesses two distinct systems of chemical sensitivity performing different tasks. The first system (taste) serves for perception of alimentary substances -- metabolites, or dangerous compounds. The second one (olfaction) perceives metabolically neutral substances which serve as pure signals. All other features of these two systems of chemoreception are considered as secondary, resulting from the main task of each system. In particular, the signal function of olfaction demands sufficiently high sensitivity and a large number of individually recognizable stimuli. The physical state of olfactory stimulants (vapours or solutions) is non-essential. Arguments are presented showing the existence of contact chemoreception with pure signal function both invertebrates and in invertebrates which serves as "contact olfaction". Several problems of chemical communication in animals are discussed, in particular, the role of chemoreception systems in the perception of pheromones and also different kinds of sensory analysis involved in the recognition of chemical stimuli.