Butterfly brilliance
Dual diffraction gratings produce two colour signals
by Jonathan Sarfati
Published: 1 January 2009(GMT+10)
Photo Wikipedia
Blue morpho
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
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.
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
More biomimetics
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.
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