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A spider web that spans rivers made from the world’s toughest biological material

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

15 thoughts on “A spider web that spans rivers made from the world’s toughest biological material

  1. A few comments there:

    1. What I see in the pictures is hardly a river. More like a creek. Compare with the plants on the banks. That creek is no wider than maybe 2 meters.

    2. The only river is on pic #2, and there only the main bridge thread goes over the river. And the fact that this thread can be seen so well makes me think it’s a human-made line over the river that was re-used by the spider. The actual webs are much smaller and only at the sides.

    3. The perspective in pic #4 is very misleading. The web is not twice as large as the person.

    4. Very long bridge threads are quite common for the larger orb weavers. I’ve personally seen threads from the common garden orb weaver that were at least 6 to 8 meters long, spanning the distance between two trees.

    5. Calling the Hyptiotes web “sticky” is quite misleading. Hyptiotes is a cribellate genus, which means it does not produce glue on its threads at all, but a very fine, tangled, wooly silk. When it collapses its web over its prey, the insect is simply hopelessly entangled. Cribellate spider silk is not sticky at all.

    6. Other very large and tough orb webs are known from the Nephilidae. Their webs are at least as large, and they can stop bats in flight.

    Don’t get me wrong: It’s an intersting find, but there’s no need to sensationalize it like that.

  2. I read it yesterday in bbc news, so cool! 😀
    (though I am still waiting for something like this: http://www.dragonbreath.nl/images/Plated%20Spider.JPG)
    @ Michael:
    I don’t know in Germany, Madagascar or UK, but in Italy a two-meters-wide stream of water we definitely call a river (as long as it does not dry up for some weeks or months every year)
    “Other very large and tough orb webs are known from the Nephilidae. Their webs are at least as large”. References and/or pictures please 🙂
    But yes, perspective in pic#4 IS misleading

  3. You can cry sensationalism as much as you like, but you’re drawing conclusions from the images, which are intended to be illustrative. They’re not data!

    Everything I’ve written reflects what’s actually in the two papers, including the measurements, the stuff about rivers, and the bits about Hyptiotes. To quote the authors: “Similarly, the most adhesive silk discovered to date comes from a spider with a very unusual, highly reduced web architecture”

    And yes, Nephila webs are very impressive and I know about them and have written about them before. That doesn’t change the fact that these webs are record-breaking, even in comparison to Nephila (which is mentioned repeatedly in the paper, which I have linked to, and which is free to view).

  4. Hey Ed, in following up your spider stories I see that nearly all of them include the phrase “an arachnophobes’ worst nightmare” in the first sentence. But only one nightmare can be the worst. Aside from that grammatical problem, you do need to find a new hook for that first sentence of spider stories.

    More seriously, I greatly enjoy your blog, especially the stories about unusual animals.

  5. re #4;

    Ah, but what was once the “worst nightmare” at the time can be superceded by an even more horrific nightmare later!

    So the most recent post is always the accurate descriptor. . . .

  6. As somebody who is terrified of spiders (for good reason, but that is another story) I can think of many many worst nightmares that are spider involved.
    1) Walking through a huge web I did not see.
    1) Waking up with spider on me (in view without needing to move)
    1) Not noticing a hatching until its too late (on back porch while napping)
    1) Never mind the mental effect of actually being harmed by a dangerous spider ( I have black widows and Brown recluses in my back yard).

    So, while understanding that this is just semantics, but If I saw that web and recounted it to others, I would have said it spanned a river and was HUGE.

    Also kind of reminds me of the Gary Larson cartoon of the spider making a web at the bottom of a playground slide…

  7. Three days ago I was showing my 7-year old nephew and 5-year old niece clips of spiders from Life in the Undergrowth (bola tossing spider, spider catching a cricket using a net, and Hyptiotes). I find it all absolutely fascinating and even my squeamish ‘HELP-there’s-a-spider-in-the-kitchen-get-rid-of-it-HELP’ sister found it interesting. Nephew and niece were quite enthralled to say the least, and he now has a spider sticker book (courtesy of his evil uncle, moi) so no doubt my sister will be having a few “HELP…oh wait…it’s another sticker…” moments.

    And the niece is a real bug and worm fanatic…today she dug up a huge earthworm and ran around showing everyone (mom once again was less than enthused but she doesn’t want to quash a budding biologist—I’ve corrupted the kids–insert evil laugh here).

  8. They wonder why it is so tough – maybe because if it is so laborious to construct, its best if it requires less repair. A stronger silk means less repair from damage done by wind, birds, bats, etc.

    Michael: visit the links in the reference and you’ll find a picture of a web spanning a river. Or check the slideshow above the default image – you’ll find more pictures, including one showing a web spanning a river, and another showing a person close to the web across the “stream” which will give you better idea of scale. Even the one across the stream is pretty big.

  9. is it possible the spider share’s it’s web?

    in that way, such a large web seems more feasible with more than one spider constructing it.

  10. @M – It’s unlikely. For a start, Agnarsson actually caught some of the wild spiders and didn’t find any evidence of multiple spinners per web. Secondly, there *are* social spiders who spin communal webs but their webs are very different. They’re messier and more tangled, rather than the neat orb webs of Darwin’s bark spider.

    I’ve written about social spiders here: http://blogs.discovermagazine.com/notrocketscience/2008/07/21/social-spiders-do-better-when-hunting-with-relatives/

  11. The river-spanning webs remind me of tunnel traps for catching bats. One of my undergrad professors had an idea that since insects come out of the water -> bats find rivers good places to catch insects -> field biologists find rivers good places to catch bats. I’m a little sad they didn’t find any vertebrates in the Caerostris spiderwebs. http://polillo.www6.50megs.com/bats.html

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