Self-organizing colours of butterflies


Updating Magic Universe

Self-organizing colours of butterflies

The papilionid Emerald-patched Cattleheart, Parides sesostris. Source: Richard Prum

A multi-disciplinary study at Yale, by evolutionary biologists, physicists and various kinds of engineers, aided by X-ray scattering specialists at the Argonne National Laboratory, has clarified the way in which butterfly wings generate shimmering colours. The wings employ a physical trick of the light that’s more influential than any chemical pigments. The team, led by biologist Richard Prum, has published the findings in this week’s on-line issue of the Proceedings of the National Academy of Sciences.

A gyroid, as visualized by Alan Schoen. Image from NASA

Complex curved assemblies of molecules called gyroids do the trick, by scattering light in distinctive ways that depend on the dimensions of the gyroids, the slant of incoming light, and the angle of view. Digging a little, I find that a mathematician, Alan Schoen, discovered gyroids as possible shapes, long before their biological role was known. In 1970, while at the Electronics Research Center in Cambridge, Massachusetts, Schoen wrote a paper for NASA that began: “A preliminary account of a study of the partitioning of three-dimensional Euclidean space into two interpenetrating labyrinths by intersection-free infinite periodic minimal surfaces …” Key words there are “labyrinths” and “periodic”.

But how does a butterfly, however mathematically astute, set about building its gyroids?  According to Prum and his chums, the outer membrane of a cell in the butterfly’s wing folds into the cell’s interior from above and below to make a double gyroid. Then starchy chitin forms on the outer gyroid to solidify it before the cell dies – leaving the colour-generating crystal form on the surface of the wing.

Here’s the most relevant story in Magic Universe. It’s called “Molecular partners: letting natural processes do the chemist’s work”.

Strasbourg or Strassburg lies in the rift valley between the Vosges and the Schwarzwald, down which the Rhin or Rhein pours. The city is closer to Munich than to Paris, and repeated exchanges of territory between the French and the Germans aggravated an identity problem of the people of Alsace. It was not resolved until, after restoration to France in 1945, Strasbourg became a favoured locale for Europe-wide institutions. But history left the Alsatians better able than most to resist the brain drain to Paris, and to pursue their own ideas, whether with dogs, pottery or science.

As a 28-year-old postdoc in Strasbourg, Jean-Marie Lehn embarked in 1967 on a new kind of chemistry that was destined to become a core theme of 21st-Century research worldwide. It would straddle biology, physics and engineering. As it concerned not individual molecules, made by bonding atoms together, but the looser associations and interactions between two or more molecules, he called the innovation supramolecular chemistry.

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