Tarantula Spins Silk from Feet
Surprise: a Costa Rican tarantula can spin silk from the tips of its feet. A team of German and American scientists writing in Nature1 coaxed one of these heavy, hairy spiders to walk vertically up glass, and was astonished to find it ejecting silky threads that arrested its slipping and enabled it to cling. They thought that these spiders only used the dry-attachment method that takes advantage of intermolecular van der Waals forces, or the use of hooks on the feet. This discovery of a third method of attachment was “unsuspected,” they wrote:
We induced A. seemanni to walk on vertical glass surfaces in order to observe the contact mechanics of this challenging locomotion. When walking up vertical planes, the spider attached only the distal parts of its tarsi to the substrate. As it started to slip down the glass, silk produced by the tarsal spigots on all four pairs of legs arrested the spider’s descent and allowed it to remain attached to the vertical surface. The spider’s feet were positioned such that the silk-producing spigots were in contact with the glass, while the dense setae in adjacent regions were held off the surface.
The silk material appears similar to that produced by the spinnerets on the abdomen. The finding made them wonder how this came about: “Our discovery of secreted tarsal silk forces a reconsideration of the evolution of spider silks,” they speculated. Did the foot organs come first, followed by the abdominal spinnerets, or vice versa? “Both evolutionary hypotheses are consistent with the homology of legs and spinnerets as arthropod appendages,” they said.
Regardless of whether tarsal silk production is ancestral or secondarily derived, the silk-producing apparatus of spiders seems to be controlled by developmental modules that can be expressed in a variety of body parts.
Investigation of the genes involved in tarsal silk production should resolve whether the original function of spider silk was to increase traction or whether it was later co-opted for that purpose. Spinneret silk proteins are encoded by a gene family that has evolved through a series of gene duplications and subsequent modifications for particular tasks. If tarsal silks belong to the same gene family, then comparison of tarsal and spinneret silks should help our understanding of the ancestral function and composition of spider silk.
The writeup on National Geographic took up on this speculation that silk first evolved as an attachment mechanism for the feet, and later evolved for web spinning. This, however, requires explaining how only tarantulas retained the original function. The article ended with an Oxford scientist noting that protein (of which silk is made) is not cheap for the spider. “Even if you use very little, it still costs energy, and energy is the animals’ money,” he said. “So why put it in the feet unless you really need it?”
See also the Live Science writeup on this story, where a scientist was “completely surprised” to find spiders can do this, commenting, “This research is a great example of how much there is still to discover in the world around us.” The article contains a brief description about how abdominal silk is produced, and a table of interesting facts about tarantulas.
1Gorb et al., “Biomaterials: Silk-like secretion from tarantula feet,” Nature 443, 407(28 September 2006); doi:10.1038/443407a.
Do we really need to spin an evolutionary tale about this? The authors do not know how the intricate machinery for spinning one of the world’s ideal materials evolved at all, let alone whether it evolved first on the legs or abdomen. Here’s another example of forcing observations into a reigning paradigm. The facts merely show that these spiders are exquisitely endowed for coping with a variety of situations they might encounter. Remember that a gland is an organ, and an organ is a collection of tissues with a function, and that tissues are made of specialized cells. It is naive to envision a whole collection of specialized parts coming into existence simultaneously by some blind process of evolution. Realize, too, that it not only takes the equipment, but the know-how and reactions to use it. The spider brain also must have software to quickly turn on the silk production in the feet and simultaneously retract the setae. How many millions of years did that take to get all this right?
Instead of getting distracted by some evolutionary tall tale, focus at the design in these amazing spiders. Look at the handsome markings, the complex eyes, the coordinated walking movements, and all the other structures and functions that come together in a single complex animal that can carry on life in its niche – and reproduce all the parts for the next generation. Did you know a female tarantula can live 20 years? These critters have defensive mechanisms that rightly cause us a little trepidation (often exaggerated), but are really quite attractive creations in their own right. Imagine designing a robot this capable. Science should discover, describe and seek to understand the workings of nature. Fitting the observations into speculations about origins is a job for philosophy and theology, not Nature.