A diabolical trap

Man is trying to produce silk as versatile as the one that a spider spins. So do not destroy any cobweb that you see, says RANJIT LAL. Sit back and watch a masterpiece of architecture at work.

WE scream whenever we walk into one, we brandish the broom ruthlessly in our homes so that not a trace of them remains anywhere, and filmmakers have traditionally used them to convey moods of suspense, evil and mystery. In doing all this, we are showing gross disrespect to one of nature's most fantastic products - spider's silk.

Just think. A web with pencil-thin strands can stop a Boeing 747 dead in its tracks. Strands thinner than your hair are more than five times stronger than steel wire of similar diameter, and can stretch by over 30 per cent without snapping. Nothing we have produced - nylon, Kevlar or anything else - comes even close. Of course, we already have our greedy eyes fixed on what mass production of spider's silk could do for us. Bulletproof jackets, everlasting shoes, improved surgical sutures, bridge suspension cables, seat belts, bumpers for cars, parachute cords, artificial tendons and ligaments - the list grows as do the dreams.

Architects are studying web geometry and trying to figure out how webs absorb huge impacts and even when partially destroyed, do not collapse. And already, the silk producing genes have been patented.

Scientists are busy trying to synthesise the stuff - without much success - and are using bioengineering gene splicing techniques to produce it, using bizarre ideas. They are trying, for example, to make silk with the help of the milk of transgenic goats.

The spider silk genes have been spliced into cells taken from the udders (which are said to be somewhat similar to the spider's spinnerets!) and worked so well here that scientists could produce high quality silk through cell culture.

Now the gene has been put back into a species of goat and the challenge will be to extract the pure silk protein from the milk and spin it into fabric in much the same way as artificial fabrics are made from petrochemical solutions. But so far, spiders run the best, and the most efficient, silk factories.

Spider's silk is actually a protein substance, chemically and molecularly oriented to give it maximum strength and elasticity. Squeezed out of the spider's spinnerets - the openings of the silk glands - in liquid form (rather like toothpaste is squeezed out of a tube), it solidifies on contact with the air. The more the spider stretches it, the stronger it gets. The silk glands may occupy the entire floor of the spider's body and are of various kinds, each producing a special quality of silk.

Seven types of silk glands have been identified, though no single spider possess all types. Thus, there are special glands that produce silk for wrapping up prey, for setting up the framework of the web (the "walking lines"), for safety lines, for egg sacs and, of course, for producing the sticky trap threads. Up to four pairs of spinnerets release the silk, and some spiders are equipped with a special plate (called the cribellum) just in front of the spinnerets, which contain a row of openings through which many strands of extremely fine silk are produced. This is used, together with special hoods on the legs to produce those zigzag "hackled bands" on the web that serve as visual warnings to birds.

While cobwebs in the attic may be unsightly, an orb web pearled with dew at dawn is every nature lover's (and photographer's) delight. So how is this masterpiece of architectural engineering constructed? The spider follows a fixed routine - not using rational logical intelligence, we are told - but following instincts that may be 200 million years old, and adjusting its instinctive blueprint to ground reality.

The first line to be laid down is the most important; it is the topmost bridge line from which the whole web will be suspended. The spider crawls up to a suitable perch and unravels a thread of silk that catches the breeze. It drifts away and, hopefully, snags and sticks on a twig or branch a little distance away. (If not, the spider fixes one end of the silk on the initial post, climbs down unraveling the line as it goes, making sure it does not snag on anything, then climbs up another suitable post and anchors the bridge line at a suitable spot).

After fixing the thread, the spider runs along this first light line, relaying a tough frame line behind it. Then back it trundles along this bridge line, spinning yet another thread behind which dangles in a loop beneath the first bridge line.

Once this is affixed at both ends, it crawls to the lowest point of the loop, attaches another line here, and lowers itself along it to the ground or some firm anchoring point. Then the cord is pulled tight and anchored, thus completing a basic Y shaped frame that comprises the primary radii of the web and whose centre is the hub.

In much the same way as this "first fork" was made, the spider spins more radial spokes and frame threads, laying them across the triangles of the first fork. Using this frame as scaffolding, it radiates outwards from the hub, spinning more radial threads. Now, from the hub, it spins a spiral scaffolding till it nearly reaches the edge of the frame. All this has been done using non- sticky silk.

Now the spider turns around and stars spiralling inwards again, laying the deadly sticky thread behind, tautening it at the points of attachments to the spokes and releasing it with a twang which breaks up the glue into tiny beads - deadly for any insect that touches it. The spiral laid earlier is consumed as the spider proceeds inwards. A "free zone" is left near the hub, between the sticky spirals and the centre. Here the spider will lie in wait, its body held away from the web and legs coated with a film of oil - just in case.

The spider takes under an hour to spin the web and does so every evening, consuming the old web in order to conserve protein.

Not all spiders spin orb webs. Some collectively erect massive hammock and sheet webs into which crawling and jumping insects trip and get hopelessly entangled. (These use non-sticky thread). Invisible for the most, these webs are best appreciated on dew- laden mornings. Some spiders have of course, dispensed with web- building altogether. Gladiator and ogre-faced spiders throw sticky silken nets over unsuspecting prey as they pass beneath them; the deadly bolas spider swings a deadly blob of glue suspended on a silk thread at a passing moth and snares it. To improve their chances, it is thought that these spiders wear and spread perfumes that are irresistible to their victims.

One day, no doubt, we too will produce silk as good as spiders do. But till then at least, if you do walk into a web, do not scream and hack it to shreds with your stick. Back off, sit down, and watch this diabolical trap at work.

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