Blue Morpho Butterfly - Source: Wikipedia

Dual Diffraction Gratings Produce Two Colour Signals

Photonic Structures in Butterflies

Some butterflies, such as the blue morpho (Morpho menelaus) of South America and the male mountain blue don (Papilio ulysses) of northern Australia are known for their brilliant iridescent blues. But their spectacular colours are not caused by pigments but by their scales forming a diffraction grating.1 These are evenly-spaced ridges or grooves that break up white light into all its component colours, but at a given angle, destructive interference cancels out all out except for the required colour, which is intense due to constructive interference. These scales have been calledsub-micrometre photonic structures, because they can manipulate light waves. The very deep black on the borders of the butterfly wings is likewise not due to a black pigment but due to photonic structures that trap light.2,3

This research has inspired the design of very effective ‘Super Black’ coatings, and might inspire other sorts of coatings that produce striking colours without the chemical waste in production of pigments and dyes.4 This is yet another example ofbiomimetics: human technology copying nature—in reality, taking lessons from the Designer of nature.5

Dual Gratings

Recent research shows that the dorsal wings of Lamprolenis nitida have two blazed diffraction gratings interspersed on single scales, which give two main colour signals.6

This was a novel discovery, since ‘Multiple independent signals from separate photonic structures within the same sub-micrometre device are currently unknown in animals.’ The scales form a pattern of cross ribs and flutes which repeat in two different intervals, hence the different signals.

The researchers say, ‘Multiple signals increase the complexity and specificity of the optical signature, thus enhancing the information conveyed. This could be particularly important during intrasexual encounters, in which iridescent male wing colours are employed as threat displays.’ They point out that males would produce strong signals even in the poorly illuminated forests where they live, where sunlight breaks through the canopy only sporadically. And they would help the females find the right species in a species-rich environment….

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