The control of carrier frequency in cricket calls: a refutation of the subalar-tegminal resonance/auditory feedback model.

The subalar-tegminal resonance/auditory feedback hypothesis attempts to explain how crickets control the carrier frequency (f(C)), the loudness and the spectral purity of their calls. This model contrasts with the 'clockwork cricket' or escapement model by proposing that f(C) is not controlled by the resonance of the cricket's radiators (the harps) but is instead controlled neurally. It suggests that crickets are capable of driving their harps to vibrate at any frequency and that they use a tunable Helmholtz-like resonator consisting of the tegmina and the air within the subalar space to amplify and filter the f(C). This model predicts that f(C) is variable, that call loudness is related to tegminal position (and subalar volume) and that low-density gases should cause f(C) to increase. In Anurogryllus arboreus, f(C) is not constant and varied by as much as 0.8 % between pulses. Within each sound pulse, the average f(C) typically decreased from the first to the last third of a sound pulse by 9 %. When crickets called in a mixture of heliox and air, f(C) increased 1.07- to 1.14-fold above the value in air. However, if the subalar space were part of a Helmholtz-like resonator, then its resonant frequency should have increased by 40-50 %. Moreover, similar increases occurred in species that lack a subalar space (oecanthines). Experimental reduction of the subalar volume of singing crickets resulted neither in a change in f(C) nor in a change in loudness. Nor did crickets attempt to restore the subalar volume to its original value. These results disprove the presence of a subalar-tegminal resonator. The free resonance of freshly excised Gryllus rubens tegmina shifted by 1.09-fold when moved between air and a mixture of helium and air. Auditory feedback cannot be the cause of this shift, which is similar to the f(C) shifts in intact individuals of other species. Calculations show that the harp is 3.9-1.8 times more massive than the air that moves en masse with the vibrating harps. Replacing air with heliox-air lowers the mass of the vibrating system sufficiently to account for the f(C) shifts. These results re-affirm the 'clockwork cricket' (escapement) hypothesis. However, as realized by others, the harps should be viewed as narrow-band variable-frequency oscillators whose tuning may be controlled by factors that vary the effective mass.