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.

Technical articles:

• "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).

• "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 layer."
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, p. 38.
• "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 it."

"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, p. 47.
• "Researchers at Tufts University in Medford, Mass., are studying the optical properties of butterfly wings to determine how they collect and distribute heat.

"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.]
http://insects.ucr.edu/ent173/tem96.html
• "Both Morpho and Papilio exhibit daily solar basking. Solar basking relies on wing scales, which are thin-film structures, to produce heat by optical interference."

http://www.tufts.edu/as/geo/bm/experiment.html

Literature skeptical of thin films as solar collectors:

Technical literature:

• "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).

Nontechnical literature:

• "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 tiger beetles."

Wade Roush, 'Butterfly biomechanics -- Wing scales may help beat the heat', Science 269: 1816, Sept. 29, 1995.

Care to respond, or have citations to add? dkoon@stlawu.edu