New Scientist reports:
They have a deadly bite, but a soft footfall. Tarantulas, it turns out, can spin silk with their feet.
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 figure out how tarantulas make their way safely up vertical surfaces, Adam Summers of the University of California, Irvine, and colleagues analysed the footprints of Costa Rican zebra tarantulas (Aphonopelma seemanni) as they climbed a glass wall. This revealed that the spiders left fragments of sticky silk a few micrometres in diameter and up to 2.5 centimetres long.
On looking closely at the spiders' feet the researchers found microscopic spigots that resembled the creatures' abdominal silk-producing spinnerets. "With all the work that's been done on spider feet it's amazing to find something like this. Somehow it has been missed before," says Summers.
The discovery of these structures raises an interesting evolutionary question, as abdominal spinnerets are widely considered to be the remnants of ancient appendages. "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.
So far Summers and colleagues have found foot spigots only in tarantulas, so it is possible that they are a relatively recent adaptation to supplement the claws and pads. Identifying the genes involved in tarsal silk production will help determine whether they evolved to increase traction, or if they were co-opted from an organ with some other function. Testing these hypotheses will require detailed surveys of all spider species, says Summers, looking for any that might also have silken toes.
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