Fig. 9. The four-bar model predictions vary depending on relative link lengths and starting angles. The linkage mechanism is operational in two regions between input rotations of 0 and 360° (region from 0 to 180° shown; the range from 180–360° is the mirror image of 0 to 180° and is never used by mantis shrimp). The horizontal lines at output rotations of 0° and 180° indicate that a change in input rotation does not yield any output rotation (i.e. the linkage mechanism is non-operational). (A) An input rotation between 40° and 120° yields an output rotation depending on relative link lengths. Green traces show the predicted behavior based on the link lengths of a relaxed raptorial appendage (i.e. Link 4 extensor muscle is not contracted). Blue traces show the predicted behavior of the relaxed appendages if Link 4 is constrained to the average shortened length observed in video images. Red traces illustrate the range of behaviors given the range of link lengths measured in loaded appendages from video images. The thick black line provides the linkage model behavior given the average link lengths measured from the loaded images (red lines; also shown in Fig. 10). (B) The predicted model behavior of four individuals (each color represents a different individual) given measured inputs and link lengths from high-speed video sequences.