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D.N. LAPSHIN AND D.D. VORONTSOV
THE MECHANISM OF DYNAMIC FREQUENCY TUNING IN THE HEARING SYSTEM OF NOCTUID MOTHS (NOCTUIDAE, LEPIDOPTERA)
Moths Blepharita (Crino) satura Schiff. were tested with short (2.5 ms) acoustic pulses delayed by 1, 6 or 12 ms from a moth's own click under conditions of tethered flight. We studied the shape of the auditory threshold curves depending on the stimulating pulse delay. The increase in moth's click emission rate was taken as an indication of the response. At 1 ms delay the minimal thresholds (tuning optimum) were recorded in the frequency range from 40 to 60 kHz. The increase of the delay value up to 12 ms resulted in shift of the tuning optimum down to 20 kHz. At 6 ms delay the optimum had an intermediate position (30 kHz on average). The smooth shift of the frequency tuning curve could be explained by the modification of the tympanic membrane resonance which depends on the tension of the membrane by the muscles of metathorax. The shapes of individual threshold curves suggest that resonance modes of some mechanical system other than tympanic membrane, presumably including scoloparium, ligament and tympanic nerve, also play considerable role in determining the resulting frequency tuning of the auditory system. At frequencies above 70 kHz the auditory thresholds to a great extent are determined by an active tuning of the auditory system to the carrier frequency of incoming signal.