The secret to walking up walls for most spiders and insects is pretty simple— teeny tiny hairs on the bottom of their feet that cling to a vertical surface. Tarantulas have the same hairs, but they’re also pretty hefty spiders, so just tiny hairs alone aren’t enough to provide the same gravity-defying stickiness. For tarantulas, their secret also involves excreting sticky web material from their feet.
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.