- University of Toronto
Summary
The dogbane tiger moth (Cycnia tenera Hübner; Arctiidae) responds to ultrasonic, artificial bat echolocation signals by emitting stereotyped trains of high‐frequency, rapidly repeated clicks. By comparing this response in intact and headless moths, the role of protocerebral auditory inter‐neurones suggested by other studies was examined. Individual moths were tested intact and decapitated, and their response differences analysed. Response latency and threshold (dB) did not alter with the removal of the head but response duration and responsiveness to stimulus trains were significantly reduced in headless moths. These data are interpreted as suggesting the existence of a reflex arc connecting the moth's tympanic organ (ear) with its sound‐producing structure (tymbal), and as providing preliminary evidence that the role of higher‐order interneurones is primarily that of response reinforcement.
Methodology
Specimens of Ctenera were reared from eggs collected from gravid females at the Queen’s University Biology Station, Chaffey’s Lock Ontario (44’34”; 79’ 15’W). The larvae were fed on local populations of dogbane (Apocynum androsaemifolium) and pupae were stored at 5’C in environmental rooms (1 6 h dark : 8 h light) until required. Diapause was broken 6 months later and adults were used within 3 days of emergence.
Acoustic response
Unanaesthetized moths were attached to the head of a dissecting pin with adhesive wax (Cenco Softseal Tackiwax) and positioned 10 cm over an aluminized Mylar, electrostatic speaker (Carleton University Science Workshops) (Fullard, 1979). Stimuli broadcast by the speakers were generated by an Exact 126 signal generator as 10-ms 4OkHz unshaped tone bursts. These bursts were delivered to the moth under red-light conditions in one of two forms, single burst (Fig. I) or trains of bursts at 100-ms intervals lasting for 5 s (Fig. 3). Stimulus intensities were computed using continual tones of equal amplitude measured with a Bruel and Kjaer %-inch (6.3-mm) 4135 condenser microphone with preamplifier mounted on a 2204 Measuring Amplifier. All dB levels reported are re. 20pPa. Moth sound production was monitored by a Bruel and Kjaer %-inch microphone Measuring Amplifier coupled to a Tektronix 564 storage oscilloscope. Signals were simultaneously recorded onto a Uher Report 4000 tape recorder (the characteristics of this recording system severely distort stimulus/response frequency characteristics but adequately preserve temporal parameters).
Ablation experiments
Each moth was tested first intact and then after decapitation. Response to stimulation in both states was measured (Fig. 1) as: (A) threshold, as the minimum sound pressure level (dB SPL r.m.s. re. 20pPa) required to just elicit sound production from the moth; (B) latency, as the time (ms) elapsed from stimulus onset to response (to avoid the possibility that latency was dependent on stimulus intensity, latencies were measured at two SPL: threshold and threshold +20dB); (C) response duration, as the length of time (ms) following delivery of the stimulus that the moth responded. Response duration was measured for two stimulus types: single burst and burst train (see Results). Statistical comparisons of the data were performed using either a parametric t-test (normality determined by D’Agostino’s ( 197 1) test (Zar, 1974)) or by the non-parametric Mann-Whitney U-test (Sokal & Rohlf, 1969) when the data were not normally distributed.