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The biggest spider web in the world

Species: Caerostris darwini
Habitat: Madagascar, alarming arachnophobes and mayflies in equal measure



The spider attaches a line of silk to the tree branch she is standing on, by the side of a river, and bungee-jumps into space. Dangling in mid-air, she begins spewing out silk. And more silk. And still more silk.

Eventually she has released more than 25 metres of continuous strands, which drift away downwind, across the river. Suddenly she stops, and begins reeling the line back in. It pulls taut. Success! The other end has tangled itself in a bush on the far bank.

This is the first step in the construction of the world's biggest spider web, which will hang above a tropical river. Perched in the centre of her vast web, the Darwin's bark spider can feast on huge numbers of insects after they emerge from the water.

Silk architect
The Darwin's bark spider was discovered in Madagascar only last year, by Matjaž Kuntner of the Slovenian Academy of Sciences and Arts in Ljubljana and Ingi Agnarsson of the University of Puerto Rico in San Juan. With Kuntner's colleague Matjaž Gregoric and Todd Blackledge of the University of Akron in Ohio, they have carried out more field studies to find out how the spiders build their webs and what they are for.

The Darwin's bark has a built-in advantage. Its silk is the toughest of any spider, which is particularly remarkable as spider silks are tough anyway, and stronger than many artificial substances.

The river-spanning lines of silk are the longest section of the web. The spider's method of building them – trailing silk into the open air and hoping for the best – is similar to the common spider trick of "ballooning". Spiders who want to travel long distances release long strands of silk that act like kites and pull the spiders into the air. Ballooning is found in most species of spider, suggesting it has an ancient evolutionary origin – the bridging lines may have developed from it.

Once the bridging line is in place, the spider walks out along it and reinforces both ends. Then she adds one vertical thread beneath the bridging line, forming a "T" shape. This vertical thread becomes the basis for the web proper, a classic orb web that can have an area of 2.7 square metres.

Web of mystery
So what does this huge net catch? Kuntner and colleagues staked out 46 webs and found that most of the prey was small insects like beetles, damselflies, dragonflies and wasps.

In a separate paper, Blackledge has shown that female orb-web spiders rely on occasionally capturing exceptionally big animals. These supply a huge amount of food in one go, so the spiders have plenty of energy to devote to laying eggs.

There was no sign of the Darwin's bark spiders catching big game, though. The researchers tested what the webs could catch in the most direct way possible: by lobbing different prey animals at them from half a metre away. Nothing bigger than a dragonfly got stuck: larger insects and frogs all got clean away.

It may be that the spiders get big meals by catching insects like mayflies in bulk when they emerge from the river. The alternative is that the team just didn't watch them for long enough to see them catch anything big. "We aren't yet sure if C. darwini is a really neat exception to most orb spiders, or if we simply don't know enough about what they eat," Blackledge says. Video at http://www.newscientist.com/article/dn21124-zoologger-the-biggest-spid
er-web-in-the-world.html

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