Spider Silk, By Leslie Brunetta and Catherine L Craig

Say "spider" and, if you can bear to think of them, you probably freely associate them with "webs". But most spiders (there are about 40,000 species) don't spin webs at all. Silk, yes, but that's a different matter - because the range of uses to which spiders put their silk goes way beyond webs. Spiders began as burrowers, lining their lairs with silk, and a particularly ingenious contrivance is the hinged trapdoor that the evolutionarily primitive Mesothele spiders still use to shut out the world.

Leslie Brunetta, a science writer, and Catherine L Craig, an evolutionary biologist and spider expert, have combined to explore the diversity of spiders and the lifestyles made possible by their production of silk. The co-authors wax lyrical on the freedom the invention of dragline silk gave to spiders. Abseiling around on their strong lines, evading predators with vertiginous drops, spanning chasms with lines flung out on the breeze, spiders have come far in evolutiuon from their burrowing origins.

Silk has proved one of evolution's most versatile technologies. On the Beagle, Darwin saw hordes of baby spiders (spiderlings) a hundred kilometres from land off South America, floating on silken threads to new habitats. One species constructs a triangular hang-glider from silken threads.

Way out on a limb is the bolas spider, which has the answer to the tricky problem of moths. Moths are good at evading spider capture because their powdery wing-scales fall off on the web. The bolas imitates the sex-attractant hormone of a few moth species, luring them and clobbering them with a large sticky silken wrecking ball.

Many biologists these days are busy making cases for their pet creatures as the model species to demonstrate evolution in action. The reason is the wonderful convergence of genomics and palaeontology. Fossils give you the actual form of a creature at a fairly accurately known date in evolutionary history; the genome gives you a printout of the divergences that demarcate species, with much of the evolutionary evidence still there in fossil genes. Put the two together and evolutionary history comes into focus.

Spiders make good model species because silk defines their lifestyle and its composition is completely defined by the DNA sequence. To create silk requires a protein with many repetitive sequences of the two simplest amino acids, and such sequences are prone to copying errors in replicating DNA. This makes the silk genes mutagenic hotspots. Hence the great adaptability of spiders and their silk.

Spider silk is of more than academic interest. Decades of research have been devoted to the attempt to replicate it for technical use. Spider silk is Kevlar-plus – amazingly strong in tension and highly extensible. At one point, the US military was developing it for flak jackets. Apart from problems in matching the quality of natural spider silk, it is so extensible that it would stop a bullet all right - but at some distance the other side of the body!

In 2002 headlines announced a commercial silk-from-milk project, with spider silk mass produced by genetically engineered goats. The silk would be extracted from the goats' milk and spun. But these synthetic silks were much inferior to the natural product (and because of their aggressive nature, spiders cannot be farmed). The work goes on: in spider silk, nature has acquired the secret of rapid, continuous product improvement - a trick too neat for human technologists not to want to unravel.

Peter Forbes's 'Dazzled and Deceived' is published by Yale

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