Their findings, published in February as a cover article in the Journal of the Royal Society Interface, show that the spiders use a combination of strategies to produce their colors, with a tough cuticle layer providing protection.
“We don’t yet know exactly why these spiders have this coloration,” Kariko said. “There are many visual predators, like chameleons, in the forests where these spiders are found, so it’s possible this may be a warning or protective coloring. With this paper, we’ve made some inroads into how they make these colors, but the why is still a mystery we hope to eventually unravel.”
Early in the research, Kariko teamed with James Weaver of the Wyss Institute and Mathias Kolle, then in the lab of Professor Joanna Aizenberg, now a professor of MIT. Kolle performed spectroscopy measurements.
The group quickly realized they also needed someone with both special skills in material science and the manual dexterity to work with the tiny spiders, which led them to Wyss postdoctoral fellow Ling Li.
Li, who is now a professor in Virginia Tech’s Department of Mechanical Engineering, used an array of imaging techniques to examine the colors in precise detail.
The team expanded to include Jaakko Timonen, then of SEAS, now at Finland’s Aalto University, who conducted detailed fluorescence imaging of the specimens, as well as Carolyn Marks, biological imaging scientist at the Center for Nanoscale Systems, who helped examine very thin slices of the spider.
It quickly became clear to the researchers that the silver color was the result of a material similar to that found in reflective fish scales.
The structure functions, Kolle said, by stacking a series of highly reflective 100-nanometer thick plates (about 1/1,000th the width of a human hair). Each plate reflects light at a slightly different wavelength and those wavelengths either cancel each other out or add up to produce color.
“Ling’s analysis brought this out beautifully,” Kolle said. “We were able to image these platelets and show that they have a specific thickness, but there is no specific control of the spacing between them. That means some areas might filter out red and reflect it strongly, and other areas might do the same for blue or for green. When you add all that up, you get the silver color.”
“We were able to show that this silver color is structural,” Li added. “So that explains why the color doesn’t fade away — it’s built into the structure.”
The team also found that the red, which is nonstructural, holds up in ethanol because the pigment is trapped in an array of microspheres only about one micron in diameter.