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Glue of orb-weaver spider in Japan uses moths' defenses against them

Glue of orb-weaver spider in Japan uses moths' defenses against them
The Japanese spider species Cytarachne akirai produces specialized glue that sticks to scaled moth wings. Photo by Sarah Han/University of Akron

Jan. 29 (UPI) -- The tiny, easily shed scales that coat the wings of moths help them evade the webs of many spiders, but not the specialized glue deployed by some orb-weaver spiders.

According to a new study, the glue that coats the silk threads of certain spider species quickly thins out and spreads out across the scales and base of the wing. After spreading, the glue quickly hardens to adhere to the entire wing mass.

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"You have this normal defense of the moth that's all of a sudden being utilized against it," lead researcher Candido Diaz, an evolutionary biologist at Vassar College, said in a news release. "Without the scales to increase the surface area, to cause the glue to spread further, the glue would actually be very weak and useless."

Diaz, who conducted the research while at the University of Akron, realized the glue of an orb-weaver spider found in Japan was different when she attempted to examine samples that had been sitting around in storage for some time.

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Because most spider glues absorb moisture from the air, they remain wet for long periods of time. Not the glue produced by Cyrtarachne akirai, found living among rice paddies in Japan.

"I had so many samples that I was holding on to, to be like, when I'm less busy, later in the semester, I'm going to study these. And then they were all dry and useless," said Diaz. "I was like, nothing in the literature explains this."

Diaz and her research partners went to the source to find out how the glue works, capturing high-speed videos of moth wings coming into contact with the glue covered webs of C. akirai. Researchers compared the videos to footage of moths wings interacting with the glue produced by a more common orb-weaver, the furrow spider, Larinioides cornutus.

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The furrow spider's glue was thick and gooey and failed to spread out much when it came into contact with a moth wing. The glue produced by the Japanese orb-weaver is thinner and quickly spreads out. Pulled by capillary forces -- the same forces that cause water to spread across paper towels -- the glue travels through the ridges and grooves that texture moth scales.

"It starts to ride those lines along the scales, and so it doesn't just radiate straight out," said Diaz. "It goes left and right, back and forth, kind of filling up the entire channel and all the spaces."

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When scientists exposed the more typical orb-weaver glue to high humidity, it became too weak to maintain a bond as it thinned. The specialized glue works differently.

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In an earlier study, scientists captured images of the naturally runny spider glue using infrared spectroscopy. The images revealed proteins condensing in the center of glue droplets, while a water layer quickly formed along the outer edge. Scientists suspect this dynamic encourages rapid evaporation as the glue spreads across wings nooks and crannies.

During the latest lab tests, described this week in the Journal of the Royal Society Interface, researchers struggled to untangle moth wings from C. akirai wings. It took roughly 30 minutes longer to free wings from the Japanese orb-weaver's glue than it did to separate L. cornutus wings.

When scientists tested the two glues on wings stripped of all their scales, the more typical spider glue proved much more adhesive than the specialized glue. The moth scales actually work to strengthen the bond of the glue produced by C. akirai.

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The evolutionary battle between spider and moth has, in the case of C. akirai, inspired a predatory tool that is enhanced by the target's defensive design.

"The large size and low viscosity of C. akirai glue droplets function together to use the three-dimensional topography of the moth's scales against itself via capillary forces," scientists wrote in their paper.

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