A spider discovered deep in the jungles of Madagascar spins the largest webs in the world, using silk that's tougher than any known biological substance.

Named Caerostris darwini, or Darwin's bark spider, the inch-wide arachnid's webs can cover 30-square-foot areas, hanging in midair from 80-foot-long anchor lines.

The webs' size generates enormous structural stresses, magnified by the struggles of trapped prey. Strands must "absorb massive kinetic energy before breaking," and are "10 times better than Kevlar," wrote University of Puerto Rico zoologist Igni Agnarsson in Public Library of Science One.

Agnarsson and Slovenian Academy of Sciences biologist Matjaž Kuntner discovered C. darwini in 2008. It's similar in many ways to Caerostris species found in Africa, but those spiders live at the edges of forest clearings. In Madagascar, where animals have taken kaleidoscopic forms since the island split from mainland Africa 165 million years ago, C. darwini evolved to exploit the airspace above streams and rivers.

A few other spiders build stream-side webs, but none "routinely utilize as habitat the air column immediately above sizeable rivers and up to several meters above water," wrote Agnarsson and Kuntner in an August Journal of Arachnology article. The spiders' superior gossamer likely evolved in tandem with C. darwini's migration to Madagascar's rivers, and is twice as elastic as silk from other web-weaving spiders.

That elasticity is key to the silk's toughness, and its molecular underpinnings remain to be studied. The researchers also plan to study how C. darwini build their webs, likely by casting threads that drift over water and catch branches on distant shores, providing a central structural element.

Another intriguing question is how these relatively small spiders maintain such immense, energetically taxing structures. The webs certainly provide rich sources of food; Agnarsson and Kuntner witnessed catches of dozens of insects at a time. They didn't see the spiders catch birds or bats, but say it's possible.

By the time these questions are answered, some other spider may hold the World's Toughest Material title. The researchers point out that there are more than 40,000 arachnid species, manufacturing some 200,000 types of silk. Scientists have studied only a few dozen.

Images: 1) Man looking at C. darwini web and a C. darwini female./Journal of Arachnology. 2) C. darwini webs spanning a Madagascar river./Public Library of Science One.

See Also:

*Citations: "Bioprospecting Finds the Toughest Biological Material: Extraordinary Silk from a Giant Riverine Orb Spider." By Ingi Agnarsson, Matjaž Kuntner, Todd A. Blackledge. PLoS One, Vol. 5 No. 9, September 16, 2010. *

"Web gigantism in Darwin's bark spider, a new species from Madagascar (Araneidae: Caerostris)." By Matjaž Kuntner and Ingi Agnarsson. Journal of Arachnology, Vol. 38 Issue 2, August 2010."

Brandon Keim'sTwitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on an ecological tipping point project.