As tarantulas fall, they appear to react like Spiderman -- shooting threads of silk from their feet (spiders don't have hands).
The move lets them avoid falls that could prove fatal to their hefty bodies, say researchers in a new study.
The work, published in the Journal of Experimental Biology, follows conflicting reports over whether these fuzzy arachnids can indeed eject silk from the ends of their legs. The new study suggests they can, but researchers who found contradictory evidence in earlier work say the debate is still not settled.
The first report was by Stanislav Gorb of the University of Kiel, Germany, and colleagues in 2006, who reported "footprints" of spider silk from all four pairs of legs when the tarantulas slipped on a vertical glass surface. Gorb showed pictures of what appeared to be tiny spigots among the hairs of the spider's feet.
But in 2009, another group argued that the spider silk could simply have come from the abdominal spinnerets -- the typical source -- and have stuck to and been dragged along by the legs as the spiders climbed.
This team, led by Fernando Pérez-Miles of the University of the Republic in Montevideo, Uruguay, sealed tarantulas' abdominal spinnerets with paraffin. The researcher subsequently found no evidence of silk as the spiders climbed, suggesting that the spiders could not make silk from other parts of their body. They also detected no trace of silk glands or channels in dissections of tarantula legs.
In the new work, Claire Rind and a team of undergraduates at the University of Newcastle, U.K., studied three different types of spiders as they climbed in glass tanks lined with microscope slides and surveyed by video. The researchers slowly tilted the tank to a vertical position as the spider held on, and then shook it slightly, until the spider slipped.
Using the video information, they removed any glass slides that were contacted by the abdominal spinnerets during the experiment to eliminate the possibility of contamination. After the experiment, they examined the slides under the microscope and found silk strands at all sites where the legs slipped.
The team also examined freshly molted tarantula exoskeletons and specimens of dead tarantulas under the microscope.
"From those experiments, we actually saw those little spigots and we saw silk emerging from those spigots," Rind said. "In some cases we could see strands of silk emerging from these tall stalks."
Rind suggests that Pérez-Miles' experiments using tarantulas with sealed spinnerets were done in a tank that was too shallow and was not shaken, such that the spiders never actually slipped and needed to use the silk from their feet.
"We are very glad to see such supportive information for the previous work," Gorb, who made the original 2006 report, told Discovery News. "They did hard work showing from which spigots the silk is coming out. This is essential.
"The next step," Gorb added, "is to show the gland, where the silk is really produced."
Pérez-Miles would like to see the study repeated with blocked spinnerets. "It’s the only way to prove that the silk comes from other parts of the animal," he said, noting that the silk could easily waft and stick, even if the abdomen doesn't touch the surface.
"This paper challenges us to continue working," he said, acknowledging the need to repeat his experiments under conditions that force a slip.
Most spiders do not need this ability, Rind and Gorb said. Arachnids use dry-adhesive methods that rely on the molecular forces generated by thousands of tiny split hairs -- similar to the methods used by geckos -- to stick to vertical surfaces. But their proportions give them a greater sticking ability for their size than bulky tarantulas have.
But it's still a mystery why some tarantulas would need the safety net. "It's not clear why spiders that live in the soil in burrows have that supposed adhesive system," Pérez-Miles said.
Rind's findings may get cheers from comic book fans: They make Spider-Man's web-ejecting hands a bit more realistic.
Spider-Man was supposed to have been transformed by a black widow spider, she said. "But perhaps it was a tarantula."