1. The physiology of sound reception by tympanal organs, long hair sensillae on the anal cerci, and hair sensillae on the thorax and abdomen has been studied by electrophysiological methods in four species of Acrididae, namely Stenobothrus lineatus, Omocestus viridulis, Chorthippus parallelus and C. brunneus.

2. The discharge in the tympanal nerve of all four species on stimulation by pure tones was asynchronous and showed no equilibration or fatigue. Comparative data are given for threshold sensitivity to pure tones for the four species over the range 0-20 kc./s., and shows the sensitivity to be very similar throughout the group.

3. The tympanal organs of all four species responded to pulsed sound with synchronous volleys of spikes up to pulse repetition frequencies between 90 and 100 pulses per second; at higher rates synchronism broke down.

4. Long-hair sensillae on the anal cerci responded to gross air movements with bursts of spikes and to pure tones up to 300 c./s. with a synchronous discharge. The cereal nerves synapse with fibres in the ventral nerve cord, and the post-ganglionic discharges on continuous maximal stimulation of the cereal sensillae show accommodation. It is concluded that these cereal sensillae are similar in response and innervation to those on the cerci of the cricket, cockroach and migratory locust, but that owing to their small number and restricted frequency range they are probably not concerned with reception of natural stridulation.

5. Discharges were evoked in segmental nerves of the third thoracic ganglion in the four species in response to vibration of the substratum and stimulation by air-borne sounds. The sensillae mediating the vibration response are hair sensillae situated on the abdominal sternites.

6. The receptors mediating response to air-borne stimuli may be hair sensillae on the abdominal pleura and sternites, or segmental chordotonal sensillae. The present work indicates that the former are probably the receptors concerned, but further work to elucidate the roles played by the two types of organ is required. Data are given on the response and threshold of the hair organs.

7. The responses of all the sound receptors investigated were independent of age and sex and also, in females, of the state of the ovaries.

Now at the Anti-Locust Research Centre, London.