Forewing asymmetries during auditory avoidance in flying locusts

Flying locusts orient to sounds in their environment. Sounds similar to those produced by echolocating bats cause a flying locust to change its flight path. We used high-speed cinematography and videography to study changes in body posture and wing kinematics of tethered locusts in response to stimulation with bat-like sounds. Locusts showed both negative and positive phonotaxis to this stimulus. Within a few wingbeats of stimulus onset (between 126 and 226ms), locusts deflected their abdomens to one side, and the angle of the left and right forewings with respect to the dorsal­ventral body axis became asymmetrical during the downstroke. This forewing asymmetry, in which the forewing on the inside of the turn became more depressed, ranged from 20 to 45° (37±9.7°, mean ± s.d.) and was correlated with the direction and magnitude of abdomen deflection, a measure of steering in tethered, flying locusts. Hindwing stroke angle asymmetries were minimal or non-existent after stimulation. Coincident with changes in forewing asymmetry and abdomen deflection was a decrease in stroke amplitude (19±6.5°) of the forewing on the inside of the attempted turn. Motor patterns from forewing first basalar (M97) muscles showed an asymmetry in the timing of left and right depressor activation that ranged from 10.4 to 1.6ms (4.23±2.85ms). The number of spikes per depressor burst increased to a maximum of three spikes in the muscle on the inside of the attempted turn, and depressor frequency (wingbeat frequency) increased by approximately 2Hz (2.17±0.26Hz). We suggest that the asymmetry in forewing first basalar activity is causally related to the asymmetry in the timing of the initiation of the downstroke, resulting in an asymmetry in the ranges of the stroke angles of the forewings, which would impart a roll torque to the locust. This would augment the steering torques generated by concurrent changes in the angle of attack of the fore- and hindwings and changes in abdomen position to effect rapid avoidance manoeuvres.