The mechanism driving symmetrization is unusual. We found that symmetrization is not driven by cellular processes that usually take place in regeneration, such as cell proliferation or cell death (Huynh et al. (2011); Teng and Toyama, 2011; Mao et al., 2013). Nor is it accomplished through the regeneration of the original muscle architecture, which we could block using cytochalasin D without preventing symmetrization. Instead, symmetry recovery is driven by mechanical forces generated by the propulsion machin- ery. Inhibiting the excitation–contraction coupling or the skeletal myosin II blocked symmetrization.