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If you brush against the butterfly's wings, you will get a fine dusting of powder. The lepidopteran dust is made up of hundreds of thousands of tiny scales and is similar to a butterfly's wings. The structure and arrangement of these scales help shield the insect from the elements.
MIT engineers have captured the butterfly scales forming. For the first time, the team has continuously observed the wing scales growing and assembling as a developing butterfly transforms inside its chrysalis.
The researchers were able to watch wing scales form in specimen of the Painted Lady butterfly, thanks to some minor surgery and a clever approach. As a wing forms, the cells on its surface line up in rows. These cells quickly differentiate into two scales, producing a pattern. The scales grow thin ridges along their length to control the insect's color and help it to shed rain.
The most detailed look yet at the budding architecture of butterfly scales can be found in the team's study. New functional materials, such as iridescent windows and waterproof textiles, could be designed using the new visualization.
Anthony McDougal is a research assistant in the Department of Mechanical Engineering at MIT. The strategy could be used to give both color and self-cleaning properties to automobiles and buildings. We can learn from butterflies' structural control.
McDougal's co-authors at MIT include a professor of mechanical engineering, a research scientist, and a professor of mechanical engineering.
A firefly field.
The structure of scales and ribs varies from species to species, and is revealed by the cross-section of a butterfly's wing. The tiny features act as tiny reflectors, bouncing light around to give a butterfly its color and shine. The ridges on a wing's scales are used as miniature rain gutter and radiators, funneling moist and heat to keep the insect cool and dry.
Researchers have tried to duplicate the structural and optical properties of butterfly wings to create new solar cells, optical sensors, rain- and heat- resistant surfaces, and even paper currency that is iridescent. Knowing what processes butterflies use to grow their scales could help to direct this kind of bioinspired technology development.
The images of butterfly wings that are developing are what's known about scale formation.
"Previous studies provide compelling snapshots at select stages of development; unfortunately, they don't reveal the continuous timeline and sequence of what happens as scale structures grow." We needed to see more to understand it better.
He and his colleagues looked to continuously observe how scales grow and assemble in a butterfly. The butterfly's wings have features that are common across most lepidopteran species, so they chose to study them.
The Painted Lady were raised in individual containers. Once the caterpillar is encased in a chrysalis, the researchers cut into the paper-thin material and peeled away a small square of the developing wing to expose the scales growing underneath. They used a bioadhesive to stick a transparent coverslip over the opening, which allowed them to watch as the butterfly and its scales continued to form.
The experts in speckle-correlation phase reflection microscopy were enlisted to help visualize the transformation. Rather than shining a wide beam of light on the wing, the team applied a "speckle field" of small points of light, each shining on a specific point on the wing. To create a three-dimensional map of the wing's structures, the reflection of each tiny light can be measured in parallel with every other point in the field.
A field of illumination points is generated by a speckled field. This method can help us map a structure in 3D by isolating the light coming from different layers.
Making connections.
The team watched the formation of detailed features on individual scales in their visualization of the butterfly wing.
They observed that within a few days, cells lined up in rows, and then differentiated into two separate groups, one overlying the wing and the other tucked underneath. As they reached their final size, each scale grew with ridges resembling corrugated roofing.
McDougal says that now we can watch continuously what's happening, which gives us more information on the detail of how scales form.
The team found that ridges on scales formed in a different way. Scientists thought the grooves were caused by compression, because scales were thought to squeeze in like accordions. The scales continued to grow in size as ridges appeared on their surface, despite the fact that they were compressed. The measurements suggest that another ridge-forming mechanism is working. The group hopes to explore this and other processes in the butterfly wing, which can help to inform the design of new functional materials.
McDougal notes that the paper focuses on the surface of the butterfly wing. Underneath the surface, we can see cells putting down roots like carrots, and sending out connections to other roots. There is communication underneath the surface. Scales are forming on the surface. We can see all of it.
There is more information about the visualization of butterfly scale cell morphogenesis. The DOI is 10.1073/pnas.
The National Academy of Sciences has a journal.
There are videos that show the growth of butterfly wing scales.
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