All spiders spin silk from their abdomens and most use it to catch prey. But for the first time, scientists have discovered a type of spider – the zebra tarantula – that produces silk from its legs, and uses it for climbing.
There is an old joke that if Spider-Man has the powers of a spider, he really ought to be shooting webs from somewhere less salubrious than his hands. In the films and comic books, Peter Parker is empowered with the powers of a human-sized arachnid through a spider bite. He effortlessly scales walls and ceilings and shoots sticky webs from his wrists. Now, scientists have found a type of spider that does just that.
Like Spider-Man, most spiders can climb sheer surfaces and they do so with two techniques. The most obvious are small claws, called tarsi, that grip onto rough surfaces. Going down in scale, their feet also end in thousands of tiny hairs. These hairs make such close contacts with the microscopic troughs and crests of seemingly smooth surfaces that they stick using the same forces that hold individual molecules together. .
But Stanislav Gorb and colleagues from the Max Planck Institute, Stuttgart, have found that one species of spider uses a third method, which exploits that most characteristic of spider traits – silk. All spider species spin silk from appropriately named organs on their rear ends called spinnerets. But uniquely to spiders, the Costa Rican zebra tarantula (Aphonopelma seemanni) from spins silk from its feet as well.
Gorb watched the tarantulas climbing up glass plates, and saw that they left behind silken footprints – dozens of fibres, just a thousandth of a millimetre wide. As the spider climbs, four of its legs leave the glass plate at any one time. As the legs land, they begin to slip but small nozzles secrete a viscous silken fluid that rapidly hardens and adheres to the surface. The silk acts as a tether, firmly holding the spider to the pane.
Spider silk is fantastically versatile and webs alone consist of several different types. As well as catching food, spiders use silk to create egg sacs, protect themselves and even travel by catching the wind. But this is the first time that silk has been documented as a rappelling aid and it throws up some questions about how it came to be used this way.
The tarantula group includes the largest of all spiders, some the size of a human hand. Gorb suggests that they may have evolved an extra source of traction to support their large bulk and prevent harmful falls. Alternatively, rather than being a new innovation, the zebra tarantula’s silken footsteps might reflect how the first spiders spun silk, with the specialised spinnerets only evolving later.
Finding how many species spin silk from their feet and the genes that control this will give the scientists further insights into how this incredible ability evolved.