By Matt Walker
Editor, Earth News
Darwin's bark spider web, in close up
A newly discovered species of spider living in Madagascar makes the world's longest known web, spanning 25m.
The spider also makes the largest orb web yet found for any spider, and constructs it out of the most tough biomaterial yet known, say scientists.
Darwin's bark spider, a species new to science, weaves its huge web over flowing rivers, stretching from bank to bank.
It is so big that it can catch 30 or more prey insects at any one time.
Darwin's bark spider weaves what experts call an orb web, the most familiar spider web design.
But this web is unusual as it is the largest orb web yet known to be made by any living spider, with the largest web measuring 2.8m².
The previous largest webs are spun by orb spiders belonging to the genus Nephila. Last year a new species of giant orb weaving spider (Nephila komaci) was discovered in Africa and Madagascar that can spin webs up to 1m across.
But even that web is dwarfed by those spun by Darwin's bark spider.
"They build their web with the orb suspended directly above a river or the water body of a lake, a habitat that no other spider can use," says Professor Ingi Agnarsson, the director of the Museum of Zoology at the University of Puerto Rico, in San Juan who made the discovery with colleagues.
The spider's web dwarfs scientists
That allows the spiders to catch insects flying over water, and explains why the web is so long.
To reach from one bank to the other, the spider must weave anchoring lines of up to 25m.
It also helps explain why the spider must weave it from such tough material, as the huge web must support its own weight and that of any prey it captures.
Prof Agnarsson and colleague Matjaz Kuntner, who also both work at the Smithsonian Institution in Washington DC, US, discovered the new spider species, which has the scientific name of Caerostris darwini, around the Namorona river in the Ranomafana NP and Fianarantsoa Province in eastern Madagascar.
Details of the spider and its behaviour are published in the Journal of Arachnology.
As in other related species, females dwarf the males.
The researchers found many webs spun by the spider, with most hanging vertically above the river, with some at an angles of up to 50º.
"Some of the webs qualify, to the best of our knowledge, as the largest spider webs ever documented," say the researchers in the journal.
Up to 32 unwrapped prey items, mainly mayflies, were found in a single web.
How the spiders spin such a huge web above water, and how they anchor their drag lines on either side of a river, is currently being researched.
Many of the webs showed obvious signs of damage and repair, while others had large open holes, suggesting that the spider maintains each web for several days.
Most orb weaving spiders take down and reconstruct their webs each day.
Monitoring a giant web
The spiders are able to weave such large webs, held up by such long drag lines, by using the toughest, most energy-absorbing silk ever discovered, tougher than any other known biological, and most man-made, materials.
Spider silks combine high strength with elasticity and are therefore already exceptionally tough, being able to absorb three times more energy before breaking than Kevlar, a material often used in bulletproof vests, say the researchers.
However, Darwin's bark spider weaves silk that is about 100% tougher than any other known silk, making it the toughest biological material known, say the researchers.
The have published details of the web's toughness in another scientific journal PLoS One.
Other spiders are capable of weaving giant webs.
For example, one huge web complex was found in 2007 in Texas.
But this web complex was not the work of one large spider.
Rather, millions of small ones weaved a series of interlaced webs that ended up covering an area twice the size of a football field.