Those who travel through the hilly rainforests of Madagascar can witness an amazing sight. Looking down at streams, rivers and lakes, they can see enormous spider webs stretching directly above these water bodies. These webs and the spiders who build them have received a lot of attention from researchers around the world. At the Jovan Hadži Institute of Biology at the Slovene Academy of Sciences and Arts, Slovene researchers and their American colleagues have also made them the subject of their research.
The spider that builds these enormous webs belongs to an incredibly diverse group of bark spiders, whose representatives are widespread in the Old World tropics. Despite the diversity of this group, all of its species belong to a single genus – Caerostris. The Darwin’s bark spider (Caerostris darwini) is the only spider among more than 42,000 spider species that builds webs directly above water. In 2010, this species was described by the Slovene researcher Matjaž Kuntner and his Icelandic colleague Ingi Agnarsson, who named it after Charles Darwin, as the discovery coincided with the 200th anniversary of his birth and the 150th anniversary of the publication of On the Origin of Species. The Darwin’s bark spider builds sizeable webs suspended above rivers and lakes, spanning up to twenty-five metres and having an orb capture area that measures up to an astonishing two metres in diameter. These extreme sizes far exceed the majority of other spider webs and represent the largest webs ever recorded, even though the spider itself is no larger than three centimetres and cannot compare in size to the largest orb-weaver spiders.
The webs of the Darwin’s bark spider above a river in the Ranomafana National Park in Madagascar (photo: Matjaž Kuntner).
Webs of the Darwin’s bark spider above a river in the Andasibe-Mantadia National Park in Madagascar (photo: personal archive).
You are probably wondering how the Darwin’s bark spider manages to cross a river or lake. When we asked the locals for their opinion, they suggested the spider might cross the river by swimming or even Tarzan-like swinging. Contrary to such suggestions, we discovered that they cross such large water bodies using the wind. At dawn or dusk, whenever the wind is the strongest, the Darwin’s bark spider builds a silk thread that spreads into tens of very thin silk threads. They have the same function as sails and are caught by even the smallest breeze. After a couple of attempts and with a little luck, the spider fairly quickly attaches the silk thread on the other bank and pulls it to test the toughness of the thread, which is also known as the bridge line, below which the spider then constructs its massive web.
But these enormous webs are not the only thing that makes this spider so special. Its silk thread is characterised by a combination of extreme toughness and elasticity that is far greater than any other spider silk and most synthetic materials. The toughness of these threads can surpass 500 J/m3, making it more than twice as tough as any other spider silk and tens of times better in quality than steel. The silk thread of the Darwin’s bark spider is thus the toughest known biological material. The exact biological role of this extremely strong silk is not yet clear. One hypothesis is that these large webs, had they not been made of such tough material, would sag too much under their own weight or even tear under the force of the wind. Yet another hypothesis is that this extraordinary silk allows the spider to catch large prey like dragonflies or even small birds and bats.
The Darwin’s bark spider with a dragonfly as its prey (photo: personal archive).
The Darwin’s bark spider (photo: personal archive).
In addition to the listed characteristics, we discovered that bark spiders build their webs in an unusual way that differentiates them from other orb-weaver spiders. Our partial results show that the co-evolution of silk’s mechanical traits and the unusual way of web building could be the reasons for the Darwin’s bark spider living in such a unique habitat. The spider silk has exceptional mechanical properties, which make it a model polymer in researching biomimetic fibres. In over 42,000 discovered spider species, scientists have tested the mechanical properties of the silk in only a few dozen randomly selected species. This is exactly why learning about the evolution of silk’s mechanical properties and web gigantism can be a key factor in determining new mechanical properties of spider silk.
The Darwin’s bark spider survives only in the remaining parts of the hilly rainforest in eastern Madagascar and like all other animals on this African island it is severely endangered due to rapid deforestation. The recent discovery of this unusual spider species that might shed some light on the vaguely researched evolution of the mechanical properties of spider silk shows the importance of preserving tropical ecosystems, which obviously hide a great many things that we have yet to discover.
Author: Matjaž Gregorič is an arachnologist and evolutionary biologist. He researches biodiversity and the mechanisms that generate it. He is particularly interested in exceptional biological materials and in the way natural and sexual selection influence animal behaviour. His research subjects are primarily spiders and their webs. He is also a film and sports enthusiast. You can follow him on Twitter at @matjazgregoric.
Translated by: Tanja Breznik.