Many types of butterflies use light-interacting structures on their wing scales to produce color. The cuticle on the scales of these butterflies’ wings is composed of nano- and microscale, transparent, chitin-and-air layered structures. Rather than absorb and reflect certain light wavelengths as pigments and dyes do, these multi scale structures cause light that hits the surface of the wing to diffract and interfere. Cross ribs that protrude from the sides of ridges on the wing scale diffract incoming light waves, causing the waves to spread as they travel through spaces between the structures. The diffracted light waves then interfere with each other so that certain color wavelengths cancel out (destructive interference) while others are intensified and reflected (constructive interference). The varying heights of the wing scale ridges appear to affect the interference such that the reflected colors are uniform when viewed from a wide range of angles. The specific color that’s reflected depends on the shape of the structures and the distance between them. This way of manipulating light results in brilliant iridescent colors, which butterflies rely upon for camouflage, thermoregulation, and signaling.
Morpho butterfly wings have tiny scales covered with microscopic ridges, cross ribs, and other structures. These play with light waves to create brilliant blues and speckles. The structure, instead of a chemical, creates the color. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
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“Brilliant iridescent colouring in male butterflies enables long-range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra-scale microstructures to overall butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles.
“Using a focused-laser technique, we investigated the absolute reflectivity and transmissivity associated with the single-scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide-angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100 degree in one plane and 15 degree in the other.
“We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected.
“Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single-scale microstructure to be n=(1.56 plus or minus 0.01) + (0.06 plus or minus 0.01)i. This figure is required for theoretical modelling of such microstructure systems.” (Vukusic et al. 1999:1403)
(i) Lamellar structure in a ridge offers constructive interference, which results in the strong reflection within a selective wavelength range. (ii) The irregularity in the ridge height eliminates the interference among the ridges, which results in the diffuse and broad reflection of a uniform colour. Thus, the combined action of interference and diffraction due to the separate lamellar structure is essential for the structural colour….(iv) The irregularity in ridge height also results in the accidental interference of scattered light in space, manifesting as the glittering speckles…” (Kinoshita et al. 2002:1420-1421)