The start of phonotactic walking in the fly Ormia ochracea: a kinematic study.

Ormia ochracea (Diptera, Tachinidae) are acoustic parasitoids of crickets that have one of the most directionally sensitive auditory systems known. We studied dynamic characteristics of walking phonotaxis in these flies in response to variations in sound source azimuth, and compared phonotaxis of flies in freely walking conditions to tethered flies walking on a treadmill. Motor patterns at the initiation of phonotaxis are not stereotyped even for similar stimulus conditions. Flies respond to directional sound sources by walking in a tight curve that combines rotation and forward translation until they are oriented towards the source direction, then continue on a straight path. Translational velocity accelerates throughout the duration of the stimulus then decelerates following stimulus offset. In contrast, rotational velocity accelerates and then decelerates within the duration of the stimulus such that flies have completed the rotational component of the response and reached their final heading before the end of the stimulus. Rotational velocity is the only response parameter that varies systematically with sound source direction (azimuth). Differences in the amplitude of rotational velocity as a function of source azimuth determine the directional orientation of phonotactic responses. The relationship between rotational velocity and source azimuth is similar to a neural measure of auditory directionality (interaural latency). There were some differences between freely walking and tethered conditions, although both showed qualitatively similar responses. Flies accelerated more slowly and attained lower maximum velocities on the treadmill, consistent with the greater inertia of the treadmill sphere relative to the flies. Also, flies tended to continue walking longer on the treadmill following cessation of the stimulus.