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Taxonomy should be based on the compilation of the highest possible number of diagnostic characters. As such, classical morphological descriptions should include behavioural characters when available. Surprisingly, the ICZN code never refers to behaviour, “work” being the closest term used in the context of ichnotaxon. The use of behavioural information has recently led to a deep revision of a small Palaearctic cicada group including new status, new synonymy and description of several new species. This example of hidden diversity revealed through acoustics illustrates and supports the validity of behavioural characters for taxonomical decisions and for species identification by experts. Taxonomy is considered as a descriptive discipline, and even if highly species-specific, sound or any other behavioural signal cannot be used as a type in replacement of a specimen. However, behaviour can turn taxonomy to experiments. It is indeed possible to test the specificity and then the validity of a behavioural diagnostic character through experiments. This is particularly the case with acoustic signals than can be manipulated and played back to target animals. Positive or negative responses to stimuli can point out the importance of the characters in species isolation and consequently in species definition by taxonomists.