Hearing and evasive behaviour in the greater wax moth, Galleria mellonella (Pyralidae)

Greater wax moths (Galleria mellonella L., Pyraloidea) use ultrasound sensitive ears to detect clicking conspecifics and echolocating bats. Pyralid ears have four sensory cells, A1-4. The audiogram of G. mellonella has best frequency at 60 kHz with a threshold around 47 dB sound pressure level. A1 and A2 have almost equal thresholds in contrast to noctuids and geometrids. A3 responds at + 12 to + 16 dB relative to the A1 threshold. The threshold data from the A-cells give no indication of frequency discrimination in greater wax moths. Tethered greater wax moths respond to ultrasound with short-latency cessation of flight at + 20 to + 25 dB relative to the A1 threshold. The behavioural threshold curve parallels the audiogram, thus further corroborating the lack of frequency discrimination. Hence, the distinction between bats and conspecifics is probably based on temporal cues. At a constant duty cycle (percentage of time where sound is on) the pulse repetition rate has no effect on the threshold for flight cessation, but stimulus duration affects both sensory and behavioural thresholds. The maximum integration time is essentially the same: 45 ms for the A1-cell and 50-60 ms for the flight cessation response. However, the slopes of the time-intensity trade-off functions are very different: - 2.1 dB per doubling of sound duration for the A1-cell threshold, and - 7.2 dB per doubling of sound duration for the behavioural threshold. The significance of the results for sexual acoustic communication as well as for bat defence is discussed.