2. Mechanical sensitivity controls evolution at multiple levels. At the core of evolutionary functional morphology is the question of whether mechanical laws act as constraints on evolution. Is structural variation limited by functional requirements? I have been testing these ideas using the mantis shrimp raptorial appendage, a power-amplified prey capture tool. My first study on this group has shown that multiple mechanisms coupled within the system show constrained evolution towards specific functions (Fig. 3d) (Anderson et al. 2014b, Evolution). This is in contrast to the general notion that multi-part mechanical systems should allow greater diversification due to multiple morphologies resulting in similar functions. My recent work has shown that the evolution of multi-part mechanical systems is likely controlled by variation in the mechanical sensitivity of the components (Anderson & Patek in review, Proc. B.). Certain components of a multi-part system will have a stronger influence on the mechanics than others. While these sensitive components will tend to show constrained evolution in order to maintain function, other, less sensitive components are free to evolve and diversify. Mechanical sensitivity represents a new paradigm for examining multi-part biomechanical systems in an evolutionary context that gives insights into how mechanics can both promote and constraint evolution at different structural levels.