Dual diffraction gratings produce two colour signals
Published: 1 January 2009(GMT+10)
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 called sub-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 of biomimetics: human technology copying nature—in reality, taking lessons from the Designer of nature.5
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.
How did these structures arise?
Even single diffraction gratings are hard to explain by a Darwinian series of small steps, each with an advantage over the previous one. A fortiori, how much harder is a dual diffraction grating to explain?
Making a pleasant change, the researchers didn’t propose a just-so evolutionary story to explain the origin of these structures; they reported on the facts, and proposed plausible functions of their current use. Indeed, even single diffraction gratings are hard to explain by a Darwinian series of small steps, each with an advantage over the previous one. A fortiori, how much harder is a dual diffraction grating to explain? This is especially so since most butterflies manage perfectly well without one, and the glasswing doesn’t even need scales at all,7 so selection pressure is not clear. Note that Darwin’s ‘theory of sexual selection’8 fails to explain the very thing Darwin concocted it for—the peacock tail!9
The researchers said that advanced human technology could benefit from copying this design:
‘The double grating of L. nitida could provide a solution to a problem with spectrometers, namely that the functional range of their grating is restricted, so that when the spectral limit is reached the grating must be mechanically swapped for another, interrupting measurements. By incorporating two gratings onto a single self-adjusting surface, this problem may be circumvented.’
Since real science works by analogy, it is fair to argue that since our diffractions require intelligent design, a fortiori, an even more advanced diffraction grating also shows the objective marks of design.
- Beautiful black and blue butterflies
- The beauty of the peacock tail and the problems with the theory of sexual selection
- Peacock tail tale failure
- Cuttlefish colour changes inspire new energy-efficient TV screen design
- Firefly lanterns inspire LED lenses