on butterfly thin films as solar collectors
The [unproven] notion that the iridescent scales of some butterfly
and moth species behave as solar collectors comes from a group of engineers
at Tufts University in Massachusetts who have repeated this claim throughout
the popular media (See below). Their conclusion is inspired by their own
work with semiconductor wafers which show anomalous heating during manufacture,
apparently due to the thin films covering them. The wafers have an altered
emissivity which can decrease the rate at which they emit their excess
heat to their surroundings by a few percent, but which can produce a significant
temperature rise in the wafer.
The researchers then looked for examples in nature where such heating
might be taking place and 'discovered' iridescent butterflies.
The iridescence of butterfly scales has long been understood as being
the result of optical interference -- in this case as a result of alternating
layers of air and chitin (the material that makes up insect exoskeletons).
While such interference can lead to 'destructive interference' of incident
light, that 'destruction' only means that the light is redirected -- transmitted
light is reflected or reflected light is transmitted. Optical interference
itself cannot cause absorption. In fact, if butterfly scales had
no iridescent coverings on them at all, one might expect there to be less,
not more light energy available to the rest of the wing for absorption.
To observe this for yourself, simply place a drop of oil on the wing
of an iridescent butterfly. (Vendors can be easily found on the Internet.)
The oil has a refractive index similar to the chitin, and replaces the
air between the chitin layers. The wing covered by the chitin turns a dull,
dark color, most likely brown or black. Some 'cover scales' are, in fact,
transparent when their irridescence is removed. Clearly, more light would
be absorbed by the underlying dark tissue if the butterfly were not irridescent.
Irridescence does seem to play a major role in evading predators, as
the butterfly can flash the bright top sides of its wings, and then fold
its wings back up displaying the dull brown undersides of its wings. The
confused predator then searches in vain for the bright butterfly of which
it caught a fleeting glance.
Literature supportive of thin films as solar collectors:
"Our study indicates that the multilayer structure of the scales causes
optical interference, which optimizes solar radiation absorption
on the wing and body."
"Most of the energy in the solar spectrum is centered around 0.5 [micron]
and 98% of the radiation lies between 0.30 and 4.0 [micron] (Malitson and
Heath 1980). This tighter spectrum offers even greater opportunity for
optical interference, provided films have appropriate thicknesses. We find
films of appropriate thickness in the wing scales of butterflies and moths."
"...[T]he structure between the grid of the upper side and the membrane
of the lower side of the scale consists of chitin laminae separated by
air gaps...This results in a thin film arrangement of alternating chitin
and air layers that often produces iridescent colors and sometimes,
as indicated by our findings, acts as a radiation capturing structure."
"[T]he butterfly Eurema lisa Boisduval & LeConte uses thin
films to reflect UV light for mating purposes (Ghiradella et al. 1972),
while capturing longer wave radiation, presumably for thermoregulation."
"Reflecting greens and blues is known to aid with camouflage and display
(Ghiradella 1991). It may also help prevent these butterfly from overheating."
"As thin film structures appear in many insects and other organisms,
it is possible that these films serve a thermoregulatory function
in a variety of situations."
Ioannis N. Miaoulis and Bradley D. Heilman, 'Butterfly Thin Films
Serve as Solar Collectors', Ann. Entomol. Soc. Am. 91(1):122-127 (1998).
The Internet: (Warning: some of these items may have since been corrected
"Measurements by engineers at Tufts University in Medford, Massachusetts,
suggest that the scales can reflect or absorb sunlight to regulate the
cold-blooded insects' body temperatures."
"[W]aves reflected off the top and bottom of the layer will be half
a wavelength out of phase (the peak of one will meet the trough of the
other), cancel one another out, and dissipate their energy within the
Wade Roush, 'Butterfly biomechanics -- Wing scales may help beat
the heat', Science 269: 1816, Sept. 29, 1995.
"Miaoulis, graduate student Bradley D. Heilman, and others...discovered
that the thickness and spacing of microscopic films in the scales of butterfly
wings were just right for absorbing heat from the sun. Now, they want to
know if the roughness of those wing scales helps even out hot and cold
spots in the wings. Such an insight could help in chip fabrication. That's
where the Tufts group hopes lepidopterists can pitch in. Says Miaoulis:
'We're operating in the crevices between fields.'"
Peter Coy (editor), 'The lab where Madame Butterfly meets Mr. Chips',
Business Week, Jan. 30, 1995, p. 75.
"By studying the semiconductors used in computers, researchers have learned
how butterflies stay warm... Butterflies collect heat by spreading their
wings, which have microscopic scales made up of thin, layered films. When
Miaoulis examined these films, he found that their thickness and spacing
was indeed optimized for heat absorption. The layered films of butterfly
wings produce shimmering colors due to the effects of optical interference.
Previous studies on thin films hadn't considered the effects of such interference,
according to Tufts researchers."
Dawn Stover, 'Butterfly chips', Popular Science, February 1995,
"If you touch a butterfly wing, you feel a fine, dry dust. It turns out
that the dust is a thin-film heat-transfer mechanism much more sophisticated
than any now on the market."
"Lepidopterists admire butterflies for many reasons. It took an engineer
to link them to silicon chips."
