THE MECHANISM OF TUNING OF THE MOLE CRICKET SINGING BURROW

1. Experimental and theoretical studies on the acoustics of the singing burrow of the mole cricket Gryllotalpa australis are reported.

2. The burrow typically consists of a bulb about 26 mm long and 20 mm in diameter, connected through a constriction of diameter about 10 mm to a flaring horn with length about 40 mm and equivalent mouth diameter about 34 mm. The mouth geometry of the burrow differs from one species to another, and the aperture may be either single, double or even multiple. The end of the bulb opposite the horn connects to a narrow exit tunnei of diameter about 8 mm and length up to 1 m. The singing cricket positions itself close to the constriction between the bulb and the horn and produces a song with a frequency around 2.5 kHz.

3. Measurements of sound pressure within the burrow when it is excited by an external sound source at the song frequency show a pressure minimum at the constriction and an amplitude and phase distribution that is consistent with resonance of the burrow at its second modal frequency. The burrow is approximately three-quarters of a wavelength long at this frequency. The same result is obtained when the burrow is excited by a dipole source located near the constriction.

4. Non-parametric model calculations confirm this conclusion and also give broad agreement with the measured response curves over a frequency range from about 1.5 to 5 kHz. The calculated curves indicate an additional resonance at about 1.2 kHz associated with the first mode of the burrow—the Helmholtz or Klipsch resonance—which is apparently not utilised by the insect. This detail is consistent with earlier measurements, and is also supported by measured responses reported here that show an increase in sound pressure with decreasing frequency below 2 kHz as predicted by the model.

5. The measured performance of the burrow is broadly consistent with the model. According to the model, the burrow geometry is close to optimal for maximal sound power radiation.