The web of life

Originally published 25 September 2005

On Thurs­day morn­ings I meet my two stu­dents, Greg and Bai­ley, at 8 AM for a walk in the woods or mead­ows. They see things my aging eyes are like­ly to miss. Togeth­er, we don’t miss much.

These bright late-Sep­tem­ber morn­ings the mead­ows are a uni­verse of galax­ies — spi­der webs made vis­i­ble by 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 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 destruc­tion. 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. Hatch­lings 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, per­haps, 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. Fos­sil silk is not pre­served, so we do not yet know when spi­ders first built webs. Indi­rect evi­dence sug­gests that by the ear­ly Cre­ta­ceous, 100 mil­lion years ago, web-build­ing was well advanced.

Fos­sil spi­ders from 100 mil­lion-year-old lime­stones clear­ly show claws adapt­ed for han­dling silk and for loco­mo­tion on spi­ral webs. The same lime­stones con­tain abun­dant insect fos­sils, evi­dence for ample spi­der prey. Even then, in the time of the dinosaurs, spi­ders pre­sum­ably spun traps of exquis­ite del­i­ca­cy — and flourished.

Across mil­lions of years, they 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 the silk 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.

And all of this — the mate­r­i­al, the design, the weaver’s skill — is some­how encod­ed with­in the spi­der’s DNA.

Bai­ley and Greg give me eyes and a fresh sense of won­der. Per­haps I repay them just a bit with a smat­ter­ing of bio­log­i­cal lore. The silky galax­ies in the morn­ing mead­ow seem even more won­der­ful when we know the prop­er­ties of the silk, the sub­tle­ty of the engi­neer­ing, and the 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 the plan­et’s first grassy mead­ows, and com­mu­ni­cat­ed across tens of mil­lions of gen­er­a­tions by the ineluctable agency of genes.