The auditory behaviour of tethered locusts flying in a wind tunnel was investigated under controlled acoustic conditions.
Reflection, attenuation and diffraction of ultrasound evoked by the locust's physical presence caused pronounced distortions of the acoustic field. Interaural pressure variations were observed that account for directional hearing at high frequencies.
Sound field measurements indicated only a minor influence of flight posture or wing position on the interaural pressure gradient.
The locusts steered away from pulsed ultrasounds that simulated bat echolocation signals. The phonotactic response was measured as ruddering by the abdomen and hind legs, resulting in a yaw torque directed away from the sound source. Wingbeat frequency increased by 15% in response to ultrasonic stimulation. This behaviour is considered to be analogous to the bat avoidance behaviour of flying crickets.
The avoidance response was observed for carrier frequencies higher than 10 kHz and for sound pressure levels (on average) higher than 45 dB SPL. Lowfrequency stimuli (<10kHz) failed to elicit any phonotactic steering at any intensity used (up to 100dB SPL). Because of its relatively low threshold of reaction, this steering behaviour is thought to be part of an early-warning system adapted to the acoustic detection of echolocating predators.
The avoidance response was suppressed when a 30 kHz (normally effective) tone was combined with a 5 kHz tone (which is ineffective alone). Two-tone suppression only occurred when the low-frequency component was 10–15 dB SPL higher than the high-frequency tone. The biological significance of two-tone suppression is discussed.
The intensity-response characteristics, the frequency sensitivity and the twotone suppression of the avoidance behaviour are discussed with respect to the auditory physiology of Locusta migratoria. The involvement of some identified auditory ascending interneurones in the avoidance behaviour is considered.