Convergent evolution of harmonic hopping: multiple origins of high-frequency calls in crickets

Harmonic hopping has been described in mammals, birds and insects as a phenomenon by which acoustic character states change in discrete steps in a form of punctuated evolution rather than in a gradual manner. In these cases, acoustic signals appear to 'jump' from low-frequency ancestral states to higher - harmonically linked - frequencies in closely related species, without apparent intermediate forms. Male crickets (Orthoptera, Gryllidae) usually produce low-frequency acoustic signals (2-8 kHz) to attract distant mating partners by rubbing their wings together. However, many species in the subfamily Eneopterinae have evolved calls with uncharacteristically high frequencies, some even reaching the ultrasonic range (>20 kHz). Here, we document for the first time high-frequency calls in the cricket genus Pseudolebinthus (Eneopterinae). Using laser Doppler vibrometry, microcomputed tomography and finite element modelling, we present acoustic evidence that the calls are indeed a result of harmonic frequency hopping from an ancestral low-frequency fundamental to its fourth harmonic. We also provide morphological and biomechanical support for a mechanism explaining the apparent frequency jump through gradual changes to the morphology of the sound-production system. Using phylogenetic analysis, we further show that harmonic hopping events in eneopterine crickets occurred multiple times in various acoustical and morphological contexts independently, thereby constituting an example of convergent evolution of an acoustic trait.