Scale coloration change following water absorption in the beetle Hoplia coerulea (Coleoptera).

The blue scales on the cuticle of the male beetle Hoplia coerulea can absorb water, with the consequence that these scales, which have been shown to be responsible for the beetle's bright blue coloration, reversibly turn to emerald green with increasing water contents. Optical measurements are shown, by analytic photonic-crystal models, to be compatible with the full filling of the scales structures with water. The natural mechanism shows the way to produce a very efficient hygrochromic material: a medium which significantly changes color when its water contents are modified.