Multilevel analysis of elastic morphology: The mantis shrimp's spring

Spring systems, whether natural or engineered, are composed of compliant and rigid regions. Biological springs are often similar to monolithic structures that distribute compliance and rigidity across the whole system. For example, to confer different amounts of compliance in distinct regions within a single structure, biological systems typically vary regional morphology through thickening or elongation. Here, we analyze the monolithic spring in mantis shrimp (Stomatopoda) raptorial appendages to rapidly acquire or process prey. We quantified the shape of cross-sections of the merus segment of the raptorial appendage. We also examined specific regions of the merus that are hypothesized to either store elastic energy or provide structural support to permit energy storage in other regions of the system. We found that while all mantis shrimp contain thicker ventral bars in distal cross-sections, differences in thickness are more pronounced in high-impact "smasher" mantis shrimp than in the slower-striking "spearer" mantis shrimp. We also found that spearer cross-sections are more circular while those of smashers are more eccentric with elongation along the dorso-ventral axis. The results suggest that the regional thickening of ventral bars provides structural support for resisting spring compression and also reduces flexural stiffness along the system's long axis. This multilevel morphological analysis offers a foundation for understanding the evolution and mechanics of monolithic systems in biology.