An Eocene insect could hear conspecific ultrasounds and bat echolocation

Hearing has evolved independently many times in the animal kingdom, and is prominent in various insects and vertebrates for conspecific communication and predator detection. Among insects, katydid (Orthoptera: Tettigoniidae) ears are unique as they have evolved outer, middle, and inner ear components, analogous in their biophysical principles to the mammalian ear. The katydid ear consists of two paired tympana located in each foreleg. These tympana receive sound externally on the tympanum surface (usually via pinnae), or internally via an ear canal (EC). The EC functions to capture conspecific calls and low frequencies, while the pinnae passively amplify higher frequency ultrasounds including bat echolocation. Together, these outer ear components provide enhanced hearing sensitivity across a dynamic range of over 100 kHz. However, despite a growing understanding of the biophysics and function of the katydid ear, its precise emergence and evolutionary history remains elusive. Here, using micro-CT scanning, we recovered geometries of the outer ear components and wings of an exceptionally well-preserved katydid fossilised in Baltic amber (~44 Ma). We show using numerical and theoretical modelling of the wings that this species was communicating at a peak frequency of 31.62 (± 2.27) kHz, and demonstrate that the ear was biophysically tuned to this signal, as well as providing hearing to higher frequency ultrasounds (> 80 kHz), likely for enhanced predator detection. The results indicate that the evolution of the unique ear of the katydid, with its broadband ultrasonic sensitivity and analogous biophysical properties to the ears of mammals, emerged in the Eocene