Acoustics of a small Australian burrowing cricket: The control of low-frequency pure-tone songs

For most insects, size determines the call frequency. This paper describes the acoustics of a small brachypterous cricket (Rufocephalus sp.; body leng th 9.6mm) producing a call with a carrier frequency of approximately 3.2kHz from a subterranean burrow. Crickets such as Gryllus campestris are approx imately twice this length and produce a call frequency close to 5kHz. The b urrow of Rufocephalus opens via a small hole with a diameter of 3.2mm. The neck of the hole at approximately 1.4mm depth opens to a vertical two-part burrow with an upper vase-shaped chamber 16.1mm in height with a diameter o f 9.4mm. This top chamber connects via a 6.4mm high (diameter 5.2mm) neck t o a more irregular chamber approximately 18mm high with a width of approxim ately 11mm. The walls of the top chamber neck and of the tipper part of the lower chamber are smooth and appear to be scaled with saliva. The song has a mean centre frequency of 3.2kHz and is made up of variable-l ength trills of pulses of mean duration 15.8ms. Many song pulses had smooth envelopes and their frequency did not vary by more than +/- 40Hz from the centre frequency, with a relative bandwidth Q(-3dB) of over 50. Other pulse s showed considerable amplitude and frequency modulation within the pulse. When driven by external sound, burrows resonated at a mean frequency of 3.5 kHz with a mean quality factor Q of 7.4. Natural-size model burrows resonat ed at similar frequencies with similar Q values. One cricket, which had pre viously called from its own burrow at 2.95kHz, sang at 3.27kHz from a burro w that resonated at the same frequency. Life-size model burrows driven by external sound resonated at similar frequ encies to the actual burrows; models three times life size resonated at one -third of this frequency. In all models, the sound pressure was more-or-les s constant throughout the top chamber but fell rapidly in the neck of the b urrow; the phase of the sound was effectively constant in the top chamber a nd neck and fell through approximately 180 degrees in passing from the neck into the lower chamber. A numerical model of the sound flow from region to region gave essentially similar results. A resonant electrical model fed from a high-impedance source with discrete tone bursts at different frequencies showed similar amplitude and frequency modulation to the various types of song pulses that were observed. It is s uggested that the high purity of the songs results from close entrainment o f the sound-producing mechanism of the insect's wings to the sharply resona nt burrow.