Mechanical Tuning of the Acoustic Receptor Organ of the Noctuid Moth

Abstract

Mechanical characteristics of the auditory organ of Noctuid moths (Prodenia eridania, Cramer) have been investigated in an effort to define the stimulus at the receptor organ. This organ contains two receptor cells of unequal sensitivity and is suspended from the tympanic membrane. The neural spike rates produced by the more sensitive of the two cells have been recorded as a function of vibration frequency while mechanically stimulating the tympanic membrane. The frequency characteristic obtained in this way is determined by the mechanical properties of the receptor organ itself and is not affected by the system of air-filled cavities surrounding it. The resulting characteristic shows a primary resonance near 2 kHz, an antiresonance near 10 kHz, and one or more higher order resonances above 10 kHz. When the tympanic membrane is exposed to acoustic waves so that the intratympanic cavities become effective, the hearing organ is most sensitive to sound frequencies between 25 and 60 kHz [K. D. Roeder, J. Insect Physiol. 12, 843 (1966)]. Thus it appears that the primary mechanical resonance at 2 kHz is attenuated by the acoustic cavity system. The remaining high-frequency response is adapted to the frequency range of the sonar signals of bats.