Meeting Abstract

98.11  Wednesday, Jan. 7  Force transmission versus speed amplification in a four bar linkage mechanism: counterintuitive results in the mantis shrimps strike CLAVERIE, Thomas*; PATEK, Sheila N.; Univ. of California, Berkeley; Univ. of California, Berkeley tclaverie@berkeley.edu

Four bar linkages are simple mechanical systems that can amplify or reduce rotation. Most biological four bar systems have been studied in the context of rotational amplification with relatively little analysis of force transmission. Mantis shrimp (Stomatopoda, Crustacea) use a four bar linkage system to power their fast predatory appendages. Some species capture elusive prey (spearers) using highly elongate appendages while others break shelled-prey (smashers) with short, massive appendages. We examined the variation in force transmission versus speed amplification in this linkage system across 14 stomatopod species. We measured the four bar linkage configuration, geometrically simulated the contraction and release of the linkage system, and calculated the resulting force and speed transmission. Most species exhibited relatively small force transmission (up to 0.4 mechanical advantage (MA)) and a large rotational amplification (typically 10-fold). The transmission of speed and force during a strike was not synchronized and followed this sequence: maximal speed, maximal force, minimal speed, minimal force and then maximal speed again. Surprisingly, the four bar model did not predict greater MA in smashers than in spearers, but species having a large MA had the lowest speed amplification. Also, species with longer predatory appendages (spearers) exhibited a maximum force transmission earlier in the strike cycle. Thus, the spatial and temporal dynamics of the four bar linkage system may be as important, or more important, than the average behavior predicted by link length ratios. These results highlight the surprising dynamics between simple mechanical systems and evolutionary variation.