The Gecko’s Foot: How Scientists are Taking a Leaf from Nature's Book. Peter Forbes
For those who fear spiders, a web large enough to enmesh a person is the stuff of nightmares. The fear of spiders is a widespread cultural phenomenon, and my interest in the silk made me question my own attitude to spiders. Primo Levi, who is always a good guide to our reactions to the natural world, summed up the symbolism of spiders like this:
The old cobwebs in cellars and attics are heavy with symbolic significance: they are the banners of desertion, absence, decay and oblivion. They veil human works, envelop them as though in a shroud, dead as the hands which through years and centuries built them.
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There is a constellation of factors that creates a general sense of unease. There are very few large, hairy poisonous spiders (the tarantula, despite the legend, is not much more poisonous to a human being than a wasp), but all spiders, by association, share in a little of the horror that these monsters can conjure up. The spider’s snaring and ambushing techniques worry some people. Something as deep-seated as spider phobia most likely has sexual connotations: the fact that the female sometimes consumes the male after mating suggests that men can associate them with women who symbolically castrate, if not devour. But women in particular are sufferers from arachnophobia. One attribute of spiders that would cause unease if it was generally known would be the fact that they have eight eyes. But most people have never seen them because they are only visible through a microscope.
Before I became seriously interested in spider silk, I had realized that not only is the web of the garden spider very beautiful but the creatures themselves, with their light speckled colouring, are much the most comely spiders you are likely to come across unless you become a dedicated arachnologist. Once I learned about the silk, my conversion to spider-worship was complete and I became ashamed of my earlier hostility.
The first documented attempt to exploit spider silk was by a Frenchman: in 1709, Xavier Saint-Hilaire Bon made gloves and stockings from the silk and presented them to Louis XIV. He wrote ‘A Dissertation on the usefulness of spider silk’. The scientist René Réaumur investigated these claims in 1710 and concluded that only egg cocoon silk is good enough for spinning but it lacks lustre (a surprising finding given that all modern research focuses on the dragline silk from which the web is hung). Réaumur estimated that it would need 27,468 female garden spiders to make 1 lb of silk. Despite this discouraging report, the Chinese Emperor requested a copy of his paper and Chinese silk experts attempted to exploit spider silk. In 1876, the Chinese Emperor gave Queen Victoria a spider-silk gown. Whether this had been a century and a half in the making, since their initial interest, we do not know. Despite the impression we have of the evanescence of spider webs, the silk is durable. In Austria in the late 18th century, there was a tradition of painting on spider webs and some of these pictures exist to this day; one of them hangs in Chester Cathedral.
There is no great problem in spinning silk from a single spider. You can just about do it yourself with some improvised kit and patience. First, wait for the webs made by the garden spider, which appear in late July, then catch a spider. The spider has to be restrained, obviously, and a Styrofoam block makes a handy mount. The spider must gently be turned onto its back and a couple of very light rubber bands used to pin four legs on each side close to the body. Once the spider is stable you can start a thread by lightly touching the spinneret under the belly with a glass rod and pulling gently.
If you want to create a reel of silk, the Styrofoam block can be mounted on a piece of wood, with an improvised reel at the other end. This could be a cotton reel on a spindle with a handle. If you happen to know anything about hand-spinning, you could collect a few bobbins of spider silk and try to braid them into a thicker thread that would bring it closer to the dimensions of usable textiles. Whatever you do with the silk, the last stage is to let the spider go, when it will instantly return to work, repairing its web.
Although you cannot see any of this without a microscope, it is worth knowing that a garden spider has three pairs of spinnerets, each with multiple spinning tubes – more than 600 in all (fig. 3.2). Without an explanation, a picture of this apparatus might appear to be some kind of technical glue nozzle system.
The industry of garden spiders is prodigious. When I brought one in to be ‘silked’, its web was damaged in the process, but a few hours after releasing the spider the web had been rebuilt. Spiders keep their webs in good repair. After a few days, a web will become tatty from insect collisions, wind, dust and the spider’s own movements across her domain. Every two or three days, the spider will consume the old web and build a new one, usually in exactly the same place, since they are highly territorial. Around 80–90% of a new web is protein recycled from the old one. This means that a spider catches food mainly to get the energy to build the web; it doesn’t need food to supply much of the material – an example of the amazing efficiency of living processes.
Attempts have been made to silk spiders on an industrial scale. Properly set up, a single golden orb-weaver can produce 300 metres of silk in one session. The problem is that spiders cannot be farmed intensively. They are aggressive, solitary creatures who, if confined in one space, eat each other.
This naturally turns the mind towards the idea of making a synthetic silk. That this might be possible was suggested as far back as 1665 by Robert Hooke:
Probably there might be a way found out, to make an artificial glutinous composition, much resembling, if not full as good, nay better, than that excrement, or whatever other substance it be, out of which the silkworm wire-draws his clew. If such a composition were found, it were certainly an easy matter to find very quick ways of drawing it out into small wires for use. I need not mention the use of such an invention.
For centuries the only way of making silk was with the silkworm. Archaeological evidence has shown this to be an ancient craft, going back to around 2600 BC in China. The silk moth is the only domesticated insect, having lost the power of flight, all pigmentation, and just about any desire to move or to do anything. The silk is produced by the caterpillar to cocoon the chrysalis and for this reason is not as strong as spider dragline silk. But it is a natural product that no synthetic has ever been able to match, although Japanese textile technologists have now come very close.
The basic process is as follows. The eggs are hatched and the caterpillars fed on mulberry leaves. They moult four times before they are ready to spin a cocoon in which the chrysalis will develop. The chrysalises within the cocoons are then killed by steam or fumigation. The cocoon silk consists of two filaments of the silk protein fibroin stuck together by another protein, sericin. To process the silk, the sericin is removed with hot water and the filaments drawn from water and combined to make yarn. The yarn undergoes stretching and is wound onto reels as raw silk.
Because of the finicky nature of the silkworms and the demanding cultivation regime, increasing the production of natural silk is not easy, and silk production has often been threatened by disease. In 1855, silkworms, particularly those in Europe, were afflicted by a parasitic disease called pébrine. This episode is the centrepiece of Alessandro Baricco’s novel Silk, which captures in delicate prose the aura we associate with the fabric:
He felt the lightness of a silken veil dropping onto him. And the hands of a woman – of a woman – drying him all over, caressing his skin; those hands and that material spun out of nothing. He never stirred, not even when he felt the hands move from his shoulders to his neck and the fingers –the silk and the fingers – climb to his lips and brush them once, slowly, then vanish.
Pasteur was called in to solve the pébrine crisis but progress was slow and this seriously focused minds on the possibility of imitating the natural process. At the time, knowledge of the chemistry of silk and all such natural substances was non-existent. Because the caterpillars grew on a diet of the leaves of the white mulberry, Count Hilaire de Chardonnet, who had worked with Pasteur on pébrine,