Spiderwebs rarely make a good first impression. Even if you aren't one of the insects they're designed to capture, a sudden coating of silk on your face can be annoying, and possibly alarming if you don't know where the spider ended up.
For those of us large enough to escape, though, spider silk is worth a second look. Not only are its creators much less dangerous to humans than commonly believed — and often more helpful than harmful — but their silk is a vastly undervalued wonder of nature. And while this supermaterial would be worth admiring even if it was useless to us, it also happens to hold huge potential for humanity.
There are lots of reasons to like (or at least tolerate) our arachnid neighbors, but if you can't make peace with spiders themselves, at least consider making an exception for their silk. Aside from capturing mosquitoes and other troublesome insects, spider silk teems with incredible capabilities, many of which humans would like to imitate. And after centuries of trying to harness the magic of spider silk, scientists are finally unraveling some of its most promising secrets.
Here's a closer look at what makes spider silk so spectacular, both as a marvel of biology and a treasure trove of biomimicry:
1. Spider silk is stronger by weight than steel.
Spider silk is lighter than cotton and up to 1,000 times thinner than human hair, yet it's also incredibly strong for such a wispy material. This outsized strength is vital for spiders, who need their silk to withstand an array of destructive forces, from the frantic flapping of trapped insects to powerful blasts of wind and rain.
Still, for animals of our size, it's hard to grasp the proportional strength of spider silk unless we frame it in familiar terms. Comparing it with steel might sound absurd, for example, but on a per-weight basis, spider silk is stronger. It may lack the stiffness of steel, but it has similar tensile strength and a higher strength-to-density ratio.
"Quantitatively, spider silk is five times stronger than steel of the same diameter," explains a fact sheet from the University of Bristol School of Chemistry. It also draws comparisons with Kevlar, which has a higher strength rating but a lower fracture toughness than certain spider silks, according to the American Chemical Society (ACS). Spider silk is highly elastic, too, in some cases stretching four times its original length without breaking, and retains its strength below minus 40 degrees Celsius.
It has even been suggested — but not tested, obviously — that a pencil-width strand of spider silk could stop a Boeing 747 in flight. In a more natural flex, however, the Darwin's bark spider of Madagascar can stretch its dragline silk up to 25 meters (82 feet) across large rivers, forming the world's largest-known spiderwebs.
2. Spider silk is surprisingly diverse.
A large orb weaver wraps up its prey with swathing silk in Australia. (Photo: Graham Winterflood [CC BY-SA 2.0]/Flickr)
Unlike silk-making insects, which tend to produce only one kind of silk, spiders make many varieties, each specialized for its own range of purposes. No one is sure how many types exist, as biologist and spider-silk expert Cheryl Hayashi recently told the Associated Press, but researchers have identified several basic categories of spider silk, each produced by a different silk gland. An individual spider can typically make at least three or four kinds of silk, and some orb weavers can make seven.
Here are seven known types of silk glands, and what each silk is used for:
- Achniform: Produces swathing silk, for wrapping and immobilizing prey.
- Aggregate: Produces droplets of "glue" for the outer part of sticky silk.
- Ampullate (major): Produces non-sticky draglines, the strongest type of spider silk. Dragline silk is used for several purposes, including the non-sticky spokes of a web and the support lines that spiders use like an elevator.
- Ampullate (minor): Silk from the minor ampullate gland isn't as strong as draglines from the major gland, but it's just as tough due to its higher elasticity. It's used in many ways, from web building to wrapping prey.
- Cylindriform: Produces the stiffer silk for protective egg sacs.
- Flagelliform: Produces the stretchy core fibers of a web's capturing lines. These fibers are coated with glue from the aggregate gland, and their elasticity allows time for the glue to work before prey can bounce off the web.
- Pyriform: Produces attaching threads, which form the attachment disks that anchor a thread of silk to a surface or to another thread.
Hayashi has collected silk glands from dozens of spider species, but she and other scientists have still only scratched the surface, she tells the AP, noting there are more than 48,000 spider species known to science around the world.
3. Spiders make silk kites, slingshots, submarines and more.
Silk gives spiders a wide range of housing options, from iconic spiral webs to tubes, funnels, trap doors and even submarines. The latter are mostly built by semiaquatic species like the beach-dwelling Bob Marley spider, which makes air chambers to ride out high tide, but there is one known species — the diving bell spider — that spends almost its entire life underwater. It only leaves its air chamber to grab prey or replenish the air supply, but even that doesn't happen very often, since the silk bubble can draw in dissolved oxygen from the water outside.
