Why Don’t Spiders Get Stuck in Their Webs?

Oh, what a tangled web we weave, when first we practice to deceive.

–Sir Walter Scott, Marmion: A Tale of Flodden Field

Despite filling a large number of us with arachnophobia, spiders are marvels of engineering and pest extermination.

The form of eight creepy-crawly legs surrounding a bulbous center with many eyes and fangs is one of the most recognizable on the planet, but just as ubiquitous is the tangled geometry of their silken webs.

The name “spider” comes from the Proto-Germanic spin-þron, which means “spinner,” so we have linked the arthropods to their webs for as long as we’ve labeled them. Though some spiders have evolved to live without webs and others craft differently shaped traps, the classic vision of a web comes from a subgroup called orb-weavers.

It’s easy to see why they garnered such a moniker, as the webs radiate around a central orb like planets orbiting a star.

Tracing the evolution of spider webs is fascinating. The structures come from silk made of protein that the arachnids develop inside their bodies. Scientists believe silk production started as a means of protection for their bodies and eggs, long before it became a means for setting traps. As insects began to fly, spiders started to use silk as guide or signal lines. Next, likely, came the advent of bush meshes, the success of which led to the aerial webs we see today.

We tend to think of these snares as sticky strings that snag snacks from the sky. While webs are gluey, that attribute is only part of the story.

How do spiders manage to ensnare prey while keeping themselves free from the trap?

The answer is multi-faceted, and one that botanists have only recently begun to understand with certainty.

For a long time, people assumed spiders must produce an oily substance that allowed them to traipse around on their sticky silk.

When we began to study them, however, we discovered something more complex. As the use of silk had progressed, spiders had evolved the ability to create different types of the substance! The silk that surrounds eggs is not the same as the silk that constitutes a web. The webs are not uniform, either. As it turns out, the central orbs and the backbone spokes feature one type of silk – often called “dragline” – and the concentric circles another – sometimes called “flag lines” or “spiral lines.”

Each type of silk comes from a distinct gland within the spider’s body; some species can produce seven different types of silk!

As the framework of the web, draglines are incredibly strong, up to five times as robust as steel proportionally. When a spider crafts a web, it sends guidelines from a perch, which it eventually replaces with the super-strong draglines. These branches are not at all sticky. The spider uses this network as a cheat code to move quickly about its grid.

The spiral lines are not as tensile as the draglines. They are designed to flex, providing the lines with the ability to absorb the impact of a flying insect.

If the spiral lines catch the food and the draglines are the non-sticky ones, how does the spider get its prey and remain unshackled?

These sticky segments don’t emerge from a spider’s spinneret uniformly sticky.

Instead, they are dotted with clingy globules. It’s not necessarily the silk that snags a bug but the nearly invisible glue!

Once again, the web’s engineer has a leg up on the competition. The spider can tiptoe through the quagmire to its prey.

The story might end at this combination of a non-sticky framework and strategically placed adhesive droplets. You might notice, however, as the image above denotes, the distance between drops is quite tiny, on the scale of micrometers. Can spiders really navigate through such minefields?

As it turns out, arachnids are not immune to their own webs. Not satisfied simply with the advantages of different types of silk, a group of scientists studied the movement of spiders on their webs with precise equipment. They uncovered a complex leg biology that provided yet another advantage to the spider. Their appendages are covered with setae, structures resembling hair or bristles. The setae reduce surface area for a spider’s leg, making contact with glue less likely to trap it. Saying that spiders “tiptoe” through their webs isn’t just a metaphor!

Further analysis showed that the setae feature an anti-stick chemical coating. So, the age-old thought about being covered in oil wasn’t as far-fetched as we initially concluded! The source of this coating is still unknown. Spiders do not seem to contain oil-producing glands, as they do for silk. Whether the coating is created alongside normal leg cell production or is applied in some other manner is an open question.

Whatever the answer, this careful dance between civil engineering and body chemistry allows spiders to prance among their webs with impunity. They can munch on bugs without worrying about self-entrapment!

BONUS FACT: Catching food in a web might be an evolutionary development to save energy. If the prey comes to the spider, the spider doesn’t need to waste fuel chasing it down. However, the silk requires protein and energy to create, potentially undoing the gains, especially in a period without meals. Spiders have adapted to mitigate this issue over the eons: after a web loses its structure or stickiness, the spider eats the silk. Yummy protein recycling!