Stopping a walking locust with sound: an analysis of variation in behavioural threshold.

Pulses of 3 kHz sound, varying in intensity from 46 dB SPL to 113 dB SPL (reference value = 20 microPa), were presented to fifth instar larvae of the locust Locusta migratoria migratorioides (R. and F.) walking on a recording treadmill. Walking locusts either responded to the sound by stopping within 2.1 s of its presentation or they continued walking apparently unaffected by it. The latency of the stopping response was correlated with stimulus intensity: the higher the stimulus intensity, the sooner the locust stopped walking. Pauses induced by sounds were longer than those that occurred 'spontaneously' and increased in duration with increasing stimulus intensity. This relationship, however, was modulated by walking speed: the faster the locust was walking, the shorter the following pause for a given stimulus intensity. The intensity of sound needed to elicit a response depended on the length of time the locust had spent in uninterrupted walking before the stimulus was presented. The stopping threshold for sound stimuli presented 3 s into the walking bout was 84 dB SPL (SE +/- 1.56), and at 20 s it was significantly less, at 76 dB SPL (SE +/- 1.56). This effect of time was significant even when other factors, such as differences between individual locusts, were allowed for in the regression analysis. This shift in the behavioural threshold could not be explained simply by on-going changes in walking behaviour, for although the speed of walking was correlated with stopping threshold its effect was not statistically significant when differences between individuals and time of stimulus presentation were allowed for in the regression analysis.(ABSTRACT TRUNCATED AT 250 WORDS)