Comparative Analysis of Morphological and Acoustic Correlates of Bush-Cricket Tympanic Membranes

Bush-crickets exhibit remarkable auditory diversity, having a pair of tympanic membranes (tympana) on each foreleg. These tympana could be exposed or covered by cuticular flaps known as auditory pinnae. These membranes receive sound directly and via an acoustic tracheal tube. Despite extensive research on auditory tuning, the relationships between the tympanal morphology, body size, and acoustic traits of bush-crickets remain unclear. In this study, micro-computed tomography reconstructions, artificial-intelligence-assisted segmentation, and phylogenetically informed comparative regressions in 18 selected bush-cricket species were used to investigate how tympanal surface area and mean cross-sectional thickness relate to pronotum length (a proxy for body size), carrier frequency, and the presence of auditory pinnae. Tympanal surface area scaled positively with pronotum length, whereas thickness was independent of size. Carrier frequency showed a weak negative relationship with tympanal thickness, although this association was not statistically significant. Among 14 species with well-developed auditory pinnae, tympanal dimensions showed no correlation with peak resonance frequency, suggesting that pinnal cavity resonance and tympanal morphology are not tightly correlated and may evolve independently. Across species, the presence of pinnae was not associated with differences in tympanal surface area, although unilateral pinnae were linked to thicker tympana. Within the 4 unilateral species, the pinna-covered tympanum remained consistently larger in area but thinner than the exposed side. Overall, these findings indicate that tympanal evolution reflects a balance of scaling constraints and configuration-specific associations with auditory pinnae.