攀登不可能的山峰（2）

CHAPTER 2 SILKEN FETTERS
A GOOD WAY TO ORDER OUR UNDERSTANDING OF ANY living creature is to imagine, fancifully and with something more than poetic licence, that it (or, if you prefer, a hypothetical 'designer' of the creature) faces a chAIn of problems or tasks. First we pose the initial problem, then we think of possible solutions that might make sense. Then we look at what the creatures actually do. That often leads us to notice a new problem facing animals of this kind, and the chain continues. I did this in the second chapter of The Blind Watchmaker, with respect to bats and their sophisticated echo-ranging techniques. Here I shall follow the same strategy in this chapter on spider webs. Notice that the progression of problem leading to problem is not to be thought of as marching through one animal's lifetime. If it is a temporal progression at all the time scale is evolutionary, but it may sometimes be not a temporal but a logical progression.
Our fundamental task is to find an efficient method of catching insects for food. One possibility is the flying swift solution. Take to the air like the prey themselves. Fly extremely fast with the mouth open, aiming accurately with keen eyes. This method works for swifts and swallows, but it absorbs costly investment in equipment for high-speed flying and manoeuvring and a high-tech guidance system. The same is true of the bat solution, which is the nocturnal equivalent using sound echoes instead of light rays for guiding the missile.
A completely different possibility is the 'sit and wait' solution. Mantises, chameleons and certain other lizards that have evolved independently and convergently to be like chameleons make a go of this solution by being highly camouflaged and by moving in an agonizingly slow and stealthy manner until the final, explosive strike with arms or tongue. The reach of the chameleon's tongue enables it to catch a fly anywhere within a radius comparable to its own body length. The reach of the mantis's grappling arms is proportionately of the same order of magnitude. You might think that this design could be improved by lengthening the radius of capture even further. But tongues and arms that were much longer than the body's own length would be prohibitively costly to build and maintain: the extra flies they'd catch wouldn't pay for them. Can we think of a cheaper way to extend the 'reach' or radius of capture?
Why not build a net? Nets have to be made of some material and it won't be free. But unlike a chameleon's tongue the net material doesn't have to move, so doesn't need bulky muscle tissue. It can be gossamer-thin and can therefore, at low cost, be spun out to cover a much larger area. If you took the meaty protein that would otherwise have been used up in muscular arms or tongue, and reprocessed it as silk, it would go a very long way, much further than the reach of a chameleon's tongue. There is no reason why the net should not occupy an area 100 times that of the body, yet still be economically made out of secretions from small glands in the body.Silk is a widespread commodity among arthropods (the major division of the animal kingdom to which both insects and spiders belong).
Stick caterpillars belay themselves to a tree with a single thread of the stuff. Weaver ants stitch leaves together using silk extruded by their larvae, held in their jaws as living shuttles (Figure 2.1). Many caterpillars swaddle themselves in a cocoon of silk before growing into a winged adult. Tent caterpillars smother their trees with gossamer. A single domestic silkworm spins nearly a mile of silk when it builds its cocoon. But although silkworms are the basis of our own silk industry, it is really spiders that are the virtuoso silk producers of the animal kingdom, and it is surprising that spider silk is not more used by humanity. It is used for making precision cross-hairs in microscopes. In his beautiful book Self-Made Man, the zoologist and artist Jonathan Kingdon speculates that spider silk may have inspired human children to invent one of our most vital pieces of technology, string. Birds, too, recognize the good qualities of spider silk as a material: 165 species (belonging to twenty-three independent families, which suggests that it has been discovered many times independently) are known to incorporate spider silk into the fabric of their nests. A typical orb-weaving spider, the garden cross spider Araneus diadematus produces six different kinds of silk from its rearend nozzles, made in separate glands in its abdomen, and it switches between the different types for different purposes. Spiders used silk long before they evolved the ability to build orb webs. Even jumping spiders, who never build webs, leap into the air with a silk safety line attached, like mountaineers roped to their most recent secure foothold.

Silk thread, then, is anciently available in the spider tool-kit, and it is eminently suited to the weaving of an insect-carching net. We can think of a net as a means of being in lots of places at once. On its own scale, the spider is like a swallow with a whale's gape. Or like a chameleon with a fifty-foot tongue. A spider web is superbly economical. Whereas a chameleon's muscular tongue surely accounts for a substantial fraction of its total body weight, the weight of silk in a spider's web—all twenty metres of it in a big web—is less than a thousandth part of the weight of the spider's body. Moreover, the spider recycles silk after use by eating it, so very little is wasted. But net technology raises problems of its own.

A non-trivial problem for a spider in its web is to make sure that the prey, after hurtling into the web, sticks there. There are two dangers. The insect could easily tear the web and shoot straight through. This problem could be solved by making the silk very elastic, but this aggravates the second of the two dangers: the insect now bounces straight back out of the web as if from a trampoline. The ideal silk, the fibre of a research chemist's dreams, would stretch a very long way to absorb the impact of a fast-flying insect; yet at the same time, to avoid the trampoline effect, would be gently buffered in recoil. At least some kinds of spider silk have just these properties, thanks to the remarkably complicated structure of the silk itself, elucidated by Professor Fritz Vollrath and his colleagues at Oxford, and now at Aarhus, Denmark. The silk shown enlarged in Figures 2.2 and 2.3 is actually much longer than it looks, because most of its length is coiled up inside watery beadlets. It is like a necklace whose beads contain reeled-in surplus thread. The reeling in is done by a mechanism not fully understood, but the result is not in doubt. The web threads are capable of stretching out to ten times their resting length, and they also recoil slowly enough not to bounce the prey out of the web.

