Modularity and scaling in fast movements: power amplification in mantis shrimp

Extremely fast animal actions are accomplished with mechanisms that reduce the duration of movement. This process is known as power amplification. While many studies have examined the morphology and performance of power-amplified systems, little is known about their development and evolution. Here we examine scaling and modularity in the powerful predatory appendages of a mantis shrimp, Gonodactylaceus falcatus (Crustacea, Stomatopoda). We propose that power-amplified systems can be divided into three units: an engine (e.g., muscle), an amplifier (e.g., spring) and a tool (e.g., hammer). We tested whether these units are developmentally independent using geometric morphometric techniques which quantitatively compare shapes. Additionally, we tested whether shape and several mechanical features are correlated with size and sex. We found that the morphological regions that represent the engine, amplifier, and tool belong to independent developmental modules. In both sexes, body size ...