Insects are prodigious users of chemical signals and cues, which play diverse and fundamental roles in the transfer of information both within and between species. Indeed, it is likely that no other group of animals makes such sophisticated use of chemical signaling in their biology. This chapter begins by defining the different classes of signals (Section 27.1), before describing the nature of intraspecific chemical signals (pheromones) (Section 27.2), the information content of such pheromones (Section 27.3), their biosynthesis (Section 27.4) and the mechanisms regulating their production (Section 27.5), as well as their sensory perception by conspecifics (Section 27.6). In the next section (Section 27.7) interspecific signals are discussed (allelochemicals), followed by their mechanisms of production and release (Section 27.8). Section 27.9 concerns defensive compounds, and the chapter ends with chemical mimicry (Section 27.10).
Defining chemical signals
Chemical signals and cues have been collectively called semiochemicals, derived from the Greek word “semeon” for signal. However, it has been suggested that the term “infochemical” may be more appropriate, based on the argument that nomenclature should be based on a “cost–benefit analysis” rather than the actual source of the signal. While there has not been complete acceptance of either term and both are used in the current literature, we will use infochemical when referring to “a chemical substance, which in a natural context, is implicated in the transfer of information during an interaction between two individuals that results in a behavioral and/or physiological response in one or both.” In this chapter the term “signal” is applied to an infochemical produced by an emitter which has been shaped by evolution to transmit a specific message to the intended receiver. An example of this would be the release of a sex pheromone for the specific purpose of attracting a mate. The term “cue” is used to describe an infochemical that conveys information to a receiver, but was not shaped by natural selection for this purpose – that is, it is exploited by receivers, often to the detriment of the emitter. For example, the sex pheromone emitted by an insect to attract a mate may also be exploited as a kairomone by a predator or parasitoid.