"Engineers aren't supposed to be messing with butterflies, sea anemones,
or prairie dogs. Miaoulis, however, thinks it is the key to the future
of engineering -- just as he thinks science itself, far from ending, will
blossom in the interstices between two disciplines, such as engineering
and biology. 'I tell all my students that there is treasure hidden in the
crevices between two disciplines, such as engineering and biology,' he
says. 'There are zillions of questions for us in there.'"
John Yemma, 'The merits of meddling', Boston Globe Magazine, May
12, 1996, p. 8.
"Imagine, [Miaoulis] mused, a creature that had evolved a skin of some
sort whose thickness was tuned to reflect the warmth of the sun -- while
perhaps a close cousin inhabiting cooler climes might have an almost identical
skin mere millionths of an inch thicker or thinner, tuned instead to absorb
that warmth. It was such a clever solution to the problem of heating control,
declared Miaoulis suddenly, that nature must already have hit upon
"An unlayered chunk of chitin absorbs about 4 percent of the heat hitting
it. The chitin-and-air scales that make up a butterfly wing can absorb
as much as 96 percent."
"'Thin-film absorption is the main way butterflies warm themselves
by the sun,' says Miaoulis. 'Their wings are little solar collectors.'"
David H. Freedman, 'The Butterfly Solution', Discover, April 1997,
"Researchers at Tufts University in Medford, Mass., are studying the optical
properties of butterfly wings to determine how they collect and distribute
"The researchers measured how much light individual wing scales reflect
over a range of wavelengths. Although the reflectivity of butterfly wings
has been well studied, says Peter Y. Wong, 'what we're looking at is what's
absorbed.' As expected, the butterfly scales reflect light most strongly
in the range of wavelengths corresponding to their observed colors. The
scales reflect very little infrared light and must therefore absorb most
of it as heat."
C. Wu, 'Butterfly sparkle characterized for chips', Science News,
152:375 (Dec. 13, 1997).
"[R]esearchers at Tufts University in Maryland [sic] are studying the structure
of butterfly wings to find out how they dissipate heat and scatter light.
They are hoping to copy the tricks butterflies have evolved for thermoregulation
and use them to keep chips within their working temperatures.
"Wong and colleagues found that much of the [solar] heat is absorbed
and conducted away by the ridges [on the wing]. By mimicking this structure,
the researchers hope to be able to siphon off heat in a chip or dissipate
it over a wide area."
Mark Ward, 'Waiting in the wings: Butterflies have some cool tips
for chip designers', New Scientist, Jan. 31, 1998, p. 9.
"Could butterflies take credit for the next great advance in computers?
Perhaps, if researchers at Tufts University figure out how the structure
of iridescent butterfly wings regulates the insect's body temperature."
Jennifer Mateyaschuk, 'Researchers Aflutter', Information Week,
Feb. 16, 1998.
"It was suggested recently that the physical properties of the scales of
moths and butterflies are suited to reflect or absorb sunlight to regulate
body temperature. Measurements show that small changes in the thickness
of the stacks of scales can have large effects on the amount of absorbed
solar energy...When the thickness of the chitin layers on the individual
scales is close to one quarter of a wavelength of light, waves reflected
from the top and bottom would be half a wavelength out of phase and therefore,
cancel out dissipating their energy into the chitin layer."
[What would happen when the phase of those two reflecting waves were
in synch is that the reflected wave would be nil, but the incident
solar radiation would be transmitted instead. Optical interference
is not a mechanism that itself can produce absorption.]
"Both Morpho and Papilio exhibit daily solar basking. Solar basking relies
on wing scales, which are thin-film structures, to produce heat by optical
Literature skeptical of thin films as solar collectors:
"The authors [Miaoulis and Heilman, op. cit.] measure the reflectance of light at normal incidence and reflection, and compare this to the predictions of a model which assumes that the reflection is due to alternating layers of air and chitin. Then they calculate the absorption of the wing, assuming zero transmittance and assuming that the normal specular reflectance equals the total fraction of solar radiation reflected, despite the strong angular dependence of the reflectance spectrum documented by one of the authors elsewhere (Tada et al. 1998). This gives an indirect and highly suspect value for the fraction of light absorbed by the butterflies, and does not say whether this fraction is larger or smaller than what would be absorbed without the iridescent scales. Perhaps the scales act more like a 'sunblock' than a 'solar collector'.
"In fact, there is no need for a thermoregulatory explanation for the optics of Morpho and others: camouflage, courtship, and display (Ghiradella 1991) probably suffice to explain why some butterflies are so shiny."
Daniel Koon, "Comment on 'Butterfly Thin Films Serve as Solar Collectors'", Ann. Entomol. Soc. Am., 92, 459 (199).
"Other scientists, however, note that no one has yet shown that the amount
of heat absorbed through the scales makes an appreciable difference. 'Just
because a feature would seem to serve a given function well doesn't mean
it's something that matters to the organism,' cautions Thomas Schultz,
a behavioral ecologist at Denison University in Granville, Ohio. Schultz
has conducted absorption experiments -- with negative results -- on iridescent
Wade Roush, 'Butterfly biomechanics -- Wing scales may help beat
the heat', Science 269: 1816, Sept. 29, 1995.
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