Tarantulas eject silk from feet
Tarantulas eject silk through their feet to anchor themselves to slippery, vertical surfaces, say scientists.
The discovery, published in the Journal of Experimental Biology, shows how these large but very fragile spiders avoid potentially lethal falls.
The team designed an experiment to dislodge the spiders' feet slightly, by gently shaking a glass tank as they climbed up the side.
Examining glass slides from under the tarantulas' feet revealed their secret.
Like all spiders, tarantulas have attachment hairs on their feet. But, for very large tarantulas, this system for sticking is stretched to its limit.
End Quote Dr Claire Rind University of Newcastle
We saw up to 30 silken threads at the point where the tarantula's foot slipped”
The idea that they had this Spiderman-like ability was first proposed in 2006 by researchers in Germany, who published a paper on the topic in the journal Nature.
But this was quickly refuted by another group that claimed the silk came from the spiders' spinnerets - their specialised silk-spinning organs - and had simply brushed on to the tarantulas' feet.
So Claire Rind and her colleagues from the University of Newcastle set out to design an experiment to test the theory.Shaking spider
"They're awesome creatures, and they have this beautiful way of moving that really fascinated me," she told BBC News.
She and her colleagues designed an experimental "arena" for their three Chilean rose tarantulas. This consisted of a glass tank with microscope slides stuck to the bottom.
They placed each tarantula on to the floor of the tank and upended it so that the spider was suspended from its feet on the vertical glass pane.
The crucial test was to find out what happened when they made the spider feel unsteady, so they gave the tank a very gentle shake.
"We couldn't see any traces of silk with the naked eye," said Dr Rind, "but when we removed the slides and examined them under the microscope, we saw up to 30 silken threads at the point where the tarantula's foot slipped.
"So our experiments to dislodge the spider made it release silk through its feet."
The next stage of the experiment was to make sure that the silk had come from the spiders' feet.
This is where Dr Rind's own pet Mexican flame knee tarantula, named Fluffy, came into the study.
Tarantulas moult their skins periodically as they grow and Dr Rind had collected all of the Fluffy's moulted skins.
She examined these - along with moults from the Chilean rose spiders involved in the study and a moult from an Indian ornamental spider - with a powerful electron microscope and saw tiny silk-producing structures, or spigots, sticking out beyond the microscopic hairs on the spiders' feet.
Looking at the tarantula family tree, Dr Rind found that the three species involved in the research were very distantly related.
"So it's likely that all tarantulas produce silk threads from their feet," she said.
"Tarantulas are quite a primitive, ancient species."
They seem to produce just two types of silk - one from their spinnerets and one from their feet.
"Many modern spiders' spinnerets produce several different types of silk, with several different functions."