A narrow ear canal reduces sound velocity to 1 create additional acoustic inputs in a micro-scale insect ear

Located in the forelegs, katydid ears are unique among arthropods in having outer, middle and inner component, analogous to the mammalian ear. Unlike mammals, sound is received externally, and internally via a narrow ear canal (EC) derived from the respiratory tracheal system. Inside the EC sound travels slower than in free air, causing temporal and pressure differences between external and internal inputs. The delay is suspected to arise as sound propagation changes from adiabatic to isothermal, imposed by EC geometry. If true, a reduction in sound velocity should persist independently of the gas composition in the EC. Integrating experimental (laser Doppler vibrometry, micro-CT) and numerical methods, we demonstrate that the narrow radius of the EC is the major cause of the signal time delay. Results imply that the EC is asymmetrically bifurcated, creating four notable auditory paths for each ear. Implication of methods and findings in avian hearing are discussed.