Mechanics of a `simple' ear: tympanal vibrations in noctuid moths

Anatomically, the ears of moths are considered to be among the simplest ears found in animals. Microscanning laser vibrometry was used to examine the surface vibrations of the entire tympanal region of the ears of the noctuid moths Agrotis exclamationis, Noctua pronuba, Xestia c-nigrum and Xestia triangulum. During stimulation with ultrasound at intensities known to activate receptor neurones, the tympanum vibrates with maximum deflection amplitudes at the location where the receptor cells attach. In the reportedly heterogeneous tympana of noctuid moths, this attachment site is an opaque zone that is surrounded by a transparent, thinner cuticular region. In response to sound pressure, this region moves relatively little compared with the opaque zone. Thus, the deflections of the moth tympanic membrane are not those of a simple circular drum. The acoustic sensitivity of the ear of N. pronuba, as measured on the attachment site, is 100+/-14 nm Pa(-1) (N=10), corresponding to tympanal motion of a mere 200 pm at sound pressure levels near the neural threshold.