Silk can be useful for transportation, too. Many spiders make silk sails, which let them travel long distances by riding the wind, known as "ballooning." This is a common way for spiderlings to disperse from their birthplace, but some species also use air travel as adults. Even without wind, spiders might still manage to fly by harnessing the Earth's electric field. And for shorter trips, some orb weavers use silk to slingshot themselves at prey, relying on the silk's elastic recoil to accelerate like a rocket.
And in one of the strangest-looking uses of spider silk, a species from the Amazon rainforest makes little silken towers surrounded by a tiny picket fence. Little is known about the builders, which are nicknamed Silkhenge spiders since the structures vaguely resemble Stonehenge. Researchers have at least learned what the Silkhenge itself is for, though: It seems to be a protective playpen for the spider's babies.
4. Silk goes from liquid to solid as it leaves a spider's body.
Silk glands hold a fluid known as "spinning dope," with proteins called spidroins arranged in a liquid crystalline solution. This travels via tiny tubes from the silk gland to the spinneret, where the proteins start to align and partly solidify the dope. Fluid from multiple silk glands can lead to the same spinneret, letting the spider make silk with specific properties for a particular task, according to the University of Bristol School of Chemistry. When it leaves the spinneret, the liquid dope is solid silk.
Spider silk's properties come not just from the proteins, but also from the way a spider spins them, as scientists noted in a 2011 research review. When people take spidroins from spiders and try to recreate spider silk, the resulting fibers "show completely different mechanical properties compared to fibers spun by spiders, indicating that the spinning process is also crucial," they wrote.
That's illustrated by cribellate spiders, a large group of species with a specialized organ called a cribellum, which makes silk with "mechanical stickiness" instead of the liquid glue of other spiders. Unlike a typical spinneret, the cribellum has thousands of tiny spigots, all producing extremely thin threads that spiders comb with specialized leg bristles into a single, wooly fiber. Instead of glue, nanofibers from this silk seem to trap prey by fusing with a waxy coating on an insect's body.
5. Some spiders replace their webs daily, but recycle the silk.
Orb weavers tend to build their iconic webs in relatively open areas, which boosts their chances of catching prey — and their chances of sustaining web damage. These spiders often replace their webs every day, sometimes even if they still seem perfectly fine, before spending their evenings waiting for prey.
That may sound wasteful, especially considering all the protein spiders must use to produce silk in the first place. Yet even if an orb weaver fails to catch any insects overnight, it still usually has enough silk proteins to tear down that web and build a new one for the following night. That's because the spider eats the silk as it removes the old web, recycling the proteins for its next attempt.
6. Spiders 'tune' and pluck their silk like a guitar.
Anyone who has watched a spider in her web knows she pays close attention to even slight vibrations, which might indicate trapped prey. In recent years, however, scientists have found this is a lot more complex than it looks. When compared with other materials, spider silk can be uniquely tuned to a wide range of harmonics, according to researchers from the Oxford Silk Group at Oxford University.
Spiders "tune" their silk like a guitar, the researchers explain, adjusting its inherent properties as well as the tensions and connections of the threads in their webs. Organs on the spiders' legs then let them feel nanometer vibrations in the silk, which convey surprisingly detailed information on multiple topics. "The sound of silk can tell them what type of meal is entangled in their net and about the intentions and quality of a prospective mate," Beth Mortimer of the Oxford Silk Group said in a statement about the findings. "By plucking the silk like a guitar string and listening to the 'echoes,' the spider can also assess the condition of its web."
Aside from shedding more light on the impressive powers of spiders, scientists are also keen to learn from a material that combines extreme toughness with the ability to transmit detailed data. "These are traits that would be very useful in lightweight engineering," according to Fritz Vollrath of the Oxford Silk Group, "and might lead to novel, built-in 'intelligent' sensors and actuators."
7. Some spider silk seems to have antimicrobial properties.
This kind of interest is hardly new, as humans have been co-opting spider silk for thousands of years. Polynesian anglers have long relied on its toughness to help them catch fish, for example, a method still used in some places. Ancient Greek and Roman soldiers used cobwebs to stop wounds from bleeding, while people in the Carpathian Mountains treated wounds with the silk tubes of purseweb spiders. Its toughness and elasticity likely made it well-suited for covering wounds, but spider silk was reportedly thought to have antiseptic properties, too.
