This is an arachnophobe’s worst nightmare: the largest spider web in the world. It belongs to the Darwin’s bark spider, which spins its gargantuan trap over entire rivers and lakes. Its shape – a simple ‘orb web’ – is normal enough, but its size is anything but. The main anchor thread that holds the web in place to both riverbanks can be as long as 25 metres and the main sticky core can be as large as 2.8 square metres.

With a web that big, it’s no surprise that Darwin’s bark spider uses the toughest silk of any species. It can resist twice as much force as any other spider silk before rupturing, and over 10 times more than a similarly sized piece of Kevlar. It’s not just the apex of spider silk – it’s the toughest biological material ever found.

Ingi Agnarsson from the University of Puerto Rico first discovered Darwin’s bark spider (Caerostris darwini) in 2001, when he was still a graduate student. While travelling through Madagascar’s Ranamofana National Park, he found the giant webs crisscrossing streams and rivers. His initial reaction was a simple "Wow!". "We knew we had found something special and wanted to return to Madagascar to research them," he says. He did so in 2008 and 2010, capturing live spiders, measuring the webs, and analysing their extraordinary silk with spider specialist Todd Blackledge.

Agnarsson found that the silk of Darwin’s bark spider is twice as elastic as any other spider silk. This, combined with its high strength, allows it to absorb huge amounts of energy without cracking, the very definition of toughness. For technically minded readers, the fibres resisted an average of 350 MJ/m3 before rupturing, and some threads withstood 520 MJ/m3 For everyone else, these values are more than 10 times tougher than Kevlar. They’re so large that Agnarsson makes a special point of saying that he didn’t screw up his measurements! He worked with a team of experienced spider scientists, and each of six captured spiders produced silk of comparable toughness.

For the moment, it’s not clear why the spider needs such tough silk. The rivers it spins above are frequented by birds and bats, and while it’s tempting to suggest that a spider could catch such large prey, there’s no evidence for this. The majority of victims in the giant webs were large insects like mayflies and dragonflies. Maybe the webs allow the spider to monopolise insects that spend their young life in rivers, trapping them as they emerge from the water in a way that riverbank spiders simply can’t compete with.

There must be some benefit, for producing that much silk would be a costly exercise. The spider itself isn't very big; the female dwarfs the male and even she only measures around 3-4 cm with legs outstretched. How she spins her web over entire rivers is a mystery, and one that Agnarsson is working on right now. He thinks it involves “bridging”, a spider technique that involves releasing strands of silk into the air and hoping that one latches onto a faraway spot.

Even so, building must be laborious. While most orb-weavers dismantle and re-build their webs every day, Darwin’s bark spider keeps each web for several; Agnarsson found that some webs had big holes in them and other obvious signs of damage. When webs take so long to build, it pays for them to be exceptionally tough. The quality of the silk probably co-evolved with the behaviour of its spinner.

Agnarsson describes his work as “bioprospecting” – searching for new materials among natural sources. Spider webs make good candidates for such an activity. With over 41,000 species, each spinning many types of silk, there are more than 200,000 different silks to analyse. The silks can be very different. Compared to primitive species like the trapdoor spiders, hunters like the black widow and spitting spiders spin threads that are 20 times tougher and 10 times stronger.

But this wealth of natural materials remains largely untapped. Only a few handfuls of species have been studied and they’ve been chosen almost at random. Agnarsson likens the approach to blind fishing and suggests that scientists are better off using other aspects of an animal’s behaviour as clues about which species to focus on.

In this case, the link was simple – a giant spider web is probably made of extraordinary spider silk. There are other examples. The stickiest silk on record belongs to Hyptiotes, a spider that spins a bizarre triangular web, which it stretches taut with one leg. When an insect flies into it, Hyptiotes releases its web, which springs back and envelops the insect.

Tough though the silk of Darwin’s bark spider is, Agnarsson thinks that there are tougher materials yet to be found. For a start, this species uses other types of silk, which may have different properties. The team also tested the silks by slowly tugging on them until they cracked. This is a fairly artificial challenge. Webs are adapted to stop insects that fly into them at high speeds and some studies have found that when spider silk is tested against such rapid forces, it proves to be even tougher.

Reference: PLoS ONE http://dx.doi.org/10.1371/journal.pone.0011234; Journal of Arachnology http://dx.doi.org/10.1636/B09-113.1