Vanessa Hamilton

Updated at 2021-06-14 17:23:44 UTC

The world's biggest spider web can span an entire river


The Darwin's bark spider is small and inconspicuous, but it spins a web 25m across using one of the toughest materials known to exist

Assuming you do not suffer from arachnophobia, you might well admire the intricate webs that spiders weave to catch their meals. But one species spins a web more impressive than any other. This web spans several meters and is spun in silk tougher than Kevlar.Spider webs come in all shapes and sizes. Different spiders favor funnels, sheets, tubes or a tangle of lines to rival any Mission Impossible laser maze. But if asked to draw the classic spider web, most of us would sketch an orb web.This is the name given to webs with a spiraling, circular pattern of interconnecting threads. 

Within an orb web, there are different types of silk: the strong scaffolding lines provide structure and the sticky capture threads trap prey.It is not surprising that orb webs are the most familiar to us, as a lot of spiders make them. That includes most of the nearly 3000 members of the Araneidae family, commonly known as the orb weavers and the third largest spider family on the planet.In the northern hemisphere, some of the most regularly encountered webs belong to the European garden cross spider. The species is named, not for its temperament, but for the white cross marking on its bulbous abdomen.Its conspicuous webs can reach over 40cm in diameter. The spiders weave them at night, ready for a day of catching common garden insects like flies, wasps and butterflies.They are particularly noticeable in the autumn, when the verdant foliage of summer dies back. 

The new generation of spiders mature in this season. Dew highlights the silken lines of their woven constructions, which often hang between shrubs or over doorways.In warmer regions of the world, both the spiders and webs can be bigger.The largest webs in Australia belong to golden orb weavers of the Nephila genus, named for the golden appearance of their silk. These webs can reach up to 1m in diameter and hang between trees or sign posts.The spiders themselves can span a human hand with their legs. Their webs are very strong and there are records of particularly large spiders feeding on birds that become entangled in them.The largest golden orb weaver hit the headlines in 2009 when scientists formally introduced it to the public. The body of a female Nephila komaci can be up to 4cm long, with legs more than 10cm long.

 They weave giant webs that can reach more than 1m in diameter.The species is very rare: it has only been found in two locations in Madagascar and Maputaland, southern Africa.Then in 2010, an international team of biologists announced that they had found the world's largest spider webs, also in Madagascar. The species responsible was officially described on the 150th anniversary of the publication of Charles Darwin's On the Origin of Species, so it was named Darwin's bark spider.


A Darwin's bark spider (Caerostris darwini) (Credit: Matjaž Gregori?)

It is hard to understand why nobody noticed these spiders before. A single web can span a river 25m wide.To build it, the female sprays a continuous line of silk from one bank of the river. Air currents carry it across to the other side to create a bridge. In the centre of this bridge, the spider constructs a spiraling orb web that can reach almost 3m in diameter.You might expect the creator of this colossal web to have the proportions of a fictional giant, but Darwin's bark spider is no Shelob."C. darwini females measure about 1.5 cm in body size, and weigh about 0.5 gram, while males are much smaller, weighing 10 times less," says Matjaž Gregori? of the Slovenian Academy of Sciences and Arts in Ljubljana, who has been researching the spiders since their discovery. 

They are also camouflaged to match tree bark.How and why would such small spiders produce the huge quantities of silk needed for their supersize webs?"Why their webs are so big is a good question and we don't have a definite answer yet," says Gregori?.To find out, he and his colleagues have been researching evolutionary relationships between spiders, which should shed some light on how web weaving evolved."The whole genus Caerostris seems to have silk tougher than other spiders, and a bit of a unique web building behaviour," he says. While Nephila spiders weave webs with dense patterns of lower quality silk, Caerostris create sparse webs with very tough silk.Both techniques provide a strong web that can catch prey. 

But by positioning their webs directly above rivers, Darwin's bark spiders can capture dozens of dragonflies, mayflies and other energy-rich insects that live over the water."We hypothesize that this combination was a predisposition for the spider to being able to conquer the unique habitat. No other spider utilizes the air column above open water, so probably, web size and material properties went hand in hand with habitat adaptation," says Gregori?.To find out if this idea is correct, biologists are studying the spiders and their silk. To test the properties of the silk they use a tensile testing machine, which Gregori? describes as: "a machine that grabs a thread at both ends and slowly stretches it until the material fails, measuring the force as it's happening."Using this method, they have found that the silk is stronger than steel. It has also been referred to as the toughest biomaterial known.They may sound similar, but there is a difference between toughness and strength. Strong materials are resistant to stress, but tough materials are stretchy, meaning they can absorb more energy before breaking. 

This is essential in a spider's web, as each strand has to resist the impact and thrashing movements of prey without disturbing the rest of the structure.The combination of strength and toughness is highly sought after by engineers seeking to make new materials, from body armour to fishing nets. Scientists have set their sights on creating "fibres of the future" from spider silk, and there is no bigger target than the webs of Darwin's bark spider.