Act88F⁶ heterozygotes bearing one copy of Act88F^A295S do not present obvious defects in average body weight nor in the development, morphology or thin filament length in indirect flight muscle, but present the following defects in flight performance, as compared to controls: a significant decrease in flight index assessed by standard flight assays; significant decreases in overall and angular velocity and in centripetal, but not overall, acceleration in digital inline holography experiments. Act88F⁶ homozygotes bearing two copies of Act88F^A295S present a fully penetrant flightless phenotype, which is associated with hyper-contracted and torn dorsal longitudinal indirect flight muscles, but not with significant changes in thin filament length, as compared to controls.