Extremely fast animal actions are accomplished with mechanisms that reduce the duration of movement. This process is known as power amplification. Although 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 that 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 was positively correlated with the size of each region. Shape, however, changed allometrically with appendage size only in the amplifier (both sexes) and tool (males). These morphological changes were correlated with strike force and spring force (amplifier), but not spring stiffness (amplifier). Overall, the results indicate that each functional unit belongs to different developmental modules in a power-amplified system, potentially allowing independent evolution of the engine, amplifier, and tool.