The resonance properties of the physical gill ofCorixa punctata and their significance in sound reception
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- Prager, J. & Streng, R. J. Comp. Physiol. (1982) 148: 323. doi:10.1007/BF00679017
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The air bubble comprising the physical gill of the Corixinae is a stimulus-conducting apparatus external to the tympanal organs, which transforms water-borne sound into airborne sound. Like all air bubbles under water, it is a system capable of oscillation and resonance; the resonant frequency depends primarily on the volume of the air bubble.
A computer-controlled method was developed by which the resonance properties of the respiratory air bubble ofCorixa punctata could be measured in the freely moving animal under controlled acoustic conditions.
The resonant frequencyfrof the air bubble varies with air volume V according to the relationfr∝V−2.3.
When the air supply is replenished by taking in air at the water surface, the enlarged volume of the air bubble brings about a stepwise reduction in resonant frequency. Thereafter, the resonant frequency gradually rises as the volume slowly shrinks, until the animal again takes in air.
In the temperature range 4 °C to 16 °C resonant frequencies between 1.35 kHz and 3.35 kHz are observed. The resonant frequency of an “average” animal at constant temperature varies owing to the change in air-bubble volume, by ±500 Hz about the mean (central) frequency. The distributions of the resonant frequencies shift toward lower frequencies as the water temperature rises. The central frequency falls about linearly from 2.4 kHz at 4 °C to 1.95 kHz at 16 °C (Figs. 6–8).
The resonant peak of the air bubble is ca. 14 dB above the low-frequency level, as calculated from the logarithmic decrement of 0.6±0.1. As compared with spherical air bubbles, the oscillations of the respiratory air bubble are considerably more damped.
When caused to resonate by external alternating pressures, the air bubble of the physical gill exhibits internal pressure alternation of greatest amplitude in the region of its resonant frequency. The range over which the resonant frequency varies coincides with the range of frequencies to which the mesothoracic tympanal organ is especially sensitive; the latter bandwidth is extended by the bilateral asymmetry of the sense cell A1 (Fig. 9).
The results are discussed in relation to the sound sensitivity of the tympanal organs and the function of the physical gill in sound production.