Invertebrates
Darwin's Bark Spider
Caerostris darwini · also called darwin's bark spider
Darwin’s bark spider, from the rivers of Madagascar, builds the biggest web of any spider on Earth — an orb up to nearly three square metres, slung across the open water on anchor lines that can stretch 25 metres from bank to bank. And it spins those lines from the toughest material any living thing is known to make.
Overview
Darwin’s bark spider lives along rivers and streams in eastern Madagascar, where large trees on both banks can anchor its web. The female’s body is about 2 cm, mottled to match bark; the male is tiny, around 6 mm. It catches mayflies, dragonflies, and other insects emerging from the water below.
A web across a river
The orb can span up to about 2.8 square metres — several times any other spider’s — and it hangs a few metres above open water on bridge lines over 25 metres long. To get the first line across, the spider floats a thread on the wind until it catches the far bank.
Record-breaking silk
Its dragline silk absorbs more energy before breaking than any other spider silk tested — roughly twice as tough as the next best, and by that measure far tougher than steel or Kevlar of the same weight. A web strung across a river must survive wind, spray, debris, and hard insect impacts.
Discovery
The species was named in 2009, in the year marking 200 years since Darwin’s birth and 150 years since On the Origin of Species.
Status
Darwin’s bark spider has not been assessed. It is known from only a few Madagascan sites and depends on both the waterway and the anchoring trees. It is harmless to people.
Darwin's Bark Spider: common questions
How big is Darwin's bark spider's web, really?
The circular sticky part of the web, the orb itself, has been measured at up to about 2.8 square metres, which is larger than a doormat and several times the area of a typical garden orb-weaver's web. What makes the whole structure even more impressive is where it sits. The orb is suspended over open running water on bridge lines that can exceed 25 metres, so the total span of silk, from one bank to the other and down to the web, is far greater than the orb alone. No other spider is known to build anything approaching this size.
How does a spider get a silk line across a whole river?
It uses the wind. The spider releases a long, fine thread from its spinnerets and lets air currents carry the free end out across the water. When that end catches on vegetation on the opposite bank, the spider has a bridge, which it then walks along, laying down thicker, stronger silk to reinforce it. Only after this bridge line is secure does it build the rest of the web hanging beneath. Getting the first line across is the hard part and can take several attempts, and it is why the spider is always found where suitable trees stand on both banks.
Why is its silk the toughest known biological material?
Toughness in materials science means the total amount of energy something can absorb before it breaks, combining strength and stretchiness. Darwin's bark spider's dragline silk is both strong and highly extensible, so it can take a large impact without failing, and tests put it at roughly twice the toughness of the silk from any other spider studied, including the famously strong silk of golden orb-weavers. A web strung across a river has to survive wind, spray, floating debris, and the impact of large insects hitting it at speed, which is thought to be the selective pressure behind such extreme silk.
Is Darwin's bark spider dangerous to people?
No. It has no medically significant venom, it is not aggressive, and it lives out over water where people rarely come into contact with it. The female is a fairly large spider, but a bite, which would only happen if the animal were handled or trapped against skin, would amount to little more than a bee sting. The species is of interest for its record-breaking web and silk, not as a hazard, and the main practical difficulty for researchers is reaching the webs, which are often strung high above a flowing river.