Each scale on a morpho's upper wing carries rows of longitudinal ridges bearing stacked lamellae, spaced at intervals comparable to the wavelength of visible light. Light reflecting from successive layers interferes constructively at blue wavelengths and destructively elsewhere. The result is a structural colour: intense, metallic, and dependent on the geometry of the scale rather than on any chemical that absorbs light.
The underlying wing membrane and the pigment present are brown. Damage or crush the scales and the blue disappears, because the structure that produced it is gone. The effect also explains the flicker seen as a morpho flies, since the reflected hue and brightness change with the angle between the observer, the wing and the light source.
The function is debated. The flashing has been interpreted as startling to predators, as making an erratic flight path harder to track, and as a long-distance signal between males, which are more strongly coloured than females in many species. Morpho nanostructures have been studied as templates for reflective coatings and vapour sensors, though commercial applications remain limited.