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 used them traditionally to convey moods of suspense, evil and mystery. In doing so, we are all showing the grossest disrespect for one of nature’s most fantastic products: spider’s silk.
Just think: It is said that a web with pencil-thin strands could stop a Boeing 747 dead in its tracks. Strands thinner than your hair are more than five times as strong as steel wire of similar diametre, 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 are 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 they say are 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, most efficient silk factories.
Spider’s silk is actually a protein substance, chemically and molecularly so oriented so as to give it maximum strength and elasticity. It is 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 and 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 its special quality of silk. Seven types of silk glands have been identified, though no single spider, possesses all seven. 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. Upto 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 hooks on the legs of these spiders 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 depending on the 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 this as she goes, making sure it does not snag on anything, then climbs up another suitable post and anchors the bridge line at a suitable spot.)
Affixing the thread to the first spot, the spider runs along this first light line, relaying a tough frame line behind her. Then back she trundles along this bridge line, spinning yet another thread behind her which dangles in a loop beneath the first bridge line. Once this is affixed at both ends, she crawls to the lowest point of the loop, attaches another line here, and lowers herself along it to the ground or some firm anchoring point. She pulls the cord tight and anchors it, 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, she spins more radial spokes and frame threads, laying them across the triangles of the first fork. Using this frame as scaffolding, she radiates outwards from the hub, spinning more radial threads. Now, from the hub she spins a spiral scaffolding till she reaches nearly the edge of the frame. All this has been done using non-sticky silk. Now she turns around and starts spiraling inwards again, laying the deadly sticky thread behind her, tautening it at its 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 a she proceeds inwards. She leaves a “free zone” near the hub, between the sticky spirals and the center itself. Here she will lie in wait, her body held away from the web, her legs coated with a film of oil — just in case. She takes under an hour to spin her web, and does so every evening, consuming the old web in order to conserve protein.
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.
[Source:-.The.Indian.Express]