- University of Toronto
- Carleton University
By examining the mechanical properties of the tympanum of the noctuid moth, Noctua pronuba, Windmill et al. (2006) suggested that this insect increases (up-tunes) the frequencies of its best hearing when exposed to high intensity sounds (HIS) resembling the echolocation calls of attacking bats. We tested whether this biophysical phenomenon was encoded in the neural responses of this moth’s most sensitive auditory receptor (A1 cell) before and after exposure to HIS. We measured: (1) the number of A1 action potentials (spikes) per stimulus pulse; (2) the proportion of A1 spike periods below that determined to elicit evasive flight maneuvers and, (3) the change in A1 cell firing (spike number, interspike interval, stimulus/spike latency) over a duration of time similar to that in which up-tuning lasts. We observed no significant spiking response changes in the predicted direction to any of the frequencies tested following exposure to HIS and we observed only two of the 24 predicted time-dependent changes to A1 firing. These results indicate that tympanal up-tuning does not result in a change to this moth’s auditory frequency sensitivity and we suggest either sensillar resonances or increases in thoracic muscle tension following exposure to HIS as alternative explanations.
These studies were independently performed at two separate locations in eastern Ontario, Canada: the Queen’s University Biological Station (QUBS) and the Department of Biology, Carleton University, Ottawa (CARL). Noctua pronuba were collected from ultraviolet lights during the months of June to August, 2007 (QUBS) and 2008 (CARL). Moth auditory nerves (IIIN1B; Nüesch 1957) were exposed using a modified ventral (QUBS) or dorsal (CARL) dissection (Roeder and Treat 1957; Fullard et al. 2003). Unanesthetized moths were restrained by pinning the wings to a block of modeling clay after which the scales were removed from the appropriate portion of the thorax and the dissection was completed. Minimal dissection was performed with only a small amount of saline in order to avoid possible surgical artefacts to the neural responses and moths were not decapitated for this study following Windmill et al.’s (2006) observation that such an operation eliminates tympanal up-tuning. Leaving the head on in these moths did not result in extraneous neural activity that interfered with analyzing the easily discernible A1 spikes (Fig. 1a).