If Spider-Man really could do “whatever a spider can”, he ought to shoot webs from somewhere less salubrious than his hands. All spiders spin silk from their rear ends, using special organs called spinnerets. But one group – the tarantulas – can also shoot silk from their feet, and they use this ability to climb up sheer vertical surfaces.
Tarantulas have been kept as pets for decades, but their silk-spinning feet were only discovered in 2006 by Stanislav Gorb from the Max Planck Institute. Gorb watched Costa Rican zebra 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.
It was a fascinating story, but three years later, Fernando Pérez-Miles claimed that Gorb had got it wrong. He found that zebra tarantulas could no longer secrete silk if their spinnerets (the ones on their backsides) were sealed. He argued that the spiders brush the spinnerets with their hind legs when they walk; it’s this motion that releases silk threads.
Now, Claire Rind from Newcastle University has weighed in on the matter, and she sides with Gorb. Rind bought three Chilean rose tarantulas from local pet stores, placed them on glass slides, and filmed them as she gently raised the glass to a vertical position. The spiders didn’t fall, even when Rind gently shook the glass. Their legs slipped slightly, but they soon regained their footing, and every time, they left tiny silken threads behind. That’s essentially what Gorb found; to seal the case, Rind had to find the structures that secrete the silk.
She gathered moulted skins from three species of tarantula, including her own recently deceased pet – a Mexican flame-knee tarantula called Fluffy. Under an electron microscope, Rind saw strands of silk emerging from the tips of several hairs (‘setae’) on the feet.
Setae usually end in brush-like tips, and they’re part of a spider’s climbing equipment. The tips make close contact with the microscopic bumps that cover every surface (even smooth ones), and they stick using the same forces that hold molecules together. But among the brush-like setae, Rind found others that were tall, ribbed and tapered. Each had a small hole at its tip, and one of them even had a droplet of silk coming out of it. Tarantulas can spin silk from their feet, and here was the evidence.
The silk-spinning hairs (Rind calls them ‘spigots’) are much taller than the surrounding setae, like skyscrapers in a low-rise suburb. This allows them to produce silk without gumming up the spiders’ feet. They’re spaced out so they don’t stop the other setae from touching the surface, and they’re ribbed so that they don’t get crushed when the spider walks.
Gorb suggested that tarantulas evolved silk-spinning feet because they need a third method of sticking to surfaces, beyond their claws and their setae. They are among the largest of all spiders, and they would be killed by falls that smaller species would shrug off. The spigots on their feet give them an extra lifeline – a way of avoiding a fatal fall when all other ways fail.
Rind agrees, and she thinks this could explain why Pérez-Miles didn’t find any silken threads in his experiments. He placed his spiders in a shallow tank and never tired to shake them off. They didn’t need their silken lifelines – their setae and claws were good enough.
The ability to spin silk from their feet could have evolved several times among the larger tarantulas, to help them support their large bulk. If that’s the case, the smallest tarantulas (some are less than a millimetre long) shouldn’t have any spigots, and that’s something for other scientists to check. But Rind thinks that this story is an unlikely one.
She worked with three tarantula species– the Chilean rose, the Mexican flame-knee, and the Indian ornamental – that are all distant relatives. It’s more likely that the ancestor of all tarantulas could spin silk from its feet. In fact, it’s even possible that tarantula foot spigots might represent the silk-spinning organs of the earliest spiders. Perhaps they were precursors to the more sophisticated spinnerets, which could have evolved from modified legs.
Reference: Rind, Birkett, Duncan & Ranken. 2011. Tarantulas cling to smooth vertical surfaces by secreting silk from their feet. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.055657
Gorb, Niederegger, Hayashi, Summers, Vatsch & Walther. 2006. Biomaterials: Silk-like secretion from tarantula feet. Nature http://dx.doi.org/10.1038/443407a
Pérez-Miles, Panzera, Ortiz-Villatoro & Perdomo. 2009. Silk production from tarantula feet questioned. http://dx.doi.org/10.1038/nature08404
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