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.
The next two tasks are to put in the rest of the spokes radiating out from the centre, and the outer frame round the edge. The spider often ingeniously manages to combine the two at the same time, using staggeringly cunning techniques of wielding double and even triple threads, which are later dragged apart as the spider walks along the existing spokes. In the original draft of this chapter I explained exactly how this cats-cradle wizardry is performed but it made my head spin to do so. When one of my editors complained that it made his head spin to read it, I was reluctantly persuaded to leave it out. The upshot of this phase of the spider's operation is a complete wheel with twenty-five or thirty spokes (the number varies from species to {45} species, and from individual to individual), and the basic skeleton of the web is in place. But the web is still, like a bicycle wheel, mostly empty space which a fly could pass right through. Even if the fly did hit one of the threads it would not be caught because they are not sticky. What is needed now is lots of threads passing across the radial spokes. There are various ways in which these could be inserted. For instance, the spider could deal with each gap between spokes in turn, zigzagging from side to side as she makes her way from the hub to the rim, then turning and filling the next gap, and so on. But this would involve numerous changes of direction, and changes of direction waste energy and time. A better solution is to go round and round the web in a spiral, and this is what spiders mostly do, although they also double back occasionally too.
But, whether you zigzag or spiral, there are other problems. Laying the sticky thread that is actually going to do the business of catching insects is a precision matter. The spacing of the mesh must be just right. The junctions with the radial spokes must be deftly positioned so that the spokes are not pulled into an ugly mess leaving holes for prey to fly through. If the spider tried to achieve this delicate positioning while balancing on the spokes alone, her own weight would be likely to pull them out of true, and the sticky spiral thread would be joined in the wrong place with the wrong tension. Moreover, near the outer rim of the web the gap between spokes will often be too big for the spiders legs to span. Both these problems might be reduced by starting the spiral at the hub and working outwards. Near the hub the gaps are narrow, and the spokes less liable to distortion by the spider's weight because they support each other. As you spiral outwards the gaps between spokes necessarily widen, but no matter: as you come to lay each ring of the spiral, the previous, inner ring offers bridging support between the widening gaps. But the trouble with this idea is that the type of thread that is good for catching insects is very thin and elastic. It doesn't offer much support. When the whole spiral is eventually in place the web is quite robust but, during the construction process, we are talking about an incomplete and therefore weak web.
This is the main problem with laying the fine, capture spiral, but it is not the only one. Remember that, although the radiating spokes are {46} non-sticky and relatively friendly to spiders’ feet, we have now moved on to talk about the sticky silk which is specifically designed to trap prey. We have already seen that spiders are not totally immune to the stickiness of their own webs. Even if they were, using each turn of the spiral as a bridge while building the next turn might rob it of some of its precious stickiness. So, although it seems like a neat idea to build the sticky spiral from the hub outwards, walking on the previous ring, there may be, literally and metaphorically, a catch.
The spider is equal to these difficulties. Her solution is one that might occur to a human builder: temporary scaffolding. She does build a spiral from the hub outwards. But it is not the final, sticky gossamer, trapping spiral. It is a special, ‘auxiliary’ spiral which she uses once only, to help her subsequently build the sticky spiral. The auxiliary spiral is not sticky, and it is rather more widely spaced than the eventual sticky spiral. It wouldn't do for catching insects. But it is stronger than the sticky spiral will be. It stiffens and supports the web, and it gives the spider safe conduct between spokes when she finally comes to build the authentic, sticky, spiral. The auxiliary spiral takes only seven or eight turns round the web to reach from hub to rim. Having completed it, the spider switches off her non-sticky silk glands and unmasks her serious batteries: the spigots that specialize only in deadly sticky silk. She retraces her spiral steps back from rim to hub, moving in tighter and more uniformly spaced coils than on the outward journey. She uses the temporary, auxiliary spiral not only as scaffolding and support but as a sighting (actually feeling) guide. And as she goes she cuts the auxiliary spiral stage by stage, after each stage has served its purpose. As each spoke is crossed, the new, fine, sticky spiral is carefully joined to it, often in an elegant junction reminiscent of those in chicken-wire or in a fisherman's net. The temporary scaffolding is not wasteful of silk, by the way, because the fragments of it remain attached to the spokes where they are later eaten, along with the rest of the web when the spider eventually dismantles it. She doesn't eat the auxiliary silk immediately, presumably because to detach the individual fragments from the spokes would waste time.
When the spider reaches the hub on her in-going spiral journey, the web is all but complete. There is some adjusting of tensions to be {47} done: a skilled, precision job rather like tuning a stringed instrument. She stands at the hub and tugs gently with her legs to feel the tension, makes any little lengthenings or shortenings that seem necessary, then turns and repeats the manoeuvre from another angle. Some spiders knit a complicated piece of crochet work around the hub, which may be used to fine-tune the tension in the web.
