The Tuned Singing Burrow of Mole Crickets

Abstract

During burrow digging by the mole cricket Scapteriscus acletus Rehn and Hubbard, the burrow is enlarged and shaped in a series of digging cycles, each lasting 1–2min, and song chirps are produced after every cycle. The song becomes up to 18 dB louder and acoustically purer during burrow building. The shape of the song pulse envelope changes so that its average power density becomes greater and its build-up and decay become more gradual. The forewings show a single resonance of the harp (Cu 1 cell) regions at 2.5-3.0 kHz: the normal song carrier frequency is 2.5-2.7kHz. Removal of the outer parts of the forewings does not affect the radiated sound power. The singing burrow has an exponential horn opening at the soil surface and an internal bulb. The calculated cut-off frequency of the horn is 1–34 kHz and that of the first resonance is 2.5-2.7 kHz. The volume of the bulb is close to that required to tune the throat: obliteration of the burrow bulb causes a substantial decrease in the radiated sound pressure levels before singing ceases. The horn mouth acts as a finite sound source, with large acoustic reactance. The acoustics of the burrow were measured with a small doublet source inserted at the position at which the cricket sings. At this position, the burrow resonates at 2.75-3 kHz, with a quality factor (Q), measured both by the - 3 dB bandwidth and by the build-up and decay of tone bursts, of 2.5-2.7. The sound pressure produced by the doublet source is up to 24 dB louder when it is in the burrow than when it is in free air. Further in or out from the insect's singing position, the effective gain of the burrow is reduced and other resonances and anti-resonances are observed. Obliteration of the burrow bulb reduces the effective gain of the burrow by 6–10 dB and the burrow ceases to resonate at 2.75 kHz. It is proposed that the tuned burrow acts as a resistive load on the vibrating harps, so allowing good coupling between the small wings and the surrounding medium while allowing a pure carrier frequency to be radiated.