The next feature that the silk needs, in order to keep the prey from escaping, is stickiness. The substance that coats the silk in the reelingin system we've just been talking about is not just watery. It is also sticky. One touch, and it is hard for an insect to escape. But not all spiders achieve stickiness in the same way. A different group called the cribellate spiders produce multi-stranded silk from a special silk gun called a cribellum. The spider then combs out the multi-stranded silk by passing it through a custom-built comb mounted on the spider's shin. Multi-stranded silk that is 'hackled' in this way puffs out into a tangly thicket (Figure 2.4). The entanglement is too small to see with the naked eye but it is just right for snagging insect legs. Hackled 'cribellate' threads behave as if they were sticky, like the gluey threads that we dealt with before. They just achieve their stickiness in a different way. In one respect, cribellate spiders have an advantage. Their threads remain sticky for longer. The non-hackling, glue-using spiders have to rebuild their gluey web anew every morning. Admit-d tedly-and almost incredibly—this can constitute less than an hour's work, but every minute counts when you face natural selection.

But now, sticky threads pose a new and an ironic problem. Whether coated with glue or hackled into a tangle, threads sticky enough to snare an insect are tricky for a spider herself to negotiate. Spiders have no magic immunity, but evolutionary technology has come up with a mixture of partial solutions to the 'own goal' hazard. The legs of glue- using spiders are anointed with a special oil which provides some protection from the stickiness. This has been demonstrated by dipping spiders' legs in ether, which strips off the oily shield and with it the protection. A second partial solution that spiders have adopted is to make some of the threads non-sticky, namely the main spokes that radiate out from the centre of the web. The spider herself runs about on these main spokes only, using specially modified feet ending in little claws to grip the fine threads. (Male spiders build webs too. For an explanation of my sexist language, see p.40.) She avoids the sticky spiral that winds round and round on top of the scaffolding made by the spokes. This is easy to do, because she normally sits and waits at the hub of the web, so the shortest distance to any point on the web would be along a spoke anyway.
Let's turn now to the series of problems that face a spider in actually building her web.

Not all spiders are the same and, where it matters, I shall take the familiar garden spider Araneus diadematus* as representative. Our—the spider's—initial problem is how to lay the first thread across the gap, say between a tree and a rock, where the web is to be sited. Once the gap has been spanned by that vital first thread, the spider can use it as a bridge. But how to build the first bridge? The pedestrian way would be to walk down, round, and all the way back up again, dragging a line. Spiders sometimes do this, but isn't there a more imaginative solution to the problem? Let's fly a kite. Couldn't we somehow exploit the light and airy properties of silk itself? Yes. Here's how a spider does it if there is enough wind. She releases a single thread with, at its tip, a tiny flattened silken sail or kite. This catches the air and floats. The kite is sticky and, if it happens to land on a firm surface the other side of the gap, it adheres. If the kite does not make a touch the spider hauls it back, recycles the precious silk by eating it, and tries again with a new kite. Eventually a serviceable bridge is thrown across the gap and the spider secures her own end of the thread by sticking it down. The bridge is now ready for crossing.

This first bridge is unlikely to be taut because the length of the thread will be whatever it chances to be: it is not tailor-made for the particular gap. The spider can now either shorten it to serve as one edge of the web; or she might drag it down into a V to form two of the major spokes of the web. The problem here is that, although it could be pulled down into a V, the V is unlikely to be deep enough to make two respectably long spokes. The spider's own solution to this problem is not to change the bridge itself but to use it as a support while she replaces it by a new and longer thread. Here is how she does it. Standing at one end of the bridge, she initiates a new line from her rear end, and fastens it down securely. Then she severs the existing bridge by biting it through, keeping hold of the cut end in her feet. She walks across, supported by the remains of the cut bridge in front, and by her new line which she pays out behind. She is a living link in her own bridge, moving steadily across its span. As for that part of the original bridge that she has already crossed, it has served its purpose, so she eats it. In this astonishing fashion, eating her old bridge as she goes along and creating a new one behind, she crosses from one side to the other. Moreover, her rear end is paying out silk at a faster rate than her front end is eating it. So the new bridge is, in a carefully controlled fashion, longer than the old one. Now securely fastened at both ends, it sags down the right distance to be pulled into a V and form the hub of the web.

To do this, she moves back to the centre of the new bridge and her own weight pulls it from a sagging curve to a taut V.The two arms of the V are well placed to make two of the major spokes of the web. There is little doubt about which is the next spoke to build. Clearly it would be a good idea to drop a perpendicular down from the point of the V in order to secure the future hub in place from below and keep the V taut even when the spider's own weight is not at the point. The spider fixes a new thread to the point of the V, and reels herself down like a plumb-line to the ground, or some other suitable surface, where she fastens the vertical thread. The three major spokes of the web are now neatly in place, and it looks like a Y.