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by JOHN H. LIENHARD
Spider Webs


Barn Spider In Her Web



There was a time when I’d casually brush cobwebs away; now I think twice. For one thing, our garden spiders help control pests. Then there is the wondrous nature of their webs. The strands are a ten thousandth of an inch in diameter. Human hairs have 30 times that diameter and a thousand times the cross-sectional area.  It’s like thread compared with rope.



The ancient Greeks applied cobwebs to wounds and so did 19th-century doctors.  Only in this century did we learn why that works: spiders coat their silk with antiseptic agents.  Nineteenth century astronomers turned to spider silk when they needed finer cross-hairs on their telescopes.  By WW-II, gunsights and bombsights, range finders and transits, telescopes and microscopes all used spider silk.  Demand outran supply.

Spider silk has amazing structural properties.  It’s stronger than steel, yet stretches to 140 percent of its length.  At the same time, it absorbs energy and doesn’t bounce back.  A fly, hitting a web, cannot trampoline off it.  And the silk stays tough at low temperatures.  

Spiders have six spigots for spinning silk, and they mix their fluids to regulate the composition.  They can make one kind of silk for catching flies and another to shape a parachute that'll carry them away on the wind.

A spider might spend an hour weaving its web to trap bugs.  A day later the gossamer cross members will be broken while the main guy lines stay intact.  The spider eats the protein-rich broken web, then spins new silk from the recycled strands.

It’d be a splendid material to use in manufacturing.  People have tried setting up spider-silk farms but spiders don't like being crowded.  Put too many in a closed space, and they solve the problem by eating each other.



If we're to have the stuff of spider silk, we have to synthesize it and nature can help us do that. In 2000, a Canadian company, Nexia Biotechnologies, took over an old farm in Quebec.  There they raised Nubian goats -- ordinary, playful, friendly animals, practically pets. However, they stirred one spider gene into the goat DNA. It was programmed to activate only within the female's mammary glands and only when she was lactating.

Even then, the goats produced seemingly normal, drinkable milk.  A fairly simple process produced an astonishing product from the milk.  Skim off the fat.  Then add salt and a special protein curdles out of the milk. Add water to that protein, and you get essentially the same fluid that's spun by a spider.

This is fairly routine technology, and it's done without messing up the goats.  No Jeff Goldblum turning into a fly here, no Island of Dr. Moreau.  These are the same low-technology goats that have provided milk and cheese for millennia.  It’s not terribly different from processes we use to produce hybrid grain.


Spiny-Backed Orb Weaver
Researchers at Shinshu University in Japan have come up with another angle.  They inject genes of golden silk orb weaver spiders into silkworm eggs.  The resulting silkworms produce a kind of spider-silk-enriched fiber.  The Okamoto Corporation funds the work in the historic town of Nara.  They're marketing strong warm socks made from the fiber.

With substantial supplies of spider silk, more applications arise. We can make remarkable super-light clothing, biodegradable sutures, body armor, ligament and tendon prostheses, any number of super-light structural materials.  (Nexia calls it Bio-steel.)  Here’s a potential replacement for so many environmentally unfriendly fibers.  Spider silk is pure protein.  And it’s the ultimate biodegradable material. 

But back to structure: Last September, I opened the back door one evening and found myself facing a huge spider web, woven across the door. In the middle sat a plump barn spider, or
Araneus Cavaticus. This was the same spider that E. B. White immortalized in his children’s book, Charlotte’s Web.

I backed away, closed the door, and thought about my own personal Charlotte. Next morning she was gone. That night, the web was back. Each night, for six weeks, she sealed us in at bedtime. Then she ate her web at dawn and retired to the eaves of our house to sleep throughout the day.

Sometimes I’d stay up and watch the complex structural engineering of my spider weaving her web.  First she swung down from the gutter on a single strand of silk.  Then she found her way back up, dangling that thread behind her and attached it to another part of the gutter.  Now a single catenary strand hung like a support cable on a suspension bridge.

Next my Charlotte dropped from the bottom of the catenary to the deck below, with a new strand. She anchored it and tightened it to shape the letter Y.  That done, she returned to the middle and spun out new strands, anchoring each to create the spokes of a wheel.  These were all non-sticky strands, and so was the one she spun next.  She returned to the center and began to spin a spiral of Archimedes upon the spokes -- moving outward, around and around.

Now a surprise, she spiraled back in, laying down strands coated with glue, while she ate the previous structural spiral so she could recycle it. When I looked closely at this last sticky spiral, I saw that surface tension had pulled the glue into a string of beads. They’re what catch and hold any errant flying bug.

After a month and a half of this, my friendly spider laid her eggs in her crevice and walked off, never to be seen again. I find I miss her. But she leaves me asking what the difference really is among Gothic cathedrals, skyscrapers, and spider webs. Seen in the language of analytical mechanics it’s hard to see any one as more grand than the other. 

In any case, one kind of structure is less grand when we first look at it. Many of nature’s seemingly random actions prove to be organized in mathematical ways that we’d never suspect at first. Mathematician Krešo Josic has one of those for us to consider. It is foraging and it’s no spider webThough it’s driven by logic, its structure hides within a very random appearance.  



Sources:

Berenbaum, M., Spin Control. The Sciences, Vol. 35, No. 5, September/October 1995, pp. 13-15.

Preston-Mafham, R., and Preston-Mafham, K., Spiders of the World. New York: Facts On File Publications, 1984.

Kamel Salama, UH Mechanical Engineering Department, makes a useful caveat.  It is that, on such a small scale, many materials show enormous strength since they don't have the usual inclusions and imperfections of large specimens.  Extremely small diameter steel wires, for example, will resist higher stresses than will spider silk, even though the breaking stress of larger steel specimens is less than that of spider silk.  The challenge is thus not just to replicate the material, but also to produce it on a large scale without imperfections.

Osborne, L., Got Silk. New York Times Magazine, June 16, 2002, pp. 48-5

See the Japanese genetically-altered silkworms here.

Here may be seen a video promoting the new spider-silk socks.

Here is a fine movie showing construction of the structural portion of the web. It omits the last stage of laying out the sticky fibers. 

Photos by John Lienhard


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