Extreme refractive index wing scale beads cause the bright colors of pierid butterflies*

Common pieird butterflies are brightly colored, ranging from white to red, caused by various pterin pigments concentrated in scattering ellipsoidal beads arranged in the wing scales. Given the sparsity of the beads in the wing scales, the high brightness suggests a scattering strength of the beads that significantly surpasses that of typical cuticular chitin beads. We have analyzed the optical signature of the pierids highly saturated pigmentary colors by using Jamin-Lebedeff interference microscopy combined with Kramers-Kronig theory and light scattering modeling to show that extreme pterin pigment concentrations cause a very high refractive index of the beads with values above 2 across the visible wavelength range. The special arrangment of ellipsoids and their chemical composition results in a highly light scattering medium.