The Silk Underfoot

In a study published online September 27 in Nature, Stanislav Gorb and colleagues report evidence that Costa Rican zebra tarantulas, Aphonopelma seemanni, secrete silk from their feet. They propose that the material helps the spiders cling to smooth vertical surfaces. The paper's title takes the more cautious form: “Silk-like secretion from tarantula feet.”

That small anatomical relocation carries a large question. Spiders are known for producing silk from abdominal spinnerets, using it for such tasks as prey capture, protection, reproduction and dispersal. A silk-producing foot would add both another location and an intriguing job: traction.

Following the footprints

The researchers studied adult females obtained from pet shops in Stuttgart, Germany. Their supplementary methods describe filming the animals walking in glass dishes, then turning a dish vertically to examine traces at the feet. They also collected filament-bearing footprints on glass coverslips attached to a vertical surface.

Crucially, they say they characterized only traces they had observed under a binocular microscope to originate from the leg surface. This was not simply a matter of finding a thread somewhere near a spider and promoting coincidence into anatomy.

The published figure documents fibres left as a spider slid down vertical glass, traces from a foot on a coverslip, and microscopic structures the researchers identify as silk-producing spigots. To examine the filaments with cryoscanning electron microscopy, they pressed a foot against glass, pulled it away and rapidly froze the sample. The method was intended to avoid distortions caused by drying.

Together, these observations underpin the team's interpretation of a foot-based attachment mechanism. They do not turn every tarantula foot into a demonstrated silk factory. This is a report about one species under laboratory conditions, not a census of spider capabilities.

A new job, or an old one?

The evolutionary temptation is delightful: perhaps silk began not as a trap but as a way to keep one's footing. Yet a present-day function does not supply its own birth certificate.

Nature's accompanying editorial summary frames the alternatives as a question for genetic investigation: did silk originally increase traction, or was it later recruited for that purpose? The climbing observations alone cannot decide which history occurred. A thread can connect a spider to glass without connecting us directly to its remote ancestors.

The coherenceist lesson is not that an exception automatically overturns a framework. It is that a useful framework must remain open to evidence arriving from the wrong address. Abdominal spinnerets do not become imaginary because researchers report a possible additional source at the feet. The account becomes more demanding: identify the material, establish its origin and function, and test how broadly the finding holds.

There is room here for both delight and restraint. A familiar animal may have offered an unfamiliar answer to an ordinary problem. We need not settle the origin of spider silk to appreciate the possibility that, for this large and rather vulnerable climber, invention is happening underfoot.