Fig. 4. Kinematic properties of turns initiated in response to changes in illumination. (A) Scatter analysis of turn kinematics for spontaneous routine turns (N=166, green), turns elicited by dark flashes (N=506, red) and short latency acoustic startle responses (N=269, blue). Dark-flash evoked turns form a distinct cluster, with bend angles exceeding those achieved by acoustic startle responses, but with much slower angular velocity. (B) Latency distribution for turns initiated during a 500 ms light flash (N=631, grey squares) or under constant illumination (N=244, dark squares). Turns peak around 200 ms after the increment in lighting. (C) Turns initiated in response to a 1000 ms dark flash (N=236, dark squares) have a longer latency, peaking 300 ms after the reduction in lighting. Turns initiated during constant illumination (N=63, light squares) show uniform distribution. (D–G) Comparison of movement kinematics for five motor patterns. Turns elicited by light flashes (`LF turn', N=111), routine turns (`Rout. turn', N=66), long latency acoustic startles (`LLC', N=96), short latency acoustic startles (`SLC', N=382) and turns elicited by dark flashes (`DF turn', N=104); values all means ± s.d. Light-flash turns were indistinguishable from routine turns for all kinematic parameters scored. Kinematics of long latency startles differed from routine turns for all kinematic parameters scored. Turns elicited by dark flashes were distinct from short latency acoustic startle responses. Although turn magnitude is extreme for dark flashes, turns are relatively slow, but occur over a protracted duration (two-tailed t-tests, ^**P<0.001). (H) After light extinction, larvae orient toward the area where the light was extinguished. Cones represent initial orientation of larvae, shading and numbers indicate rightward turn bias (where 100% signifies that all turns are made to the right, –100% means always left). Larvae initially facing the light with their right side show a strong right turn bias, while the opposite is true for larvae in the reverse orientation. Larvae oriented parallel to the direction of the light show no directional bias. (I) Example of a dark-flash turn, demonstrating the very large bend amplitude attained and 180° reorientation typical for these motor patterns.