Sunday, July 09, 2006

Silken snares

On misty mornings here the rough fields around the house are a universe of galaxies: spider webs made visible by beads of dew. Star-strung spirals suspended on glistening threads. Tangled silk mats in the grass. Silver funnels, with a spider waiting at each funnel's black throat.

This is architecture for the belly, silken snares set for dinner. "What refinement of art for a mess of flies!" exclaimed the great entomologist J. Henri Fabre, in his The Life of the Spider. "Nowhere, in the whole animal kingdom, has the need to eat inspired a more cunning industry."

It is worth getting up early to see the perfection of the spider's work, before wind, rain, birds, and insects wreak their destructive effect. While we slept, the virtuosos of silk were busy. Flinging gossamer strands across the void. Repairing the porches of their burrows. Extending sticky tablecloth traps. And, most spectacularly, spinning spiral webs against the sky.

The spinning of silk is the spider's greatest accomplishment, a tour de force of evolution. Spiders are born with the weaver's talent. Hatchling spiders spin webs that rival the finest work of adults. Says Fabre: "There are no masters or apprentices in their guild; all know their craft from the moment that the first thread is laid."

According to one popular hypothesis, web-building had its origin in the silky line that all spiders pay out behind them wherever they go. Originally, the line served only to help the spider find its way home. After many forays, the mass of threads near the entrance to the spider's shelter proved useful for another purpose: If an insect touched the sheet of resonant silk, the spider was alerted by vibration, and the prey was soon secured. From this accidental door mat, all future webs evolved.

This happened quite some time ago. Fossil spiders with spinnerets (silk glands) on their abdomens are known from the Devonian and Carboniferous periods of Earth history, 300 to 400 million years ago. A recent issue of Science (June 23, 2006) reports the oldest evidence yet of web building, preserved in Spanish amber dating from about 110 million years ago. The amber clearly shows a fly and a mite trapped by strands of spider silk, apparently from a spiral web. This possible instance of orb architecture coincides in the geological record with the explosive diversification of flowering plants and pollinating insects.

In another report in the same issue of Science, researchers analyzed DNA from the silk glands of different lineages of spiders to confirm that all orb-web building had a common evolutionary origin about 135 million years ago. Previously it was thought that different chemical and mechanical orb-building strategies might have arisen independently on at least two occasions -- an example of convergent evolution.

Across millions of years, spiders refined and diversified their craft. A typical garden spider can manufacture as many as eight types of silk, each especially suited for its purpose. Web silk, for example, is different from the silks used for egg sacs and for binding prey.

The familiar orb web of a garden spider consists of two types of silk. The threads radiating from the center are stiff and non-sticky; they provide a strong scaffolding for the web. The circular threads, or capture threads, are elastic and studded with glue droplets; they hold insects fast and stretch without breaking.

The capture threads of an orb web are especially remarkable. As the spider spins these threads it coats the silk with a viscous liquid. Surface tension causes the liquid to contract into droplets, the way a thin stream of water from a faucet breaks up into drops. As the drops coalesce along the thread, some of the silk is gathered up in bunches within the drops. When the thread is stretched, the silk unwinds from the droplets, like tiny key chains on spring-loaded reels, and then pulls tight back into the drops. The result is a product wonderfully suited for holding insects with virtually unbreakable bonds.

Capture threads can contract to a twentieth of their length in the web without sagging, and stretched to 3 times their length in the web without breaking. If human engineers could reproduce these astonishing properties in thread or cable of a larger size, it would be the greatest product to hit the market since Nylon.

The silky galaxies in the morning mist seem even more miraculous when we know the properties of the silk, the subtlety of the engineering, and the deep, deep history of the weaver's talent. Here, in dozens of glistening webs, the spider composes symphonies of silk. Swiveling spinnerets move in concert with agile claws, devising, measuring, laying down lines, practicing an art learned in the company of dinosaurs, perfected in concert with insects and flowering plants, and communicated across tens of millions of generations by the ineluctable agency of genes.

Further Reading

Most of J. Henri Fabre's wonderful books are out of print, but they are easy to find. Any good library will have some. I'll say a few things about Fabre himself in my post tomorrow.

Discuss this essay and more over on the Science Musings Blog.