- University of Toronto
The tympanate, arctiid moth,Cycnia tenera responds to pulsed, 30 kHz acoustic stimuli resembling bat echolocation signals by emitting trains of clicks. This phonoresponse was used to determine that this moth is maximally sensitive to stimulus pulse repetition rates of 30–50 pulses/s, rates typically emitted by bats shortly before they close with their targets. At rates both above and below this optimum moths exhibit higher thresholds and reduced responsiveness. These data suggest that C. tenera is capable of using the repetition rate emitted by an approaching bat as a cue in determining the relative proximity of the bat. The use of repetition rate information should allow this moth both an unambiguous indication of a bat at very close range as well as the ability to distinguish sources of nocturnal, high-frequency sounds not emitted by predators.
Individual Cycnia tenera were raised from eggs collected from gravid females at the Queen's University Biology Station, Chaffey's Locks, Ontario, Canada. Pupae were kept in storage at 5 ~ until use five months later. Adults emerged when pupae were exposed to 25 ~ 12:12, day/night cycle (Fullard 1982). For the acoustic trials moths were fastened with adhesive wax (Cenco Softseal Tackiwax) to the head of a dissecting pin and suspended 10 cm over a KEF T27 speaker (Tovil-Maidstone, England). Acoustic stimulus pulses were generated by a Hewlett-Packard 3311A Function Generator, shaped to 2 ms, i ms rise/fall time by a Coulbourn $84-04 envelope shaper, amplified and delivered to the speaker. Stimulus frequency was maintained at 30.0_+0.1 kHz with a Non-linear Systems FM-7 Frequency Counter. To monitor the sounds emitted by the moth a Brfiel and Kjaer 1/4-inch condenser microphone (Type 4135) coupled to a measuring amplifier (Type 2204) (system response: 20 Hz to 70 kHz + 2 dB) was positioned next to the moth's thorax. The microphone output was monitored on a Tektronix 5111 Oscilloscope to determine the onset of the moth's sounds since the emissions are almost completely inaudible to the human ear (Fullard et al. 1979). Stimulus pulse rates from 10 to 200 pulses/s were randomly selected and the acoustic stimulus was initiated. The pulse train intensity was then raised by approximately 5 dB SPL/s until the moth responded with its acoustic emissions. The speaker voltage at threshold was recorded, the moth allowed to rest for 5 min and a new stimulus pulse repetition rate selected. Threshold speaker voltages were later converted to intensitites (dB SPL) by means of a minimal audible field technique. A continuous 30 kHz sinusoidal tone was dehvered to the same amplifier/speaker system used in the trials and a Briiel and Kjaer 1/4-inch microphone used to convert recorded threshold voltages to SPL's.