Do tarantulas owe their apparently gravity-defying climbing ability to silk shot from their feet? Some researchers say they do, but arachnid specialist Rainer Foelix doesn't think so.
Foelix and his team found no evidence of silk in tarantula footprints, the researchers report in the Journal of Experimental Biology.
Foelix also thinks the "alleged" silk-oozing spigots spotted by neurobiologist Claire Rind of Newcastle University in England on tarantula feet are probably hairs involved in taste and motion sensing.
"For me, it was important to contradict [her] paper," said Foelix, who's been studying arachnids for about 40 years. "They never figured out that these 'spigots' looked exactly like chemoreceptors."
Using an electron microscope and looking at several tarantula species, Foelix compared the supposed foot spigots to spigots inside spinnerets, the spiders' silk-spinning abdominal organs. Those in the feet had a socket at the base, a blunt tip, a bent herringbone-patterned hair shaft and an interior rod -- all well-documented chemosensor characteristics, he says.
Spinneret spigots, on the other hand, had a large bulbous base, a long pine-cone-patterned shaft, and a big pore at the tip. He also didn't see strands in or coming out of ribbed hairs from tarantula feet, whereas spinneret spigots had threads in and protruding from them quite frequently.
Foelix's results "clearly demonstrate that these hairs are in fact contact chemosensory receptors," spider researcher Fernando Pérez-Miles of the Facultad de Ciencias in Montevideo, Uruguay wrote in an e-mail. Pérez-Miles doesn't think these ribbed hairs produce silk.
Foelix's team did see thin threads coming out of ribbed hairs in experiments with live tarantulas, but these strands broke easily and were present in very small quantities.
"That's another argument against the silk hypothesis," said biomechanist and study co-author Anne Peattie, then at the University of Cambridge. "If it was helping them climb, you'd expect to see more of it."
The thousands of hairs on tarantula feet provide support, but it's unlikely these tiny threads could make much functional difference, Foelix said. "A few silk threads could never provide a secure foot holding, which is just a weird idea. It’s just sad this has even ended up in the literature," he said.
But Rind disagrees. Foelix's team didn't "make quantitative measurements to come to that conclusion," she wrote in an email.
In an upcoming *Journal of Experimental Biology *study, Pérez-Miles repeated Rind's experiments with plugged spinnerets. Unlike Rind, he didn't see any silk secretions, Foelix said.
For now, no one can say conclusively whether that substance is silk or not, says Peattie. "But we think it's not."
*Images: 1) The Mexican knee flame tarantula (*Brachypelma auratum), one of the species Foelix studied. Ruben Kiel Alvarado/Wikipedia. 2) Scanning electron microscope images of spinneret spigots (left) and ribbed hairs (right). Spigots have a characteristic pine-cone look, while hairs have a herringbone pattern. Foelix/Journal of Experimental Biology.
