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Jennifer Ouellette-November 22, 2021 at 9:09 PM UTC

One of Gerard Manley Hopkins's most famous poems opens with praise of the phenomenon of iridescence. It is represented by the colorful wings of kingfishers and dragonflies in Hopkins poems, but iridescent colors can also be found in the wings of cicadas and butterflies, certain beetles, and the bright feathers of male peacocks. Now, for the first time, a group of researchers from the Massachusetts Institute of Technology has captured the unique structural growth of butterfly wings in the video-as the butterfly continues to develop in its pupae. The researchers described their findings in a new paper published in the Proceedings of the National Academy of Sciences.

As I wrote before, the bright rainbow colors in butterfly wings do not come from any pigment molecules, but from the structure of the wings. This is a naturally occurring example of what physicists call photonic crystals. The scales of chitin (a polysaccharide common in insects) are arranged like roof tiles. Essentially, they form a diffraction grating, but photonic crystals only produce light of a specific color or wavelength, while diffraction gratings will produce the entire spectrum, much like a prism.

Also known as photonic band gap materials, photonic crystals are "tunable", which means they are precisely ordered to block certain wavelengths of light while allowing other wavelengths to pass through. By changing the size of the tiles to change the structure, the crystal becomes sensitive to different wavelengths. (In fact, the rainbow weevil can control the size of its scales and fine-tune these colors according to how much chitin needs to be used.)

Even better (from the perspective of the application), the perception of color does not depend on the viewing angle. Weight scales are not just for aesthetics; they help protect insects from the elements. There are several types of artificial photonic crystals, but a better and more detailed understanding of how these structures grow in nature can help scientists design new materials with similar qualities, such as iridescent windows, self-cleaning surfaces for cars and buildings, and even Waterproof textiles. Paper money can contain encrypted rainbow patterns to frustrate counterfeiters.

Since the growth of butterfly wings was first recorded in 1938, butterfly wings have fascinated scientists. We now have more advanced imaging techniques to further clarify this complex process. "Previous research provides a convincing snapshot at a specific stage of development; unfortunately, they do not reveal the continuous timeline and sequence that occurs as the structure of scale grows," co-author Mathias Kolle, a mechanical engineer at the Massachusetts Institute of Technology Say. "We need to see more to begin to understand it better."

The team cultivated batches of colorful butterflies (Vanessa cardui) in the laboratory and carefully monitored the larvae in separate containers until the larvae shed their skins. Once the caterpillar was encapsulated in the pupa and turned into a butterfly in the end, the researchers began to document the process. They rely on several surgical methods to understand the development of the pupa's wings.

First, the researchers exposed the forewings by removing part of the cuticle with a scalpel; the pupae were anesthetized in the process. Then, they placed a thin glass cover slip on the excision area with a bioadhesive and sealed it with a hand-held dental curing light.

To image the hind wings, the MIT team grabbed the cuticle and forewings of the pupa and folded them toward the head. The hind wings and fore wings are separated by a tooth composite strip. The researchers again used glass coverslips to protect the exposed wings and provide a window for the pupae, which was sealed in place with dental composites.

However, researchers need a special kind of imaging to capture the formation of the wing, because simply shining a wide beam of light on the wing may damage the cells. Solution: Speckle-related reflection phase microscopy, which involves illuminating many tiny light spots on specific points on the wing.

"The spot field is like thousands of fireflies, they produce an illumination spot," said co-author Peter So, one of the three experts in the field of imaging of this type of imaging, and he participated in the experiment. "Using this method, we can isolate the light from different layers and reconstruct the information to effectively map the 3D structure."

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