Unique Silk Physics Keeps Dangling Spiders From Twisting Wildly

Spider silk has special properties that help spiders stay steady when they descend on their threads. _jure/iStock/Thinkstock

Imagine a spider plunging in front of your face. First, don't be alarmed — it's all in your head, remember. What do you see? Probably a spider that drops confidently in a straight line, without so much as a twist. But if you tie a little weight to the end of, say, string or dental floss, the weight will spin wildly. So, what's so special about spider silk that allows spiders to stay relatively straight while rappelling?

According to new research out of China and the U.K. and published in Applied Physics Letters, spiders' draglines — the silk strands they lower themselves on — yield a bit when twisted. The silvery threads dissipate energy, which slows the oscillations you would expect from human hair, thin wires or carbon fiber (other materials the researchers tested). That way the spider can plummet like a rock, instead of like a whirling bungee jumper.

Spider silk has long been given credit for its tensile strength. But to test its torsional properties (or twisting response), the researchers collected silk strands from two species of golden orb weavers, hung them in a cylinder and placed washers on their ends. Then they twisted the strands with a rotating turntable, and recorded oscillations with a high-speed camera. They found that the silk goes slightly out of shape when twisted, which releases up to 75 percent of its potential energy, slowing the oscillations.

The researchers think this response is related to the physical structure of the silk — which includes sheetlike amino acids that can stretch like elastic and snap back — and suspect humidity may be a factor, too. But they're still not entirely sure how the silk's torsional properties work.