1. The pattern of nervous discharge from tympanal organs and abdominal hair sensillae resulting from stimulation by natural and imitation stridulation has been studied in four species of Acrididae, namely Stenobothrus tineatus, Omocestus viridulus, Chorthippus parallelus and C. brunneus.

2. Experiments with tympanal organs stimulated repeatedly with identical sound pulses show that the resultant volleys of spikes, although synchronous with the pulse frequency of the stimulus, were variable in themselves, and that this variability was random, bearing no simple relationship to any quality of the stimulus. This variability in the structure of the impulse bursts was manifested with both natural and imitation stridulation.

3. The response of abdominal hair sensillae showed them to be capable of signalling the duration of supra-liminal stimuli, and responses signalling pulse repetition frequencies were limited to cases where the pulse rates did not exceed 5 per second. In the present species, this meant that only the normal songs of C. brunneus and C. parallelus would produce impulse bursts in tergal nerves synchronous with the pulse frequency of the stimulus.

4. On the basis of this evidence it is adduced that the characteristic of stridulation which enables inter-specific recognition to be effected is the pulse repetition rate of the songs.

5. Comparative data from the four species show that the auditory receptors throughout are very similar, and thus the pattern of nervous discharge will be, in its essential element of the rate of impulse burst, the same for all species when exposed to the same stimulus. The discrimination process must thus be a central one, and it is suggested that this problem might be examined in relation to the ‘Reafferenzprincip’ of von Hoist & Mittelstaedt (1950).

Now at the Anti-Locust Research Centre, London.