Mention of stringed instruments prompts a masculine digression. I have referred to spiders as ‘she’ in this story, not because males don't build webs — they do, and even new-born spiderlings can build miniature webs — but because females are larger and more prominent. Couple the larger size of females with the fact that spiders, of any age or sex, tend to eat anything smaller than themselves that moves, and it does raise problems for males. Spiders are food for beetles, ants, centipedes, toads, lizards, shrews and many birds. Whole groups of wasps are specialized to catch nothing but spiders and feed them to their larvae. But probably the most important predators of spiders are other spiders, and they are no respecters of species boundaries. Any spider who ventures on to the web of a larger spider is in mortal danger, but this is a danger that a male must face if he is to do what he has to do.
Exactly how the male copes with the problem varies from species to species. In some cases he wraps a fly in a silken parcel and presents it to the female. He waits until her fangs are safely sunk in the fly before he goes to work on her with his sexual apparatus. Males without a fly parcel may be eaten. On the other hand, males sometimes get away with presenting an empty parcel, or snatching the food out of her jaws and absconding with it after mating, perhaps to offer it to another female. In other species, the male relies on the fact that, immediately after a spider has moulted and before her new shell has hardened, she is more or less defenceless. This is when, if ever, there is a tide in the affairs of male spiders, and in several species copulation takes place at no other time than immediately after the female has moulted and she is soft and compliant, or at least disarmed.
Other species use a more appealing technique, the one that prompted my digression. Web spiders inhabit a world of silken tension. Si
lk lines are like extra limbs, questing antennae, almost like eyes and ears. Events are perceived via a language of tightenings and {48} loosenings, stretchings and relaxations, shifting balances of tension. The females heart-strings are of taut, well-tempered silk. If a male wishes to woo her and avoid, or at least postpone, being eaten, he had best play upon those strings. Orpheus himself had not better cause. In some cases the male stations himself right at the edge of the female's web and plucks the web as one might pluck a harp (Figure 2.5). This rhythmic twanging is something that no insect prey ever does, and it seems to propitiate the female. In many species the male increases his distance from the females web by attaching to it a special ‘mating thread’ of his own. He plucks this special string, like a jazzman with a one-string tea-chest bass. The vibrations are transmitted along the mating thread and resonate around the female's web. They suppress, or delay, her normal urge to feed, and they lure her to walk out along the mating thread to the source of the twanging, where mating takes place. The end of the story is not always happy for the male's mortal body, but his immortal genes are by now safely stowed away inside the female. The world is well supplied with spiders whose male ancestors died after mating. The world is bereft of spiders whose would-be ancestors never mated in the first place.
Before leaving this discussion of sex and silk, make what you will of the following story. There are species of spider in which the male ties
Figure 2.5 Discretion: male spider with mating thread attached to female's web. {49}
the female down, Gulliver-like, with silk before mating with her (Figure 2.6). One is tempted to guess that he is taking advantage of the temporary eclipsing of the females prey-catching instinct by her sexual drive, roping her down so that he can make good his escape when her feeding urge returns. I tell the tale that I heard told: the fact is that, after mating, the female has no difficulty in shaking off her fetters and striding off alone. Perhaps the ritual bondage is a symbolic vestige of an ancestral roping in earnest. Or maybe the female is hampered for just long enough to give the fleeing male a head start. He cannot, after all, want her to be tied down forever: she has to be free to lay her eggs, otherwise the whole dangerous enterprise was genetically in vain.
Let's return to the main topic of orb webs and how they are built and used. We left the web-building spider in the centre of her web at the end of the construction process, giving her attention to the fine-tuning. To carry on with our list of problems and solutions, a mesh that is fine enough to catch insects is too fine to allow the spider herself to pass from one side of the web to the other. The long detour to the edge of the web is often avoided by the simple device of a ‘free zone’. This is usually a ring around the hub left free of sticky spiral thread. In some species, for instance of the genus Zygiella, the web has a single radial segment left vacant. Although I have introduced this hole as if it were a conduit from one side of the web to the other, it may be less important for this purpose than you might think, for Zygiella does not sit at the hub as many spiders do. She sits in her own
Figure 2.6 Male spider tying larger female down with threads. {50}
tubular retreat off to one side, for a reason which brings me to my next spider problem.
