Aug 23 2010
Butterflies and Flashlights [Excerpts]
I once met a fellow who was an aficionado of, believe it or not, flashlights. It seemed rather mundane until I saw all the neat designs using LEDs (light emitting diodes). These semiconductor devices have been greatly improved in recent years and are finding a wide range of uses. But as is so often the case, these technological advancements were there all along in the biological world. In this case, certain butterfly species have their own elaborate optical emission system in their wings. As one researcher put it, “Who knows how much time could have been saved if we'd seen this butterfly structure 10 years ago.”
Up until a few years ago the problem with LEDs was that most of the light was not emitted. This inefficiency was resolved with two-dimensional crystals and layered reflectors called distributed Bragg reflectors (DBRs). And like these high-emission LEDs, scales on the wings of African Swallowtail butterflies make up a two-dimensional photonic crystal enhanced by a three-layer, cuticle-based DBR.
As the passage explains, the butterfly’s optical emission system is tuned to use sunlight and to maximize visibility. Here is a less technical description of the system:
The trouble with this mechanism is that while half the fluorescent light radiates away from the butterfly, the other half radiates into the wing structure. That half of the light would be lost were it not for the extraordinary structure of the scales.
[As the researcher] Vukusic discovered, the base of each scale is a highly efficient three-layered mirror--a structure known as a distributed Bragg reflector. Light from the pigment bounces between these layers, interferes constructively, and then escapes in the direction it came from.
Distributed Bragg reflectors are not perfect, however; some light always becomes trapped on the surface of the reflector and is lost. But the butterfly has another neat trick to get around this. Vukusic and his colleague Ian Hooper discovered that in each scale, sitting just above the mirror, is a slab of material filled with hollow cylinders of air that run perpendicular to the mirror. These cylindrical holes channel the light away from the reflector, preventing it from getting trapped. The slab, says Vukusic, is what optical physicists call a photonic crystal.
The end result is a highly specialized structure that converts skylight into blue-green light, captures this light, and finally channels it out to act like plumage to attract female butterflies.
Was this remarkable system constructed by the blind interplay of natural processes? Evolutionists think so. In fact they are certain it was, though beyond vague speculation they don’t know how.