Fig. 2. A four-bar linkage model and the associated points in the proposed mantis shrimp four-bar linkage model. Red circles indicate pivot points; numbers indicate links. Pivots A and D are fixed in space and form Link 1. Link 2 is the input link formed by pivots A and B. Link 3 is the coupler link formed by pivots B and C. Link 4 is the follower link formed by pivots C and D. The input angle (_in) and output angle (_out) can be calculated using the law of cosines and the length of the diagonal (green broken line connecting pivots B and D). This particular model configuration is not operational when B, C and D are collinear, thereby limiting the input range of _in (see Fig. 9 in the Results). (A) A tracing of a high-speed video image of a raptorial appendage that has completed the load phase and is spring-loaded and prepared to strike. The saddle (s) is compressed and the meral-V (v) is rotated proximally. (B) The corresponding linkage model to A. (C) A raptorial appendage in the release phase. The saddle is hyper-extended into a flattened shape. The meral-V is fully rotated and open. (D) The corresponding linkage configuration to C. m, merus; c, carpus; p, propodus; d, dactyl. Beige regions in A and C represent arthrodial membrane; gray regions indicate exoskeleton; yellow area represents the saddle.