D.N. LAPSHIN, D.D. VORONTSOV
TUNING TO AND FOLLOWING THE ACOUSTIC SIGNAL BY THE AUDITORY SYSTEM IN NOCTUID MOTHS (NOCTUIDAE, LEPIDOPTERA)
The measurement of frequency threshold curves under different experimental conditions was performed in moth Amphipyra pyramidea L. We compared data obtained from an intact animal in tethered flight experiment, and the curves measured in electrophysiological experiment. In the latter case the activity of auditory interneurons of prothoracic ganglion was recorded. In intact moths the decrease in thresholds with the frequency rise was observed above 80 kHz while in responses of auditory interneurons thresholds dramatically increased from 70 to 100 kHz by 15 dB on average. In this paper we examined the reasons for non-coincidence of results obtained in behavioral and electrophysiological experiments. It was shown that, after 30 min since electrophysiological experiment start when the effect of chloroform narcosis was reduced there was an evidence of auditory system tuning to the stimulus carrier frequency in the range from 70 to 100 kHz: auditory thresholds measured by decreasing the stimulus amplitude were lower than the ones measured by increasing the amplitude ("upper thresholds"). The maximal difference in auditory thresholds obtained using the measurement scheme described above varied from 7 dB at 70 kHz to 17 dB at 90 kHz. Basing on several features of auditory responses recorded, particularly on the dynamics of neuronal firing in the near-threshold area, we conclude that when stimulus level exceeded the "upper threshold" in our experiments some high-quality resonance within the tympanic organ shifted downwards from above 100 kHz. This effect is not observed in narcotized or heavily injured insect and this is the reason why one usually cannot demonstrate the effect in electrophysiological experiment. The range of threshold changes during frequency tuning in intact moth exceeds the difference in thresholds measured under various experimental conditions. We have also found that during experimental warming up of a moth body from 20 to 30░C auditory thresholds proportionally decreased by 7 dB on average without considerable changes in the shape of frequency threshold curve.