April 28, 2012
In what looks to be the last word on the tarantula foot-silk controversy, Fernando Perez-Miles and David Ortiz-Villatoro take a firm stand with the title of their new paper in the Journal of Experimental Biology: "Tarantulas do not shoot silk from their legs." In our last post, we outlined why Rainer Foelix and his team contend that the supposed silk spigots described by research teams supporting the pro-foot-silk position are most likely chemoreceptor hairs. Now, Perez-Miles and Ortiz-Villatoro also dispute the earlier findings of foot-silk; they contend that the silk those researchers found is actually spinneret silk.
Back in 2009, Perez-Miles, Ortiz-Villatoro and two other colleagues challenged the original claim that the zebra tarantula produces foot silk, reporting that when they sealed the tarantula's spinnerets with paraffin before having it cling to vertical pieces of glass, they couldn't find any silk. When yet another team of researchers claimed to have found foot-silk left behind on glass by four different kinds of tarantula, they suggested that the Perez-Miles team might not have shaken the glass enough. In other words, maybe the spiders didn't feel the need to produce foot silk.
Now, Perez-Miles and Ortiz-Villatoro have more closely repeated the procedure of previous foot-silk finders, with four tarantula species. When they left the spinnerets unsealed, they found silk threads in the "footprints" left behind on the glass. When they sealed the spinnerets, they found none. Their conclusion: the footprints became contaminated with spinneret silk, which easily travels from spinneret to leg because it's so light.
In combination with the facts that no one can find silk glands in the spiders' feet and that the "spigots" have more in common with chemoreceptors than with spinneret spigots, this latest piece of research makes it seem highly unlikely that anyone will find spiders whose feet produce silk.
October 7, 2011
[This post contains later amendments that may be instructive to other science writers. See the later posts on purported tarantula foot silk to see why.]
Three papers published this summer might at first seem unrelated. But read together, they pull the entire arc of spider silk evolution into sharper focus. Two papers indirectly address the evolutionary origins of spider silk production. The other demonstrates that the evolution of silk proteins has been central to spider evolution even after the extraordinary proliferation of silks that made the vertical orb web possible.
First, F. Claire Rind and colleagues reported that at least some tarantulas (which belong to the family Theraphosidae) do indeed secrete silk from their feet. This report appears to settle a controversy that first broke out in 2006, when a team of researchers led by Stanislav Gorb announced that they had persuaded a Costa Rica zebra tarantula, Aphonopelma seemanni, to walk on a nearly vertical surface covered with glass microscope slides. The researchers claimed that as the tarantula started to slip, it left behind “footprints” made up of miniscule silk fibers. If this observation held true, it could have important implications concerning the origin of spider silk production. Like all spiders, tarantulas secrete silk through abdominal spinnerets, small appendages ending in multiple spigots that are the outlets for the abdominal silk glands. Genetic studies have shown that spider spinnerets are the evolutionary descendants of the gill branches of ancient arthropod limbs. If spiders secreted silk from their limbs as well as through their spinnerets, this fact might not only cement the limb-spinneret connection but also suggest new hypotheses for the earliest origins and survival value of spider silk.
The Gorb report left some questions open, however. (more…)