Butterflies and Flashlights

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. The photonic crystal is infused with highly fluorescent pigment and contains an array of hollow air cylinders arranged in a pattern of triangular symmetry. As one paper further explains:

As in ultra–high-efficiency LEDs, these Butterflies’ DBRs support a spectral stop band that matches the peak emission from the structure above it. The DBRs reflect upwardly the downward-emitted fluorescence concurrently with non absorbed longer wavelengths pass through the PCS. The spatial separation between the DBR and PCS minimizes losses via coupling to guided modes in the DBR. Excitation for this fluorescent material appears to be optimized for the radiance from blue skylight, which peaks around 420 nm. Additionally, because the alpha-absorbance band of rhodopsin dominates the green wavelength photosensitivity of Papilio vision, the spectral form of this absorption is ideally placed for stimulation by fluorescence from conspecific wings. As with some shrimps and birds, this enhances signaling, because absorption of visually less productive short wavelengths leads to the emission of longer wavelengths that trigger photoreception.

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.

Vukusic discovered that 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.

Evolutionists speculate that perhaps these marvels happened to arise luckily via random mutations. Or perhaps self-assembly and mechanical processes such as buckling, cracking and splitting are important factors. In fact, perhaps pre existing cellular structures serendipitously provide a manufacturing framework. Could it be that “the highly complex inverse opal-type structures could appear ‘suddenly’ in evolutionary time (without having to evolve stepwise)”? Amazingly this is what one finds in evolutionary theory--unfounded speculation underwritten by dogmatic certainty.

Perhaps flashlights also appeared suddenly. Religion drives science, and it matters.