How Spiders Took Over the Sky [Book Excerpt]

Despite a trendy spider’s impressive offensive arsenal of venoms and trapping silks, defensive predator-evading ” dragline” silk is maybe its most beneficial 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 threshold of a bookshelf. It senses a sudden shift inside the air around its body, the riffling of its sensory bristles putting it on alert. Its eyes discern a big object sweeping toward it. But before the impending hand slams down on it, the spider dives over the brink 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 increasingly silk, plummeting until it lands on the floor and sprints for protection.

Often, something that seems to return out of nowhere triggers unforeseen consequences. For spiders, this event was the advent of major ampullate silk. Major ampullate, or dragline, silk is the rappelling rope spiders rely upon as they plunge throughout the air. That is one of the vital toughest materials on the earth, ready 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 were 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 inside the latter component of the 20 th century, major ampullate threads made ideal crosshairs for surveyors’ transits, telescopes, and other optical instruments

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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, referred to as frame lines, which the spiders spin from major ampullate silk.

The suspended web made possible by major ampullate silk is, to humans, probably the most spectacular manifestation of the silk’s utility. But the net’s ability to trap prey is perhaps a byproduct of major ampullate silk’s most powerful function-to allow spiders to circumvent 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 one of those personal ad, intended to intrigue potential mates. Mesotheles’ and mygalomorphs’ trail silk is generally more like a glue than a thread, a thin stream of protein as opposed to 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 injury’s way, stop at any point in midair, and climb back up its rappelling line to its original position when the risk had passed. Outfitted with major ampullate silk, spiders could move rapidly in the course of the air although they lacked wings. Once spiders were ready to make silk which could support their weight, other possibilities spread out.

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Its [major ampullate silk's] toughness granted araneomorph spiderlings and some adults the facility to ” balloon,” to be lifted into the air on updrafts and then glide along on the breeze. They are able to 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 which includes it….

With a silk puff above them, off the spiderlings glide, sometimes drifting only some feet but sometimes whisked up into air currents that carry them hundreds of kilometers. Breeze-blown gossamer is so ethereal that one legend claims that it really 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 inside the sky has dropped at mind otherworldly creatures equivalent to fairies, too. But fairies, with their piloted wings, are really more like dragonflies and somehow less endearing than the genuine-life speck-sized aeronauts who, within 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 during the air attached to ” patches of the flocculent web,” and landed on the ship’s rigging. The Beagle was sailing about a hundred kilometers off the eastern coast of South America, and Darwin believed that the spiderlings had wafted at the very least that far.

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For decades, 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 during the air is defenseless, and it has no control over where it’s going to land and whether its landing will go unnoticed or be relished by an animal already in residence. For spiderlings, the ease accruing from moving far from a decent 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 of the spiderlings.

Stegodyphus dumicola is a social species of velvet spider. Inside the late 1980s and again within the late 1990s, reports of Stegodyphus adults ballooning appeared, but it surely was not clear whether the spiders were actually ballooning or had simply been blown into the air by a gust and were looking to save themselves with their silk. Then in 2000 a gaggle of arachnologists in Namibia noticed a set of Stegodyphus adults ” tiptoeing” along the pinnacle 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 millimeters (almost half an inch) long and can be too heavy to be lifted by a single strand of silk. To the researchers’ amazement, each adult let out dozens of strands of silk, which fanned out to form a triangular sheet a meter across at the top farthest from the spider. These gossamer hang-gliders bore the spiders aloft, and the researchers overlooked them when they had risen about 30 meters (about 100 feet). By capturing a later group of tiptoers, the researchers determined that the majority if not all were females bearing fertilized eggs. Probably the most logical explanation is they’d been setting off on a quest to ascertain new colonies. Further research showed that only large colonies produced ballooners; these colonies could have been approaching their practical limits. For these adult ballooners, the hazards of aerial emigration may need been balanced by the promise of building 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 in addition 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 diverse 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 component of Jane Goodall’s team following chimpanzees during the forests of the Gombe Stream Research Center in Tanzania before gaining her PhD in Ecology and Systematics from Cornell.

Spider Silk : Evolution and 400 Million Years of Spinning, Waiting, Snagging, and Mating is on the market at Amazon .

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This post is tagged: fashion designers, materials scientists, mechanical engineers, optical instruments, traditional peoples, unforeseen consequences