How spider silk is one of the most versatile materials on Earth

Spiders spin webs out of silk, but they also use their threads as slingshots, submarines, and hang-gliders.Friday, 13 September 2019

An Orb-weaver spider web in the early morning dew.
An Orb-weaver spider web in the early morning dew.
photo by Brian Gordon Green, Nat Geo Image Collection

Spider silk is one of the most versatile materials on Earth. Actually a protein created by special organs known as spinnerets, spider silk can be used for transportation, shelter, courtship, and all kinds of creative ways to trap prey.

Some spiders can produce more than one type of silk. A common orb-web, for example, may contain at least four different kinds, each adding a different component, such as strength, flexibility, and stickiness.

Equipped with such a versatile material, spiders have evolved to create a wondrous assortment of webs. There are horizontal sheet webs that catch falling prey and vertical latticework webs that intercept flying prey. Black widow webs are messy affairs, while funnel webs and lampshade webs can resemble three-dimensional sculptures. Spiders in the Theridiosomatidae family build conical webs that can fire a spider at nearby prey like a slingshot, while ogre-faced spiders nab their meals with hand-held nets. (Read about the spider that uses its web to shoot itself faster than a rocket.)

The redback spider of Australia spins a tangled web with sticky, “gum-footed” lines that stretch straight down to the ground like a beaded curtain. When ants or crickets brush up against one of these tendrils, the line snags the prey and then snaps, drawing the helpless creature up into the air where it will dangle until the redback decides to eat it.

“Some spiders produce a silk that is low in UV reflection and is also translucent, so insects can’t see it,” says Catherine Craig, an evolutionary biologist and author of Spider Silk: Evolution and 400 Million Years of Spinning, Waiting, Snagging, and Mating.

On the other side of the spectrum, there are spider silks that reflect ultraviolet light and appear blue at certain angles. In the tropics, there are even spiders in the Nephila genus that infuse their silks with carotenoids, which, when the sun hits them, makes the webs seem as if they were dipped in liquid gold.

Bolas spiders skip web-building altogether. These clever creatures lure moths in close with pheromones and then swat the insects out of midair with a single piece of sticky, weighted silk that they swing around like a mace. Gnaphosids shoot silk at their prey like Spiderman.

And they’re not alone.

Of the close to 50,000 spider species known to science, most do not produce webs at all, says Craig. But all spiders produce silk. The ways in which they use this material are as varied as they are fascinating.

Home is where the silk is

For hundreds of millions of years, before the evolution of webs, and even before there were flies to catch in them, spiders used their silk glands for shelter.

“Spider silk is incredibly strong and flexible,” says Catherine Scott, an arachnologist at the University of Toronto Scarborough. “It also tends to be very clean and have anti-microbial properties, because spiders don’t want molds and microbes growing on their webs.”

Some spiders, like those on the 300-million-year-old Mesothelae branch of the spider family tree, dig burrows on slopes and banks and line them with layers of gauze-like silk. Next, these spiders construct circular, hobbit-hole doors complete with a silk-bound hinge. Not only does this hide the spiders from predators and enable them to pounce on unsuspecting prey, but the doors seal the predators off from the world and allow the spiders to regulate the burrow’s temperature and humidity—and even protect them against flooding.

Purseweb spiders build silken tunnels that slink up the sides of trees. Most people never notice them, though, because the structures are covered in dirt and other bits of debris.

A delicate Orb Spider web glistens at sunset over the grasslands.
A delicate Orb Spider web glistens at sunset over the grasslands.
photo by Jason Edwards, Nat Geo Image Collection

“You will almost never see the spider, because it is inside of that tunnel and it extends into the ground,” says Sebastian Echeverri, an arachnologist and science educator at the University of Pittsburgh. “And when prey walks along the tunnel, the spider feels the vibrations, and it will actually run up on the inside and bite through the silk and grab the prey with its fangs, envenomate it, and drag it back inside.”

And then there’s the diving bell spider.

“So, this is a spider that lives its entire life underwater by tying together some vegetation with its silken web,” says Echeverri.

These spiders can’t breathe underwater, though, so they make repeated trips to the surface to capture air bubbles with specially adapted hairs. Once back in their underwater vegetation dens, they then wipe these bubbles off and bring them into the web to form a tiny, oxygen-rich sanctuary where they can hide from predators and lay eggs.

“That’s a spider that just defies most spider-like things,” says Echeverri.

Getting around

While silk is an excellent building material, it can also be used for transportation.

Jumping spiders are constantly leaping across chasms, for instance. They protect themselves against falls by anchoring a silk safety line to their perch. This allows jumping spiders to crawl back to where they started if they miss their mark. It even allows change directions once they’re airborne with a quick tug of the drag-line.

Most spiders are tiny, but they can travel between trees or across enormous gaps through a process known as “bridging.” All the spider needs to do is let out a line of silk into the wind and then pull it taut once it connects to something out in the world. The arachnid doesn’t really know where it’s going, of course, but it beats crawling.

“And then I’d be remiss not to mention how spiders can fly,” says Echeverri.

Similar to bridging, numerous spider species are able to “balloon” up into the sky by releasing strands of silk that get picked up by the wind and Earth’s electrical fields. Ballooning spiders have been found floating more than two miles high and thousands of miles out at sea.

Spiders Blanket Creek in Webs—a Surreal, Rare Sight
Spiders Blanket Creek in Webs—a Surreal, Rare Sight
Long-jawed spiders spun webs that blanketed the trees and bushes of Nahal Soreq Creek near Jerusalem. These arachnids are normally solitary. But in this rare instance, there are millions of them residing along this creek. The area is also infested with mosquitoes that the spiders feast upon.

Can you smell me now?

Spider silk isn’t just strong, stretchy, and sticky—it can be stinky, too.

“We know that female spiders have pheromones on their silk,” says Scott.

In a recent study published in Proceedings of the Royal Society B: Biological Sciences, Scott showed that male black widows can detect these come-hither scents from nearly 200 feet away and use them as a compass to find a female.

What’s more, Scott’s research showed that some males can make their way to a female even faster by following the drag-lines left by the males that have come before them. The males were even able to sniff out the difference between silk left by their rivals and strands laid down by males of another closely-related species, the false widow.

Getting it on

“Silk is a communication method,” says Scott. “It works chemically, using pheromones, but then for web-building spiders, it is also a dance floor used for male courtship displays.”

Partly to woo the female and partly to convince her he is a suitor rather than dinner, males of many species will tap, pluck, and otherwise send vibrations throughout the female’s web. Males may also remodel the female’s web by laying down silk of his own or destroy whole sections of it, perhaps in an attempt to hide the female from other males in the area.

Elsewhere in the mating and reproduction game, spiders use silk to safeguard their eggs and build nursery webs to protect their spiderlings. Males of some species use silk to gift-wrap food items, which they then give to females in an attempt to woo their favour, though sometimes a spider will try to cheat the female by wrapping up a rock or seed instead.

“By the time she gets to what’s inside and realises it’s not food, he might have gotten away with a copulation,” says Scott.

Silk can also be used to tie a female up during courtship. This is called “mate binding” or the “bridal veil.” And while it may sound strange, this behaviour may make the female more receptive to mating by bringing her sensory hairs into contact with the male’s pheromone-laden silk. (See a video of spider mate binding.)

Of course, the silk binding may serve a more straight forward purpose.

“Physically restraining her can also prevent cannibalism,” says Scott.

Read More