Two sides of the same wing: ventral scales enhance dorsal wing color in the butterfly Speyeria mormonia.

Biological visual signals are often produced by complex interactions between light-absorbing and light-scattering structures, but for many signals, potential interactions between different light-interacting components have yet to be tested. Butterfly wings, for example, are thin enough that their two sides may not be optically isolated. We tested whether ventral wing scales of the Mormon fritillary, Speyeria mormonia, affect the appearance of dorsal orange patches, which are thought to be involved in sexual signaling. Using reflectance spectroscopy, we found that ventral scales, either silvered or non-silvered, make dorsal orange patches significantly brighter, with the silvered scales having the greater effect. Computational modeling indicates that both types of ventral scale enhance the chromatic perceptual signal of dorsal orange patches, with only the silvered scales also enhancing their achromatic perceptual signal. A lack of optical independence between the two sides of the wings of S. mormonia implies that the wing surfaces of butterflies have intertwined signaling functions and evolutionary histories.