ABSTRACT
The calls of male Gryllus bimaculatus were digitally recorded under four different conditions: in air; in 80 % He/20 % O2; with the tympana occluded with wax in air and finally in the helium/oxygen mixture. The principal frequency component, usually referred to as the carrier frequency, was analysed in a large sample of chirps recorded in the four conditions. In all four recording conditions, the principal frequency component was found to vary from chirp to chirp. The mean of the distribution of the principal frequency component was different in the four recording conditions. Insects with occluded tympana produced in air a greater dispersion of the principal frequency component than insects with normal functioning ears.
The spectrum of an individual chirp generally contained two frequency components, the principal component, which was related to the plectrumfile strike rate, and a second component, which was related to the free vibration of the wings. The subalar air space volume is shown to act as an acoustic resonator and is important in the filtering and amplification of the sound signal. These observations were confirmed by a model stridulatory system. The model system shows that the resonant frequency of the subalar space is dependent upon the square root of the effective volume of the space.
The results suggest that song generation in crickets is a dynamic process involving an auditory feedback control loop. The singing insects appear to be able to control the plectrum–file strike rate as well as the resonant frequency of the subalar space by changing the relative position of the wings and the abdomen, hence varying the volume.
