Tuesday, 21 September 2010

World's Biggest, Strongest Spider Webs Found

River-Spanning Spider Web

A river-spanning spider web dwarfs a park ranger in Madagascar in 2008. Made of the world's strongest known biological material, the web is the product of a new species, the Darwin's bark spider, which makes the world's largest webs of any single spider, new studies say.

Zoologist Ingi Agnarsson and colleagues have found Darwin's bark spider webs as wide as 82 feet (25 meters)—about as long as two city buses.

In Andasibe-Mantadia National Park (pictured), "the park rangers knew about them, and I think they've shown them to tourists for a while," said Agnarsson, of the University of Puerto Rico.

But the Darwin's bark spider and its record-breaking webs were unknown to science until they were documented by the team, whose findings appear this week in the Journal of Arachnology and PLoS ONE.

Darwin's Bark Spider Web

An approximately three-foot-wide (meter-wide) Darwin's bark spider web hangs above a river in Madagascar.

Though the new species' webs are overall the world's largest, other spiders might exist that create larger orbs—the spiral at the center of the web—according to study co-author Todd Blackledge, a biologist at the University of Akron in Ohio.

Despite spinning webs of Spider-Man-like size and strength, the Darwin's bark spider uses them to feed mainly on small fry—insects such as mayflies and dragonflies, according to the team, which included Blackledge, the University of Puerto Rico's Agnarsson, and Matjaž Kuntner of the Slovenian Academy of Sciences and Arts.

"In our dreams, we were hoping for bats or birds or something," Agnarsson joked.

Female Darwin's Bark Spider

The weavers of the largest Darwin's bark spider webs are almost always female, such as this spider pictured in 2008, said Agnarsson, a grantee of the National Geographic Society's Committee for Research and Exploration.

Juvenile males also weave spider webs, but once they become adults, they abandon this behavior and instead direct their energies solely to sex.

For survival, the Darwin's bark spider relies in part on its mottled, jagged appearance, which camouflages the spider against trees and—along with Charles Darwin—inspired its name. The species is known to exist only on the island of Madagascar, off Africa's southeastern coast.

Giant Web, Snack-Size Fare

Dozens of mayflies hang helpless in a Darwin's bark spider's web in 2008.

Darwin's bark spider webs are made out of two basic kinds of silk, Agnarsson explained.

"Dragline" silk is used to create the supporting strands that anchor the endpoints of an orb web to tree branches overhanging rivers or lakes and forms the radial threads in the web. Stretchier, stickier silk is used to create the spiral that captures prey.

When an insect flies into the web, it becomes stuck, and its struggles causes the silk lines to vibrate, alerting the Darwin's bark spider.

The spider then crawls to the captured insect, and envelops it in a silk cocoon to eat at its leisure.

Unlike most spiders, Darwin's bark spiders will sometimes wrap several insect corpses into a single cocoon, creating a snack pack for later consumption.

Towering Achievement

Earlier this year, study team member Matjaž Kuntner, of the Slovenian Academy of Sciences and Arts, uses a tape measure to determine how high up the center of a Darwin's bark spider web is.

Unlike their webs, Darwin's bark spiders aren't very big: Not counting its legs, an adult is usually less than 0.8 inch (2 centimeters) long, Agnarsson said.

An analysis of the Darwin's bark spider's silk indicates it's the toughest biological material discovered to date.

"'Tough' means the ability to absorb energy before breaking, and results from a combination of strength and elasticity," Agnarsson explained.

The Darwin’s bark spider's silk "is about the strength of steel but much, much tougher, because it also stretches. It's many times tougher than even Kevlar, which is one of the best man-made materials."

Behind the Web

Scientists in 2010 observe the web of an unnamed species that may be a close relative of the Darwin's bark spider and also happens to live in Madagascar's Andasibe-Mantadia National Park.

The central orb structure of this spider's web is about three feet (one meter) wide. By contrast, the orb of a Darwin's bark spider web can be nearly seven feet (two meters) across, though most are much smaller.

Darwin's Bark Booby Trap

One of the first things Agnarsson's team wondered after learning about the Darwin's bark spider was exactly how it creates webs wide enough to span bodies of water, such as this river in Madagascar's Andasibe National Park, pictured in 2010.

One of the rangers "said the spiders do a Tarzan swing, like they hang down on the silk and swing over," Agnarsson said. "We really, really tried to verify that, but it turned out to be false."

Since then, team member Matjaz Gregorič has discovered the spider's trick and will describe it in a future science paper.


Agnarsson, shown behind a Darwin's bark spider web in 2010, hopes that studies of the species will help shed light on mysteries of spider silk.

"Almost all of the spider-silk research has focused on one or two spider species," he said, referring to golden orb weavers and spiders of the Agriope genus. "Our interest is in looking much more broadly across the diversity of spiders.

"There's no reason to think that the two exemplar species that most researchers happen to be studying are among those that make the best silk of all the spiders."


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