Western Tiger Swallowtail butterfly, and a very close look at butterfly wing-color

We’ve gotten a few terrific photos of butterflies this year—some posted here and here— but none of the swallowtails has cooperated by alighting within range. When I saw one that had died and fallen to the road I carefully carried it home for the chance to get a close look.

Papilio 02 Dorsal.jpg

There are at least three very similar species of swallowtail around here—the Anise, Western Tiger, and Oregon Swallowtails. Based on the red and blue markings I’m thinking this is the Western Tiger Swallowtail, Papilio rutulus.

Finer than “frog hair”—butterfly hair!

Enlarging the macro photos shows details such as hairs on the body and along the inner edges of the wings.

Papilio40 CLOSE 1.jpg

These hairs, called tactile setae, are attached to nerve cells, which relay information about the hairs’ movement to the butterfly. … Adults have tactile setae on almost all of their body parts. In both adults and larvae [caterpillars], the setae play an important role in helping the butterfly sense the relative position of many body parts (e.g., where is the second segment of the thorax in relation to the third segment). This is especially important for flight, and there are several collections of specialized setae and nerves that help the adult sense wind, gravity, and the position of head, body, wings, legs, antennae, and other body parts. In monarchs, setae on the adult’s antennae sense both touch and smell. (from monarchwatch.com).

In the photo below, a ventral view of the lower wings where they meet at their lowest point, there is also a delicate fringe visible along the edges. This could have aerodynamic as well as sensory functions.

papilio 46 CLOSE.jpg

From pointillism to nanostructures

Parts of the markings that appear as solid areas to our eye are revealed to be pointillist creations. I suspect we would need to know much more than we do about the vision of butterflies (and their predators?), in order to understand how these markings work for them.

Papilio42 CLOSE 1.jpg

The odd squareness of the smallest dots of color is not some pixellation in the photo, but an accurate representation. It shows the shape of the overlapping scales which form the surface of butterfly wings. Here are some microphotographs of wing scales at various magnifications, from Wikipedia.

ButterflyWingScales.jpg

And here are color microphotographs showing the same squared-off dots along with the underlying scale pattern.

MicrophotographButterflyWingScales.jpg

Picture source.


It’s been known for some time that the colors of butterfly wings are partly from pigments but mostly from the microstructure of the scales, scattering light to produce the colors. Blues, greens, reds and iridescence are usually structural, while blacks and browns come from pigments. (Wikipedia).

But now we know more, and the more we know the more intricate and amazing it is. Research (published this past June) has been able to identify the light-scattering shapes from the wings of several butterfly species, and they are described as “ ’mind-bendingly weird’ three-dimensional curving structures… [resembling] a network of three-bladed boomerangs”. The name for these crystalline forms is gyroids, and they were first described

in 1970 by NASA physicist Alan Schoen in his theoretical search for ultra-light, ultra-strong materials for use in space. Gyroids have what’s known as an ‘infinitely connected triply periodic minimal surface’: for a given set of boundaries, they have the smallest possible surface area. The principle can be illustrated in soap film on a wireframe (see image below). Unlike soap film, however, the planes of a gyroid’s surface never intersect. As mathematicians showed in the decades following Schoen’s discovery, gyroids also contain no straight lines, and can never be divided into symmetrical parts. (source, text and soap-bubble photo: wiredscience.com)

Gyroid-like soap bubble.jpg

Gyroid-like soap bubble. Photo from wiredscience.com

So gyroids were introduced to humans as an imagined created form, something that is a mind-boggler for non-mathematicians to envision.

gyroid_hex.jpg

The image above is a mathematician’s representation of one of the simpler types of gyroid.

Materials scientists have learned how to make synthetic gyroids for photonic devices, such as solar cells and communication systems, that manipulate the flow of light.

gyroidProcess.jpg

A self-assembled solar cell begins with one of two polymers forming a “gyroid” shape while the other fills in the space around it. The inner polymer is dissolved away to create a mold that is filled with a conductor of electrons. The outer polymer is then burned away, the conductor is coated with a photosensitive dye, and finally the surrounding space is filled with a conductor of positive “holes”. A solar reaction takes place at all the interfaces throughout the material, and the interlocking gyroid structure efficiently carries away the current. (Source for image and caption, Cornell Univ.)

