- University of Toronto
- Cornell University
Certain tiger moths (Arctiidae) defend themselves against bats by phonoresponding to their echolocation calls with trains of ultrasonic clicks. The dogbane tiger moth, Cycnia tenera, preferentially phonoresponds to the calls produced by attacking versus searching bats, suggesting that it either recognizes some acoustic feature of this phase of the bat's echolocation calls or that it simply reacts to their increased power as the bat closes. Here, we used a habituation/generalization paradigm to demonstrate that C. tenera responds neither to the shift in echolocation call frequencies nor to the change in pulse duration that is exhibited during the bat's attack phase unless these changes are accompanied by either an increase in duty cycle or a decrease in pulse period. To separate these features, we measured the moth's phonoresponse thresholds to pulsed stimuli with variable versus constant duty cycles and demonstrate that C. tenera is most sensitive to echolocation call periods expressed by an attacking bat. We suggest that, under natural conditions, C. tenera identifies an attacking bat by recognizing the pulse period of its echolocation calls but that this feature recognition is influenced by acoustic power and can be overridden by unnaturally intense sounds.
Cycnia tenera Hübner were reared in the field from eggs collected from wild specimens captured at the Queen's University Biological Station in Chaffeys Lock, Ontario, Canada, raised to pupae on local plants (Apocynum androsaemifolium and A. cannabinum) and stored in constant-temperature rooms at 4°C with a 12 h:12 h light:dark photoperiod for 5 months. Pupae were transferred to 16 h:8 h light:dark rooms at 25°C, and adults emerged 2–3 weeks later. Adults were allowed to mature for 24–48 h and were then tested during the nocturnal part of their diel cycle.
Individual moths were fastened by their descaled mesothoracic terga to the head of a dissecting pin with a drop of molten Cenco Softseal Tackiwax (Cenco Scientific, Chicago, IL, USA) and suspended 20 cm above a Technics EAS10TH400B (Panasonic, Matsushita Electric Industrial Co. Ltd, Kadoma City, Japan) speaker in a chamber lined with sound-attenuating foam. Moths were positioned under red light and were then left in complete darkness for 20 min before playbacks began. Continual tones produced by either a Wavetek (model 23) (Willtek Communications, Ismaning, Germany) or Hewlett-Packard (Hewlett-Packard, Palo Alto, CA, USA) signal generator (model 3311A) were shaped with a 0.5 ms rise/fall time to various durations and periods (Coulbourn S84-04; Coulbourn Instruments, Allentown, PA, USA), amplified (National Semiconductor LM1875T; National Semiconductor Corp., Santa Clara, CA, USA) and broadcast from the Technics speaker. Certain of the stimulus trains were stored on a Racal Store 4D tape recorder (Racal Acoustics Ltd, Harrow, UK) running at ∼76 cm s–1 (as internally calibrated), while others were recorded as .wav files onto a PC laptop using a 500 kHz sampling rate PCMCIA card (DAQ Card-6062E; National Instruments, Austin, TX, USA) controlled by the programme BatSound Pro v.3.30 (Pettersson Elektronik AB, Uppsala, Sweden). Stimulus trains were either played back using the Racal tape recorder or the playback feature of the BatSound Pro programme and DAQ Card. Playback intensities were recorded as mV peak-to-peak and were later converted to peak equivalent dB sound pressure level (peSPL) (re 20μ Pa rms) from equal-amplitude continual tones using a Brüel and Kjær (B&K) (Nærum, Denmark) type 4135 6.35 mm microphone and type 2610 B&K measuring amplifier. The system was regularly calibrated with a B&K type 4228 pistonphone. Stimuli were presented to the moth as trains of pulses of different durations, frequencies and periods depending upon the experiment.
The phonoresponse in C. tenera (Fig. 1A) was generated by exposing moths to acoustic stimuli as generated by the aforementioned methods. Tymbal sounds are generated as trains of clicks [modulation cycles (MC) (Blest et al., 1963)] that result from the in and out buckling of the striated tymbal surface (Fullard and Fenton, 1977) and were recorded with the B&K microphone and measuring amplifier onto the Racal tape recorder. Phonoresponse recordings were played at ½ real-time tape speed into a data acquisition board (TL-2; Axon Instruments, Molecular Devices Corporation, Sunnyvale, CA, USA) at a 20 kHz sampling rate and stored on a PC, and files were subsequently analysed using the programme AxoScope 8.1 (Axon Instruments). Certain trials were recorded using the DAQ Card and stored and analysed as .wav files using BatSound Pro.