How Spiders Took Over The Sky

Despite a modern spider's impressive offensive arsenal of venoms and trapping silks, defensive, predator-evading "dragline" silk is perhaps its most valuable asset. In this week's excerpt, authors Leslie Brunetta and Catherine Craig discuss the incredible silk's evolution and ingenious implementation.

A spider sits motionless at the edge of a bookshelf. It senses a sudden shift in the air around its body, the riffling of its sensory bristles putting it on alert. Its eyes discern a huge object sweeping toward it. But before the approaching hand slams down on it, the spider dives over the edge of the shelf, descending on a shimmering filament of silk. The hand, not fast enough to intercept the dive, grabs the silk and jerks it upward. But the spider reels out more and more silk, plummeting until it lands on the floor and sprints for cover.

Often, something that seems to come out of nowhere triggers unforeseen consequences. For spiders, this event was the appearance of major ampullate silk. Major ampullate, or dragline, silk is the rappelling rope spiders depend on as they plunge through the air. It is one of the toughest materials on earth, able to withstand great stress and absorb immense amounts of energy without rupturing. Materials scientists, mechanical engineers, biochemists, arms manufacturers, surgeons, and fashion designers hanker after the secrets locked in its amino acid chains. Birds filch webs containing major ampullate silk to bind together the materials in their nests. Humans have been stealing spider webs for thousands of years. Traditional peoples of the South Pacific gather it to make fish seines, fishing lines, and waterproof hats and bags. And until platinum filaments and improved glass engraving replaced them in the latter part of the twentieth century, major ampullate threads made ideal crosshairs for surveyors' transits, telescopes, and other optical instruments

*** Major ampullate silk, named for the ampoul or flask shape of the glands producing it, makes possible spiders' trademark suspended silk webs. Like suspension bridges, araneomorph webs hang from support cables, known as frame lines, which the spiders spin from major ampullate silk.

The suspended web made possible by major ampullate silk is, to humans, the most spectacular manifestation of the silk's utility. But the web's ability to trap prey is probably a byproduct of major ampullate silk's most powerful function - to allow spiders to avoid becoming prey themselves. All spiders (including mesotheles and mygalomorphs) leave behind silk protein trails as they travel. The function of these trails is unknown: perhaps they help spiders find their way back to their burrows or serve as a kind of personal ad, intended to intrigue potential mates. Mesotheles' and mygalomorphs' trail silk is often more like a glue than a thread, a thin stream of protein rather than a coherent line. As araneomorphs diverged from mygalomorphs, this stream was replaced by a line strong enough to support the spider's body weight. So equipped, an exposed araneomorph that sensed danger while walking across a branch could drop out of harm's way, stop at any point in midair, and climb back up its rappelling line to its original position when the danger had passed. Outfitted with major ampullate silk, spiders could move rapidly through the air even though they lacked wings. Once spiders were able to make silk that could support their weight, other possibilities opened up.

*** Its [major ampullate silk's]toughness granted araneomorph spiderlings and some adults the ability to "balloon," to be lifted into the air on updrafts and then glide along on the breeze. They could find a high point, stick their tail ends up into the air, and float out puffs of major ampullate silk. The wind would catch the silk and lift the spiderlings along with it....

With a silk puff above them, off the spiderlings glide, sometimes drifting only a few feet but sometimes whisked up into air currents that carry them hundreds of kilometres. Breeze-blown gossamer is so ethereal that one legend claims that it is actually loose threads from the Virgin Mary's winding sheet, still falling to earth after her assumption. The sight of these gossamer wisps sailing along in the sky has brought to mind otherworldly creatures such as fairies, too. But fairies, with their piloted wings, are really more like dragonflies and somehow less endearing than the real-life speck-sized aeronauts who, in the words of a spiderling in E. B. White's Charlotte's Web, go "wherever the wind takes us. High, low. Near, far. East, west. North, south." The distances these threads can take the spiderlings is attested to by Charles Darwin, who on a clear November morning in 1832 stood on the deck of the Beagle, gazing upward. Thousands of tiny spiders floated through the air attached to "patches of the flocculent web," and landed on the ship's rigging. The Beagle was sailing about a hundred kilometres off the eastern coast of South America, and Darwin believed that the spiderlings had wafted at least that far.

*** For a long time, arachnologists thought that only spiderlings ballooned. Most adult spiders are too heavy to be lifted by a strand of major ampullate silk. And ballooning is risky. A spider floating through the air is defenseless, and it has no control over where it will land and whether its landing will go unnoticed or be relished by an animal already in residence. For spiderlings, the benefit accruing from moving away from a tight cluster of hungry siblings may balance this risk.

Yet it turns out that adult araneomorphs do balloon. About twenty araneomorph species are social spiders. Unlike ants and bees, who are "eusocial," social spiders do not develop distinct reproductive and worker castes. But hundreds, sometimes thousands, of these spiders live together on large, communal webs. Webmates do not eat one another, and they share web construction and repair chores, dispatch prey together, and cooperate to feed all the spiderlings.

Stegodyphus dumicola is a social species of velvet spider. In the late 1980s and again in the late 1990s, reports of Stegodyphus adults ballooning appeared, but it was not clear whether the spiders were actually ballooning or had simply been blown into the air by a gust and were trying to save themselves with their silk. Then in 2000 a group of arachnologists in Namibia noticed a group of Stegodyphus adults "tiptoeing" along the top strand of their communal web. (Tiptoeing is the arachnological term for stretching upward on the legs and sticking the tail end into the air preparatory to ballooning.) These velvet spiders average about 10 millimetres (almost half an inch) long and would be too heavy to be lifted by a single strand of silk. To the researchers' amazement, each adult let loose dozens of strands of silk, which fanned out to form a triangular sheet a meter across at the end farthest from the spider. These gossamer hang-gliders bore the spiders aloft, and the researchers lost sight of them after they had risen about 30 meters (about 100 feet). By capturing a later group of tiptoers, the researchers determined that most if not all were females bearing fertilised eggs. The most logical explanation is that they had been setting off on a quest to establish new colonies. Further research showed that only large colonies produced ballooners; these colonies may have been approaching their practical limits. For these adult ballooners, the risks of aerial emigration may have been balanced by the promise of establishing a new silk span in some uncharted land of opportunity, soon to be populated by their own offspring. For both baby and adult ballooners, major ampullate silk is the thread of promise that takes them not just down and out of danger and hardship but also up and away.

Leslie Brunetta grew up in Golden's Bridge, NY, graduated with an AB from Princeton and an MPhil from St. Catherine's College, Oxford, where she was a Fulbright Scholar. She has freelanced for various publications, including Technology Review, Sewanee Review, and The Federal Reserve Bank of Boston Regional Review.

Catherine L. Craig grew up in Ventura, CA and majored in Human Biology at Stanford. She spent six months as part of Jane Goodall's team following chimpanzees through the forests of the Gombe Stream Research centre in Tanzania before gaining her PhD in Ecology and Systematics from Cornell.

You can purchase Spider Silk: Evolution and 400 Million Years of Spinning, Waiting, Snagging and Mating at Amazon.

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