Cicada orni L. is one of the most abundant and common species of cicada in Greece. However, this species was not found during recent field work on the Greek islands of Samos and Ikaria. Instead, the very closely related C. mordoganensis Boulard was found practically everywhere on these islands. C. orni and C. mordoganensis are very closely related species which are morphologically very similar (sibling species), even the male genitalia, and the acoustic signals produced by males during courtship and pair formation have the same general pattern. In order to describe the acoustic signals produced by these sibling species, temporal and spectral analyses were made of the calling songs of the males and certain acoustic variables were measured. Based on the duration of echemes, the number of pulse units they contain, the intervals between echemes and the number of echemes per second, the song of C. mordoganensis is distinct from that of C. orni. Cluster analysis of the acoustic characteristics of C. mordoganensis from Samos and Ikaria and of C. orni from the Greek mainland (Dionysos, north of Athens) gave a clear and distinct separation of these species.
Moreover, as there has been very little divergence between these two species at the protein electrophoretic level, the acoustic divergence has evolved independently of allozyme divergence. This may imply that in these cicadas acoustic divergence, and therefore premating isolation, may have evolved rapidly and resulted in rapid speciation at low levels of general genetic differentiation.