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

Neoconocephalus tettigoniids are a model for the evolution of acoustic signals as male calls have diversified in temporal structure during the radiation of the genus. Tettigoniidae have hearing organs in the forelegs with species-specific numbers of auditory sensilla in a linear crista acustica. We investigated changes of the hearing organs during an evolutionary radiation with divergence of intraspecific acoustic signals. We compared the neuroanatomy of the crista acustica from 9 Neoconocephalus species with different temporal call features, life histories, and from different phylogenetic positions. Average numbers of auditory sensilla were species-specific, ranging between 32–35 sensilla. It is likely under sexual selection for detection of male calls and natural selection for detection of bat calls. We found statistically significant differences in sensillum numbers among species, but no relationship of crista acustica length or sensillum number with phylogenetic position or life history. Statistically significant correlations existed with call patterns: species with slow pulse rates had significantly higher numbers of auditory sensilla and a longer crista acustica. Further, species with a derived double-pulsed calls had longer cristae, and continuous callers had a higher number of auditory sensilla. Correlations with crista length were stronger than with the number of auditory sensilla. The hearing organs show considerable diversity between species despite their recent divergence and morphological and ecological similarities. Thus, they have the potential to respond to various selective pressures, including divergence of temporal and spectral signal properties. Phylogenetic constraints are unlikely to limit the evolutionary change of the auditory systems.