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spider silk

18 march 2011

"No matter where you are at this moment, you can probably spot some spider silk," begin Leslie Brunetta and Catherine Craig in Spider Silk (ix). And they're right: two frothy webs, each enfolding a dead housefly, are draped in the corners of the window right next to me as I type. This is perhaps better evidence of my sluttish housekeeping than of arachnid ingenuity, but really both are involved. Fast as anyone can clean them away, spiderwebs reappear anywhere prey congregate; and unless you douse your home with chemicals, prey tend to present themselves for spiders.

An entire book about spider silk? Not really. Spider Silk is really about how spiders, their equipment, and their behavior have evolved over 400 million years. Much of the book consists of basic exposition of the mechanisms of evolution. And much is in implicit dialogue with advocates of "Intelligent Design."

A common orb web is a fantastically intricate thing. Geometrically calibrated to provide an optimal insect trap for the least spider effort, the orb web is one of the most perplexing achievements of nature. Intelligent-design promoters seize on such items as examples of "irreducible complexity": how can a creature with a brain smaller than the dot on one of these i's I'm typing build a structure that no human engineering can reproduce?

Orb webs are minimalist in design; it's no problem for us to draw one. What we can't replicate is the silk itself, the topic of Brunetta & Craig's essay. The proteins that comprise spider silk are beyond human fabrication, almost beyond human understanding. And the little spiders in my windows, cobweb-weavers, spin several different kinds of silk: tough, springy, gluey as need be, all equally unfathomable.

Surely a benevolent Designer created . . . actually, surely we don't need to address that suggestion in a popular science book in the year 2010? If life could evolve to begin with, then surely any complication that life presents could equally well have evolved. Yet spider scientists, writing for Yale University Press, seem to be obligated to spend many pages on a basic review of evolutionary theory, and to present their claims as rebuttals of the unrebuttable.

Brunetta and Craig take the reader down an arachnid cladogram, without ever mentioning the word "cladistics" or fuzzying their prose with esoteric terminology. Here too, I am a little disappointed at the need to avoid technical language. Technical terms are sometimes clearer than ordinary language. No matter; it's clear what they're doing. But it's not clear that an uninitiated audience is going to be convinced by it. So why play to the uninitiated?

I suppose, however, it's entirely possible that someone will pick up Spider Silk because of its cool topic – maybe even after seeing a certain blockbuster comic-book-movie series – and thereby get their first introduction to Darwin and Wallace, Mendel, Crick and Watson. So much the better!

Back to cladistics. The thousands of living species of spider fall roughly into a few major groups. Mesotheles spin rudimentary silk and crouch in burrows waiting to get lucky. Mygalomorphs, including many lovely tarantulas, are burrowers too, but have a less-segmented body and more active habits. Araneomorphs spin true webs, thanks to amazing "major ampullate" silk that can support their entire weight on a thread. But some make do with the simplest designs, and others spin the artistic orbs we are so fond of (or so creeped out by when we walk through them to get into the carport in the morning). But even further down the clade are my friends in the window, whose chaotic weaving represents a departure from a more geometric past, and is a better coping strategy with the ways of their typical prey and predators.

All spiders share a common ancestor; so do mygalomorphs and araneomorphs; so do all araneomorphs; so do all cobweb spiders. By moving from node to node as these spiders become "more evolved," Brunetta and Craig present an elegant picture of the spider clades (groups that share a common ancestor apart from the rest of the larger family). But they are careful to note that all living spiders are by definition "fit," and have coped with natural selection in ways that have produced their current families – just as you may have cousins on several continents, with their several languages and cultures, all equally flourishing at the present moment.

In other words, this is a really good volume, even if you are acquainted with some of its very general background material and want to flip through those chapters quickly. The authors elaborate the evolution of the spider in ways that are current, artful, and entertaining. Some book!

Brunetta, Leslie, and Catherine L. Craig. Spider Silk: Evolution and 400 millions years of spinning, waiting, snagging, and mating. New Haven: Yale University Press, 2010.