Spiders, as we have seen, are not invulnerable to being eaten themselves, for instance by birds. Except in certain angled lights, or when dew-spangled, a web itself is quite hard to see because it is so fine. Its constructor, sitting four-square in the middle, is usually its most conspicuous feature. When you are fat and conspicuous to birds, there is much to be said for sitting off your web. On the other hand, it is in the nature of a spider's way of hunting that she sits and awaits prey for long periods, and the hub is the obvious place to sit because it is the junction of all the arterial trunk roads of non-sticky silk. Such a dilemma invites compromise, and different species compromise severally. Our Zygiella female may sit off the web, but she is never far from the centre of things. She keeps in touch by a special signal thread running from her retreat to the hub. The signal thread is under tension and it instantly transmits vibrations to the waiting spider. She races along the signal thread to the hub at a moment's notice, and from there up whichever arterial spoke will take her closest to the struggling target. The signal thread lies right down the middle of the open segment that I have already mentioned. To re-open the question of why it is open, perhaps the presence of a ladder of sticky threads would hamper her in her lightning dash to the centre of the web. Perhaps the signal thread would transmit vibrating messages less efficiently if hamstrung with crosswires.
To sit completely off the web is the compromise chosen by Zygiella, who doubtless pays the price of being a little slower off the mark when prey is struggling in the web (if you wonder why speed is important, we'll come to that soon). Another compromise is to sit it out at the hub, but to try to look as inconspicuous as possible. Spiders often build a dense mat of silk at the hub, behind which they can hide, or against which they can appear camouflaged. Some webs have a stripe or stripes of extra-densely zigzagging silk which might divert attention from the spider herself, lurking in the middle of it (but it has alternatively been suggested that such stripes actually are part of the spiders apparatus for fine-tuning the tensions in the web). Some spiders build extra silken ornaments into the web which look a bit like ‘false spiders’, and it has {51} been suggested that they serve to deflect the pecks of birds. It's also been suggested, however, that they work in a very different way. They reflect ultra-violet light (invisible to us) in such a way that, to insect eyes, they might look like patches of blue sky or, in other words, holes.
I've made mention of a spider's need to race to the scene as soon as an insect is caught in the web. Why bother? Why not just wait until the insect ceases to struggle? The answer is that struggling by insects is often effective. They do sometimes manage to break free, especially big, strong insects like wasps. And even if they don't break free they can damage the web while trying. How to prevent an insect struggling, once initially caught, is the next of our spider problems.
The basic solution is brutally simple. Rush to where the insect is and bite it to death, guided by the vibrations from its struggles. If the insect stops struggling for a moment while you are searching for it, attempt to locate it by plucking radial threads and feeling, from the tensions of the different threads, which one is loaded down by an insect. Once the prey is reached, grapple with it and try to administer a lethal or paralysing injection of nerve poison. Most spiders have sharp, hollow fangs with poison glands (a few, such as the famous black widow, are dangerous to us but the majority of common spiders can't penetrate our skin and, even if they could, they don't have enough poison to harm a big animal). Once she has sunk her poison fangs in the prey, the spider usually hangs on for up to several minutes, waiting for the struggles to cease.
I described venomous biting as the basic method of subduing a struggling victim but it is not the only method. Most of the others — as we have come to expect of spiders — involve silk. Even before delivering the bite, most web spiders wrap some extra silk around the victim to supplement the web silk that will already be entangling its limbs and body. If the prey is dangerous like a wasp the spider usually smothers it in silk, swaddling it round and round, then finally pierces the white shroud with its fangs to give the poisoned coup de grâce.
Butterflies and moths, with their huge, scaly wings, present a special problem. The scales readily slough off. If we handle a moth our fingers become dusted with a fine powder made of scales. Shedding scales helps moths escape from spider webs, for powdering seems to neutralize {52}
Figure 2.7 Ladder webs,
independently evolved:
(a) from New Guinea,
(b) from Colombia.
the stickiness of the threads. Moths when in danger typically fold their wings and drop to the ground. Whether for this reason or simply because their wings are still partially noosed so that they can't fly, when a moth escapes from a web it will
often do so by falling. This opens up a new avenue of opportunity for spiders, one that has been seized.
Michael Robinson, now Director of the National Zoo in Washington, and his wife Barbara, discovered a remarkable web in the jungle of New Guinea (Figure 2.7a). The New Guinea ladder web is basically an ordinary orb web, but with the lower side of the web stretched into a yard-long, vertical strip. The spider sits at the hub near the top. When a moth hits an orb web it has a good chance of falling free. But the New Guinea ladder spider has provided a long depth of web into which the moth can tumble. Further fluttering all the way down the web helps to use up the scaly powder and increases the chance that the moth will be detained for long enough to allow the spider to race down the ladder and thrust home its lethal bite. Soon after the Robinsons’ discovery in New Guinea, their colleague William Eberhard found a New World equivalent (Figure 2.7b) {53} in Colombia. That this ladder was invented independently of the New Guinea one is attested by a difference of detail: its hub is at the bottom of the ladder rather than the top. But it works in just the same way and apparently for the same reason: both species specialize in eating moths.
Climbing Mount Improbable Page 5