Top Spin: Why spiders are the master spinners of the natural world

Ranjit Lal
The true ustads of silk production are, of course, not insects at all-but spiders. (Source: Ranjit Lal)

It must be one of nature's most astonishing evolutionary inventions, whose patent holders are mainly insects and spiders. A wide range of insects, including caterpillars (of moths and butterflies), beetles, honeybees, bumblebees, wasps, ants, fleas and flies, produce silk, mainly for reasons of protection. Most prominent and prolific among these are, of course, butterflies and moths. From tent caterpillars, which throw huge ghostly shrouds over trees, to the famous silkworm, a single one of which can produce a line of silk nearly a kilometre long, and which we have exploited to the hilt, this astonishing substance consists of two natural protein fibres called fibroin and serecin. Five thousand years ago, the Chinese discovered that the silk spun by the larvae of the Bombyx mori moth, also known as the mulberry silkworm, could be woven into shimmering textiles and a whole industry was born.

The true ustads of silk production are, of course, not insects at all-but spiders. Spiders produced silk long before they began to spin webs, and today, your average garden spider, sitting in her fabulous orb-web can produce six different types of silk from the spinnerets (nozzles) at her rear end - depending on what she wants to use it for. One of the main purposes of a web is to capture prey - and spiders have evolved different types of web for this single purpose. The trapdoor spider digs a silk-lined tunnel in the earth, with a silken trapdoor on top and lies in wait. It senses an approaching victim above, and grabs it, dragging it down into the tunnel and slamming the trapdoor shut. Some will lay alarm or trip lines, which get triggered when a victim stumbles in. Some combine forces and build huge dome webs. The famous bolas spider uses just a single thread, tipped with a sticky knob at its end and swings this around hoping to snare a passing moth (which is seduced into the vicinity by the irresistible scent of a pheromone). The ping bell spider spins its web under water and then carries down bubbles of air trapped in its furry carapace - so that it can breathe underwater.

Once a victim is snared, the spider will rush out to it and swaddle it in a shroud of silk completely immobilising it, so it cannot fight or bite. It'll then sink its fangs in and inject its venom - and, maybe, if hungry, begin to inject digestive enzymes which will liquefy its victim so it can slurp up the "soup". Often, it will just save the straddled victim for another time. There is one species of spider which will mix venom with the silk and spit it at its victim from its mouth, rather like the spitting cobra.

Some spider moms shroud their eggs in cocoons of silk to protect them and guard these zealously. I once observed a lynx spider in the garden that had attached herself to the cocoon in which her eggs were, by a line, which she dragged along behind her - under the leaves, if she sensed a threat.

Baby spiders leave home by ballooning. When it's time to go, they will climb up to a prominent perch, stick their bottoms up and let loose a strand of silk. This catches the breeze and away they go.

Some spiders will attach a "mating line" to their intended's web and twang that rhythmically when they want a date. This reduces the bride's desire to eat the groom - she'll come out to meet him and allow him to make out with her, and, sometimes, get away. If he does get eaten afterwards, it doesn't matter too much because his genes have already got to where required. Other spiders will bring gifts - flies and cockroaches - wrapped up in silk. While the lady unravels her present, he makes out with her and then vamooses (and yes, sometimes, there's nothing inside).

Depending on its use, the properties of the silk differ. The scaffolding - or dragline - silk, used to affix the web and the lines radiating from the hub, is strong and elastic and non-sticky and used by the spider to move around. The spiral silk, used for the actual capture, is glue-coated and elastic, too. The spiral silk, produced by some species of spiders, is truly ingenious: seen magnified, the threads resemble a necklace, with spaced-out beads. Inside each bead of the necklace lies coiled up silk, waiting to be played out when an insect bongs into the web, making it stretch! When the tension is released, the silk thread reels back in, so the web won't sag.

Spider silk is six times as strong as steel wire, weight for weight, and the orb-weaver wastes none of it, eating it when rebuilding its web every morning. It is extraordinarily light, too - the silk used for a large web is one thousandth the weight of the spider.

As for us, we're still struggling to find ways to synthesise spider silk - it could have myriad uses if it could be produced in large enough quantities (bulletproof jackets, sutures, scaffolding). So far, alas, the spider still sits in the middle of her web, guarding her secret jealously.

Ranjit Lal is an author, environmentalist and bird watcher. This article appeared in the print edition on April 21, 2019, under the title ‘Down in Jungleland – Top Spin’.