Diurnal moths have larger hearing organs: evidence from comparative 3D morphometric study on geometrid moths

Tympanal organs, crucial for anti-bat defence in moths and key for taxonomy, are often overlooked due to their fragility during dissection. Using micro-CT, we analyzed the tympanal organs of 19 geometrid species, comparing diurnal and nocturnal species to understand how predators, like bats and diurnal birds or lizards, influence tympanal morphology and its allometric relationship with body size. We hypothesized that diurnal moths, with reduced anti-bat function, would have smaller tympanal organs, irrespective of body size. Allometry was tested using phylogenetic linear regression and tympanal volume was compared across diurnal and nocturnal moths relative to the abdominal volume. We used 3D geometric morphometry, followed by comparative analysis of the shape and size of ansa, a unique “mechanical” structure for geometrids. Contrary to our hypothesis, diurnal moths had significantly larger tympanal organs, with no allometric relationship with body size. Although activity patterns had no significant effect on ansa shape and size, convergence in shape among nocturnal species, suggests a possible auditory function. This study explores how daily activity patterns and predator-prey interactions may influence sensory adaptations, with larger tympanal organs of diurnal species potentially reflecting adaptations to detect lower, “non-bat” frequencies. It also highlights non-invasive imaging techniques for studying delicate anatomical features in museum specimens.