Evolutionary diversification of the auditory organ sensilla in Neoconocephalus katydids (Orthoptera: Tettigoniidae) correlates with acoustic signal diversification over phylogenetic relatedness and life history.

Neoconocephalus Tettigoniidae are a model for the evolution of acoustic signals as male calls have diversified in temporal structure during the radiation of the genus. The call divergence and phylogeny in Neoconocephalus are established, but in tettigoniids in general, accompanying evolutionary changes in hearing organs are not studied. We investigated anatomical changes of the tympanal hearing organs during the evolutionary radiation and divergence of intraspecific acoustic signals. We compared the neuroanatomy of auditory sensilla (crista acustica) from nine Neoconocephalus species for the number of auditory sensilla and the crista acustica length. These parameters were correlated with differences in temporal call features, body size, life histories and different phylogenetic positions. By this, adaptive responses to shifting frequencies of male calls and changes in their temporal patterns can be evaluated against phylogenetic constraints and allometry. All species showed well-developed auditory sensilla, on average 32-35 between species. Crista acustica length and sensillum numbers correlated with body size, but not with phylogenetic position or life history. Statistically significant correlations existed also with specific call patterns: a higher number of auditory sensilla occurred in species with continuous calls or slow pulse rates, and a longer crista acustica occurred in species with double pulses or slow pulse rates. The auditory sensilla show significant differences between species despite their recent radiation, and morphological and ecological similarities. This indicates the responses to natural and sexual selection, including divergence of temporal and spectral signal properties. Phylogenetic constraints are unlikely to limit these changes of the auditory systems.