And when Yale evolutionary ornithologist Richard Prum got curious about exactly how butterfly wing-scales twisted light, he found gyroids. His team had to use an advanced microscopy technique with nanoscale resolution, called synchrotron small angle X-ray scattering, in order to see them, but there they were. (See note at end for citation of article in PNAS.)

The butterfly’s gyroids are made of chitin, not exactly the flashy material I would associate with iridescent wings. It’s

the tough starchy material that forms the exterior of insects and crustaceans. Chitin is usually deposited on the outer membranes of cells. The Yale team wanted to know how a cell can sculpt itself into this extraordinary form, which resembles a network of three-bladed boomerangs. They found that, essentially, the outer membranes of the butterfly wing scale cells grow and fold into the interior of the cells. The membranes then form a double gyroid—or two, mirror-image networks shaped by the outer and inner cell membranes. Double gyroids are easier to self assemble but they are not as good at scattering light as a single gyroids. Chitin is then deposited in the outer gyroid to create a single solid crystal. The cell then dies, leaving behind the crystal nanostructures on the butterfly wing.

“Like engineers, butterflies grow their optically efficient single gyroids through a series of steps that make this complex shape easier to achieve. Photonic engineers are using gyroid shapes to try to create more efficient solar cells and, by mimicking nature, may be able to produce more efficient optical devices as well,” Prum said. (Source)

In an interview about the work, Richard Prum said “We’re still trying to wrap our brains around gyroids and what they are.” The shapes seem to have evolved separately in several lineages of butterflies.

”It’s a Swiss cheese,” he adds, “with spiraling channels of air traveling through it that intersect one another. But those channels actually travel in three different dimensions through the cheese, and what you end up with is this very complicated form left behind, and that form is a gyroid.”

And while the idea of butterflies with Swiss cheese wings is slightly strange, Prum says it’s a very useful one for scientist and engineers looking for the next leap forward in electronic technology.

For example, Prum says, take the fiber-optic cables that carry phone calls under the ocean. These cables carry signals in the form of colored light, but it’s very difficult to insulate them well enough to prevent light from leaking out. Current transoceanic cables have to have booster stations built along them to keep the signal strong. But a layer of gyroids around the fiber-optic cable “would act like a perfect insulation to that fiber,” Prum says. The same tiny structures that give the Emerald-patched Cattleheart its lovely green patches could also be used to keep green light from escaping a fiber-optic cable.

ButterflyScalesGreen.jpg

The vivid green color of the scales of this Papilionid butterfly are produced by optically efficient single gyroid photonic crystals. Caption and photo from www.physorg.com

Right now, it’s expensive and impractical to manufacture gyroids small enough to do that job. But butterflies hold the secret to growing them naturally. “If you could grow one, at exactly the right scale, as butterflies do,” says Prum, “you could make these things a lot easier.” (NPR interview, Jul 3, 2010)

This is a fine example of how curiosity can lead us to unexpected discoveries. The original question is one that could be used by certain Congressional anti-intellectuals in their periodic efforts to discredit basic research: “Imagine, all this work to find out what makes the color on butterfly wings! How ridiculous!” The research and technological developments that are thought “useful” by these folks had their origins in someone’s basic research, sparked by human curiosity. From butterfly wing-color to, perhaps, more efficient fiber-optic cables or solar energy collectors. It’s called bioengineering: investigating the functions and structures of nature, to derive principles and patterns for technological innovations. But for me it’s satisfying in itself, the revelation of these marvelous structures, underlying the evanescent beauty of a butterfly.

Papilio rutulus.jpg

Western Tiger Swallowtail butterfly on Buddleia bloom. Photo by terwilliger911, flickr.

Note: The article describing gyroids as the structure causing some colors in butterfly wings is:
Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales.
Vinodkumar Saranathan et al. Proceedings of the National Academy of Sciences. Published online before print June 14, 2010, doi: 10.1073/pnas.0909616107. The abstract is available free, but the article requires purchase or subscription to PNAS. There is a supplementary article here that contains some interesting images and very technical text. There’s even a movie you can watch showing a slice-by-slice trip through a certain sort of gyroid, or as the text says, though “the pentacontinuous volume of a level set core-shell double gyroid structure”.

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