Iridescence, polarisation and directionality of Morpho butterfly displays.

Morpho butterflies are widely known for their striking blue colourations, which are produced by multilayered ridges of the wing scales. Several Morpho species have a flashing iridescence, but others have a more diffuse display owing to curved wing scales. The structural colouration is even absent in species with whitish or brown colours, which suggests that there is a considerable intrageneric variation in wing scale anatomy and pigmentation. Here, we investigate four species with superficially similar blue wings: Morpho aega, M. zephyritis, M. godartii and M. helenor. Characterising the spectral and spatial reflection properties, we reveal marked interspecific differences in spectral and angle-dependent wing reflectance. We show that the superposition of scales broadens the angular range of the display, whereas the orientation of the scale ridges determines the directionality and polarisation of the reflected light. We discuss how the polarised iridescent displays of Morpho butterflies are presumably tuned to their natural environmental conditions and to the visual perception of their observers.