Unlike many vibrant insects, which depend solely on pigments for their colors, a butterfly’s radiant shimmer has a special source: the precise structure and arrangement of the microscopic scales on its wings. These small but powerful scales provide iridescence as well as body temperature maintenance and protection from the elements.
For the first time, scientists at MIT have devised a way to view and record these microscopic scales as they grow and overlap on a developing butterfly inside its chrysalis. The team raised painted lady butterflies, Vanessa Cardui, waiting for the caterpillars to envelop themselves in pupae. Once the metamorphosis began, the team cut out the cuticles of each chrysalis and covered the openings with glass coverslips, allowing them to see the developing wings through that window. The team recorded the wing’s scale development from start to finish and published their findings in Proceedings of the National Academy of Sciences.
Butterfly scales are complex microstructures, but most of what is known about their formation is based on still images of developing and mature butterfly wings. The team knew they needed a clearer and more complete view of butterfly wing development to understand how scales work.
“Previous studies provide compelling snapshots at certain stages of development; Unfortunately, they don’t reveal the continuing chronology and sequence of what happens as scale structures develop, ”co-author and mechanical engineer Matthias Kolle said in a statement. “We needed to see more of it to begin to understand it better.”
[Related: A beginner’s guide to butterfly watching]
To visualize this continuous sequence, the team used spot correlation reflection phase microscopy, a light-based imaging technique that applies a scattered field of light spots to a target. Large, focused beams of light can damage delicate butterfly wing cells, but this method creates detailed three-dimensional maps of the scales without this residual damage. Co-author and biological engineer Peter So compared speckle field microscopy to “thousands of fireflies that generate a field of lighting points.”
Using their high-resolution imaging, the scientists found that butterfly scale cells quickly lined up in rows within days of pupa forming. The cells have developed into cover scales, which are found above the wing, or ground scales, which develop below. As the cells continued to grow, the research team expected each cell to crumple and compress, like an accordion. Instead, each cell developed some sort of ripple, like the ripple on a metal roof.
The authors hope to deepen the mechanism of this ripple, seeking to use butterfly scales as a source of inspiration for the design of new materials. Butterfly scales have other fascinating properties such as water repellency and the ability to regulate temperature. Lead author and mechanical engineer Anthony McDougal said in a statement that understanding scale formation could help “give both color and self-cleaning properties to automobiles and buildings.” Now we can learn from the butterfly structural control of these complex micro-nanostructured materials. “