Tarantulas are too heavy to stick to glass, yet the largest spiders in the world regularly seem to defy physics.

The trick: Dozens of silk-oozing spigots on their feet spin near-invisible safety lines, keeping the colossal spiders stuck wherever they please.

“No one has ever accurately described these structures before,” said neurobiologist F. Claire Rind of Newcastle University in England, leader of a June 1 Journal of Experimental Biology study of the spigots. “We’re certain they’re playing a big role in preventing [tarantulas] from sliding down vertical surfaces.”

Lighter, less-primitive relatives of tarantulas use millions of tiny foot hairs, called setae, to stick to walls, glass and other treacherous vertical surfaces. The hairs don’t dig into smooth material, but instead maximize molecule-to-molecule attraction (called the Van der Waals force) to keep them from falling.

While tarantulas also use setae to stick, some weigh in at more than a third of a pound, far too heavy for the bristles on their feet to support a vertical climb.

In a quest to explain the perplexing feat, researchers suggested in 2006 that a tarantula species called Aphonopelma seemannisecreted sticky silk from its feet. But no spiders were known to secrete silk from their feet — they use abdominal structures called spinnerets — so critics refuted the claim. They argued that silk from spinnerets could have contaminated the discovery, and noted that the hypothesized silk-spinning foot structures hadn’t actually been found.

Driven by curiosity, Rind enlisted three graduate students to help her find out who was right. On the following pages, we take a look at what they discovered.

Citation: “Tarantulas cling to smooth vertical surfaces by secreting silk from their feet.” F. Claire Rind, Chris Luke Birkett, Benjamin-James A. Duncan and Alexander J. Ranken. The Journal of Experimental Biology, June 1, 2011, Vol. 214, Pg. 1874-1879. DOI: 10.1242/?jeb.055657