Movie 1.

Head and abdomen flexion. Top: zoomed top (left) and side (right) views. White points with red, magenta, cyan, green, and orange edges are the origins of thorax frame, head frame, abdomen frame, middle point of thorax-head joint, and middle point of the thorax-abdomen joint, respectively. Solid and dotted arrows show +x and +x' direction of body (red), head (magenta), and abdomen (cyan) frames, 38 respectively. Head and abdomen flexion are the angles between body +x' direction and head or abdomen +x direction. Bottom left: isometric view. Bottom right: head and abdomen flexion as a function of time.