And according to modern research, these ancient appreciators of spider silk may have been on to something. In a 2012 study, researchers exposed a Gram-positive and a Gram-negative bacterium to silk from the common house spider (Tegenaria domestica), observing how each grew with and without the silk. There was little effect in the Gram-negative test, but the silk inhibited growth of the Gram-positive bacterium, they found. The effect was temporary, suggesting the active agent is bacteriostatic rather than bactericidal, meaning it stops bacteria from growing without necessarily killing them. Since spider silk is also biodegradable, non-antigenic and non-inflammatory, this hints at a significant therapeutic potential.
More recently, scientists have figured out how to boost this natural property of spider silk, creating an artificial silk with antibiotic molecules chemically linked to the fibers. The silk can respond to the amount of bacteria in its environment, the researchers reported in 2017, releasing more antibiotics as more bacteria grow. It will be a while before this is used clinically, but it shows promise, according to the researchers, who are also looking into spider-silk scaffolds for tissue regeneration.
8. A golden age of spider silk might finally be near.
This cape was hand-embroidered from the silk of 1.2 million golden orb-weaver spiders, a process that took eight years. The bright yellow color is reportedly the natural color of the spiders' silk. (Photo: Oli Scarff/Getty Images)
Despite our long fascination with spider silk, humans have also struggled to harness its powers on a larger scale. We've had trouble farming spiders like we do with silkworms, partly due to the territorial and sometimes cannibalistic nature of its creators. And due to the fineness of their silk, it can take 400 spiders to produce one square yard of cloth. To make the spider-silk cape pictured above, for instance, a team of 80 people spent eight years collecting silk from 1.2 million wild golden orb-weaver spiders in Madagascar (which were returned to the wild afterward).
The alternative to spider farming is creating synthetic spider silk, which might be a better option anyway, both for us and for spiders. Yet this has been elusive, too, even after scientists began to reveal the chemical structure of spider silk. A spider-silk gene was first cloned in 1990, according to Science Magazine, letting researchers add it to other organisms that might be better able to mass-produce the silk. Since then, a variety of creatures have been genetically engineered to make spider-silk proteins, including plants, bacteria, silkworms and even goats. The proteins often turn out shorter and simpler than in true spider silk, though, and since none of those other creatures have spinnerets, researchers still have to spin the silk themselves.
Nonetheless, after years of frustration, the long-awaited age of synthetic spider silk may finally be near. Several companies now tout their ability to make spider-silk proteins from E. coli bacteria, yeast and silkworms, for purposes ranging from skin lotions to medical devices. We may still have to wait for bulletproof vests and other tough fabrics made from recombinant spider silk — a quest that "isn't quite there yet," Hayashi told Science in 2017 — but in the meantime, scientists have made another breakthrough with a less famous arachnid product: spider glue.
Beads of spider glue cling to a strand of capture-spiral silk. (Photo: Sarah Stellwagen [CC BY-ND 4.0]/The Conversation)
In June, two U.S. researchers published the first-ever complete sequences of two genes that let spiders produce glue, a sticky, modified silk that keeps a spider's prey stuck in its web. That's a big deal for a couple reasons, the study's authors explain. For one, they used an innovate method that could help scientists sequence more silk and glue genes, which are difficult to sequence due to their length and repetitive structure. Only about 20 complete spider-silk genes have been sequenced so far, and that "pales in comparison to what's out there," the researchers say.
On top of that, they add, spider glue should be easier to mass produce than silk, and could offer unique benefits. While it's still a challenge to mimic the way spiders turn fluid dope into silk, spider glue is a liquid at all stages, which might make it easier to produce in a lab. It could also have potential for organic pest control, says co-author Sarah Stellwagen, a postdoctoral researcher at the University of Maryland, Baltimore County, in a statement. Farmers could spray it on a barn wall to protect livestock from biting insects, for example, and later rinse it off without worrying about water pollution from pesticide-tainted runoff. It could also be sprayed on food crops, thwarting pests at no risk to human health, or in areas plagued by mosquitoes.
After all, Stellwagen points out, "This stuff evolved to capture insect prey."
Now, some 300 million years after the dawn of spiders, their silk and glue has also captured something else: our imagination. And if spiders can help us learn to make tougher fabrics, better bandages, safer pest control and other advances, maybe we can even forgive them for weaving all those webs at face level.