High Wire Artists
The more they watch spiders in action, the more scientists learn about the abilities of nature's master weavers
Richard Wolkomir and Joyce Wolkomir
At age five, George Uetz did not know he was arachnophobic. What he did know was that he had stuck his hand into a bush and a large spider had run up his arm. For years after that, the creatures terrified him. Then he enrolled in the wrong biology course in college. Spiders, it turned out, were the focus of the course. For Uetz (rhymes with "foots"), it was an epiphany: "As you find out more about them," he says, "your fear turns to fascination."
Today, George Uetz's fascination has led him to become president of the American Arachnological Society. A behavioral ecologist at the University of Cincinnati, Uetz is also one of the nation's leading arachnologists, or spider researchers. Because the creatures are both predators and prey, they make excellent model organisms for studying animal behavior and ecology. These days, Uetz spends much of his time ensconced in Mexican fields and forests amid clouds of webs—spider metropolises where hundreds of thousands of the eightleggers communally snag insect prey.
Uetz is among the growing number of scientists taking a closer look at nature's master web-weavers and, in the process, making some astonishing discoveries. "The thing that really knocks me out about spiders," he says, "is that here you have a group of some of the most recognizable creatures in the world, creatures that exhibit greater diversity of form and behavior than birds, for example. Yet hardly anything is known about them."
What is known is that spiders have a special talent for producing silk, which they use for everything from building homes and bagging meals to finding mates. So strong and lightweight is spider silk that industry hopes to exploit it for use in the development of products for humans. In one project, the U.S. Army is trying to produce the silk artificially and use it to make bulletproof vests.
Unlike insects, which have six legs, spiders have eight. Spider bodies are also divided into two sections, not the insects' three. Along with mites, ticks, scorpions and daddy long legs, they make up the class Arachnida. Of spiderdom's estimated 120,000 species, scientists have described only about 34,000 and actually studied far fewer. Not all the species weave webs, but all produce silk.
Most spiders will not bite, even when threatened, and very few species are dangerous to people. Of North America's 3,000 or so species, only two—the black widow and brown recluse—are poisonous to people. Despite the creatures' ghoulish image, such as in the movie Arachnophobia, deaths from wasp and bee stings far outnumber those from spider bites.
But perhaps not even that reassuring statistic would comfort the tourist who happens upon one of the larger arachnids. Hairy South American bird spiders, which stalk lizards and small birds, grow as big as dinner plates, spanning 10 inches from toe to bristly toe. "The biggest spiders weigh a quarter of a pound," says Uetz. At the other end of the size spectrum are the dwarf spiders, which grow no bigger than a pinhead.
Some of the spiders that do not weave webs use more conventional predator tactics to catch their dinner. Tarantulas, for example, hunt by night, lying in wait and pouncing when they sense movement nearby. Cornered, they sometimes hiss and rear up on their back legs. As do most spiders, these creatures grip prey with claws on their feet and two leglike pedipalps protruding from their front ends. They disable victims with venom injected by two fangs, then regurgitate digestive enzymes onto their prey and suck out the pre-digested material.
Jumping spiders, which are most abundant in the tropics, stalk their quarry using eyes that resemble headlights and leap for the kill. Tropical spitting spiders spew out threads as if throwing a sticky net over their victims. Brightly tinted crab spiders, which live in many parts of the world, hide in flowers and ambush their prey.
It is the creatures' gift for spinning silk, however, that most fascinates scientists. Glands produce the silk in liquid form as a protein called fibroin, which is pumped out through spigotlike spinnerets at the creature's rear. When stretched, the fibroin hardens into a thread. The silk of one species—Nephila, found in the southern United States and in the tropics—is among the strongest of natural fibers. (South Sea islanders use the webs to make bags and fish nets.) Lightweight and fine, a spider web nonetheless absorbs a force proportionally equal to the impact of a jet fighter landing on an aircraft carrier every time it snags a fly.
In the early 1700s, a French inventor fashioned gloves and stockings from spider silk, which is twice as strong as silkworm silk and much more elastic. However, commercialization of the material was stalled for years when the Academy of Sciences of Paris reported it would take the efforts of 663,522 spiders to produce just one pound of silk.
Scientists today are exploring ways to get around that problem and are looking at other possible applications for spider silk technology. At the U.S. Army's research center in Natick, Massachusetts, molecular biologist Stephen Lombardi is splicing spider silk genes into bacteria. His goal is for the bacteria to mass-produce liquid fibroin in vats for scientists to convert into usable yarn.
Lightweight bulletproof vests and helmets made of spider silk are the Army's chief interest. But Lombardi says the silk could be woven into featherweight tents, sleeping bags and other clothing. "I'd say commercialization is about five years away," says Lombardi, who is known among his coworkers as "Dr. Spiderman."
As scientists in the field have discovered, spiders make different silks for different purposes. During their travels, most spiders spool out a single strand of silk called a dragline. Strong as nylon and twice as elastic, dragline is a highly versatile tool. Roving spiders follow their dragline back to headquarters. If a spider stumbles, its dragline stops it from falling and then becomes a handy ladder for climbing back up.
Spider silk owes much of its strength and flexibility to the fact that it is a composite. Strands have stiff crystalline sections interspersed with rubbery regions, according to researchers at the University of British Columbia, who found spiders to be prodigious silk producers. Working with a common garden spider, the scientists collected silk by pushing a spider off a card so that it hung by a strand. They twirled the card to reel in the silk, and before they knew it, they had gathered a half-mile of the material.
Oxford University biologists compared two types of strands that garden spiders use in their webs. Strong but rigid drag-line silk provides the web's structural support. Onto this frame, the creature spirals a sticky prey-snagging strand that can stretch to four times its original length to absorb impact.
