[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…)