Fig. 1. Location of digitised points and experimental setup. (A) Tactile stimulation of the wing at one of two stimulus sites (open triangles, means ± 1 S.D.) elicited scratching movements that began with the tarsus of the ipsilateral hind leg standing on a rod that defined the start position (filled square, means ± 1 S.D., N=462 trials in 3 animals). In `loaded trials', a mass of 142 mg was added to the ipsilateral leg at one of three locations (a–c, open circles). In `control trials' the leg was left unloaded. The coordinate frame of reference used in all of the analyses was centred on the metathoracic coxa, with the horizontal x-axis passing through the mesothoracic coxa. (B) To track movements of the body and limb, eight points (filled circles) were digitised manually in all video frames. Stimulus location and start position of the tarsus were digitised in the first frame. (C) In the simplest representation of a movement, we reconstructed the trajectories of the points representing the proximal and distal ends of the femur, the distal end of the tarsus, and the tip of the wing. For clarity the tarsus is represented as a grey line segment. This example shows a movement made in response to a stimulus at the anterior target (open triangle) in the unloaded condition. The distal end of the femur described an arc dorsal to the body (femur–tibia joint positions), while the distal end of the tibia and tarsus moved towards the target and then in three repeated loops (tarsal positions). In this case the wing did not move (end of wing), and the coxa rotated only a little (coxa–trochanter positions).