Using a scanning electron microscope, the Oxford scientists noticed that each sticky strand is beaded with droplets of a gluey liquid. Inside each droplet, the line is coiled like a telephone cord. When a captured insect struggles, the coils unwind, giving the strands their elasticity. When the prey is subdued, the silk recoils. Meanwhile, the spider hiding nearby grips a single strand attached to the web and, like a fisherman awaiting a bite, senses any vibration in the line.
Many spiders weave protective silk cases for their eggs. The eight-eyed nursery web spider, for instance, carries her egg sac in her mouth until hatching time. Then she ties leaves together with silk to create a nursery and stands by, ready to protect her emerging young. The European water spider weaves a bell-shaped web underwater and attaches it to bottom-growing plants. The creature collects air around its body at the surface, dives and then releases the air into the web, making a bubble in which it hatches its young and spends most of its life.
Trapdoor spiders dig burrows using a spiny rake mounted on their jaws, then line the burrow and the opening with silk. They create a trapdoor at the entrance to the burrow, camouflaging their handiwork with debris. When passing insects or other prey create vibrations, the spiders throw open the hatch, seize them and pull them below.
Spiderlings use silk to fly, perching atop limbs and playing out silken strands like kite strings. The wind catches the silk and wafts the spiders off to new frontiers. On some fall days the air glistens with the young balloonists.
Two years ago, Yale University researchers discovered that some spider silks reflect ultraviolet light, which is invisible to the human eye but a virtual neon advertisement for hungry bugs. (Many flowers reflect ultraviolet light to attract pollinating insects.) Some researchers think spider webs made of ultraviolet-reflecting silk mimic blossoms, luring unsuspecting insects to their doom.
A number of spiders weave webs that do not reflect ultraviolet light. When that's the case, though, the creatures often weave designs into their webs using a type of thread that does have reflective properties. In at least one tropical species, Argiope argentata, the spider itself reflects ultraviolet light. "The spider's body colors are an important part of the design, while the legs seem to disappear," says Yale researcher Catherine Craig. She and Gary D. Bernard, a University of Washington electrical engineer, found that decorated webs capture 58 percent more insects than plain webs.
Cornell University entomologist Thomas Eisner suggests another purpose for web designs: avian traffic signals. On a Florida field trip, he noticed that two spider species—Argiope florida and the golden orb weaver (Argiope aurantia)—both weave thick patches of white silk strands into their webs. Eisner observed that songbirds chasing insects on the wing veered away to avoid crashing through these highly visible webs. Eisner, along with animal behaviorist Stephen Nowicki, found that spiders expend the extra energy to create ornamentation if they are species that weave long-lasting webs. Spiders that construct webs in the evening and take them down at dawn have fewer problems with birds and do not go to the trouble.
Eisner also has found that certain opportunistic flies actually benefit from a spider's handiwork. Many insects emit defensive chemicals when caught in a web. These effusions generally do not harm the spiders, though they can invite trouble. "I've been studying tiny flies that catch a whiff of those protective chemicals off in the woods and make a beeline for the spider's web," says Eisner. The flies then try to eat the web owner's catch. "The spiders try to chase the flies away," he says. "It's like hyenas around a lion kill in Africa."
Paul J. Watson, a former Cornell researcher now at Oxford University, has discovered why male Sierra dome spiders sometimes roll up a female's web into a ball, destroying the fruit of her labors. It's the old story of the flirtatious female and a jealous male, he explains.
Usually, the female attracts a number of suitors, which battle for her favors. When males are scarce, however, the female weaves strands perfumed with male-attracting pheromones into her web. "It is the last resort of a ten-month-old female that has only a month in her one-year life to breed," says Watson. When a male answers the call, he rolls up the web and its scented strands to keep it from attracting unwanted competition.
Master builders, spiders are also amazingly resourceful. Not long ago, NASA rocketed a spider into space to test the effect of zero gravity on web-building. Unable to use its own body weight as a guide, the spider at first wove a misshapen web. After three days the animal got its "space legs," and its webs were near-perfect.
Some of the creatures have even adapted to an urban life-style. For about ten years, George Uetz has been studying the Metepeira incrassate spiders that live and hunt in vast, interconnected spider cities in the Mexican state of Veracruz. "The largest colony we found had around 160,000 spiders," he says.
Less than one-tenth of one percent of the world's spider species are social. Indeed, most spiders aggressively defend their property from others of their kind. What's unusual about this one species in Mexico is that each spider anchors its web to its neighbors' to create a single web that often covers a distance longer than a-football field. They do this, says Uetz, because of what he calls the ricochet effect: An insect that escapes one web is apt to bounce off and become snared in a neighboring web. "Where food is abundant, it sometimes makes sense for them to go together," says Uetz. Each spider thus expends less energy bagging meals.
To the Mexican farmers living in the remote areas where Uetz works, spiders are commonplace—but scientists are not. "Sometimes we'll be working, look around and see 30 kids watching us, giggling," says Uetz. Once, when he and a colleague were photographing webs in Mexico, they heard a horse. The scientists looked up to find themselves surrounded by rifle-toting security guards. "They were serious guys," remembers Uetz.
As it turned out, a local landowner had hired the riders to chase away squatters. Uetz explained in broken Spanish that he and his associate were merely studying spiders. That seemed to satisfy the guards, who let the scientists go unharmed. For George Uetz, however, those few tense moments were almost as scary as that time long ago when a spider ran up his arm.
Richard and Joyce Wolkomir share their Vermont home with spiders and enjoy having a little wildlife in the house.