Butterfly Scales

Catch a butterfly, and you’ll notice a dust-like residue left behind on your fingers. These are tiny scales from the butterfly’s wing. Under a microscope, those scales overlap like shingles all over the wing. Their downstream edges tilt upward, leaving narrow gaps between one scale and the next. Experiments show that, although butterflies can fly without their scales, these tiny features make a big difference in their efficiency.

When air flows over the scales, tiny vortices form in the gaps between. These laminar vortices act like roller bearings, helping the flow overhead move along with less friction and, thus, less drag. Compared to a smooth surface, the scales reduce skin friction on the wing by 26-45%. (Image credit: butterfly – E. Minuskin, scales – N. Slegers et al., experiment – S. Gautam; research credit: N. Slegers et al. and S. Gautam; via Physics Today)