14 DAY TRIAL // Tarantulas are the biggest spiders in the world, included in the Theraphosidae family.
The largest, Goliath bird eater, found on Amazon forest, has the diameter of a plate.
People can be horrified by their appearance, but tarantulas are much less harmful than poisonous spiders. Their venom is much less powerful than that of many spiders.
And they are found only in tropical, subtropical and desert areas.
Tarantulas were already known to produce silk used to protect their eggs and conceal their nests.
But new studies revealed that tarantulas produce sticky silk from their feet to walk on slippery vertical surfaces.
Specialists already knew those silk producing glands opening through ducts called spinnerets on the outside of a tarantula's ( and generally spiders') abdomen.
Spiders were known to produce silk from their abdomen to form webs only for protection and capturing prey.
Stanislav Gorb of the Max-Planck-Institute for Metals Research in Stuttgart, Germany and his colleagues found that zebra tarantulas (Antrodiaetus seemanni) (photo) from Costa Rica secrete tiny bits of silk from nozzle-like structures in their feet to walk up vertical glass surfaces.
These tethers allow the arachnids to scale vertical surfaces.
The discovery forces scientists to reconsider where spiders first produced silk: on their feet or on their abdomen.
"Silk production was long thought to have evolved from glands found on the legs of early spiders." says Fritz Vollrath, a zoologist and expert on spider silk at the University of Oxford in England.
"It's not a surprise in a way that there are actually some [modern spiders] that still, if you wish, spin with their feet," Vollrath said.
"We know that all the extremities of ancestor arachnids probably had this possibility to adhere during locomotion, for example, or during prey capture." Gorban said.
"It is thought that abdominal spinnerets could be vestigial legs," says Todd Blackledge, who works on spider silk at the University of Akron in Ohio.
The spinnerets have jointed segments and have been shown to move in sync with the legs when spiders walk.
"It could mean that silk production actually originated in the feet to increase traction, with the diversity of spinneret silk evolving later," said Adam Summers of UC Irvin.
"Alternatively, the foot secretions may have evolved independently in tarantulas to help the relatively large spiders move around safely," he adds.
Though ground dwelling, these spiders can normally hang on to vertical surfaces by using thousands of spatulate hairs and small claws, which can latch onto rough surfaces for an added adhesive.
"Like geckos (lizards), spiders rely on weak molecular attractions called van der Waals forces generated by tiny hairs on their legs to attach to vertical surfaces," Gorb says.
Tarantulas use these mechanisms but likely add the silk for better traction.
When a tarantula starts slipping, it sends out strands of silk from the ends of its legs.
The tethers put the brakes on the spider's descent, leaving behind what looked like a series of "footprints" consisting of dozens of thin, silk fibers with diameters 10 times smaller than human hair.
Scientists put tarantulas to walk on vertical glass surfaces.
The scientists noticed that when the spider started to slip down the surface, it produced silk from its legs, allowing it to adhere to the glass for more than 20 minutes.
The silk secretions were clearly visible on the glass.
Using scanning electron microscopy, the scientists also were able to see the openings on the legs that resemble the silk-producing spigots on spider abdominal spinnerets.
Gorb's team discovered the tarantulas' silk-spinning abilities by examining glass plates scattered vertically in a tarantula terrarium.
When studied under powerful microscopes, the plates revealed silken remnants where the spiders had walked.
Gorb says that the silk is likely secreted as a fluid that quickly solidifies, so that as the spider steps, the silk comes out as a thread.
These threads tether the spider to the surface and, with every step the spider makes, the threads break in sequence "like peeling off Scotch tape from a surface," Gorb added.
Scientists have yet to discover the mechanism that allows the spider to control silk generation.
And which are the locomotion situations when tarantulas use the silky secretion?
"You can imagine if it's running over the surface, this mechanism will probably not be possible to use, because the silk needs some time to solidify," Gorb said.
If the common ancestor of spiders had spinnerets in its feet, as many scientists hypothesize, then the feature is apparently carried over only in the tarantulas.
One explanation may be the relative weight of tarantulas which weigh on average 0.18 to 0.25 ounce (5 to 7 grams). The next largest spiders are only about 0.07 ounce (2 grams).
To crawl vertically and cling upside down, most spiders use minute claws and pads on their feet or "tarsi".
These work on rough surfaces, but may fail on smooth or dirty ones.
While this is not a problem for small spiders that can survive long falls, for a heavy tarantula a slip could be fatal.
"To my knowledge, no other animals are using silk for locomotion," said Gorb.
As foot silk is found only in tarantulas, it is possible that they are a relatively recent adaptation to supplement the claws and pads.
The zebra tarantulas may need the foot silk to move over large, slippery leaves in their native rain forest habitat in Costa Rica.
"Protein is not cheap," Gorban said (silk is a protein).
"Even if you use very little, it still costs energy, and energy is the animals' money ? So why put it in the feet unless you really need it?"
Another issue is to investigate whether the silk produced by the feet is the same as that produced by the abdomen or not. Spiders can produce seven different kinds of silk (including the new tarsal type).
Genes involved in silk production from the feet will be investigated and so, scientists determine whether silk was originally used for traction or whether that was a secondary usage that came later.
"Testing these hypotheses will require detailed surveys of all spider species," says Summers, "looking for any that might also have silken toes."