Tuning threads and sticky situations: the science of spider webs

In amongst the jack-o'-lanterns, ghosts and goblins, spider webs always make a strong showing at this time of year. 

But spider webs are much more than just a spooky way to spice up your Halloween decorations. 

Nature's best engineers have devised one of the strongest, most elaborate, and most fascinating materials known to humankind.

What's so fascinating about cobwebs?

First of all, there's the incredible engineering behind silk — it is one of the strongest natural materials in the world: tougher than steel and yet lighter than cotton. 

What is truly amazing about spider silk is how many of its properties can be manipulated by spiders. Spiders can control the stickiness and tensile strength of their silk depending on its intended use, whether it's a line used to suspend upon and descend down your neck, or a sticky thread intended to catch prey.

Then there are the intricate design principles at work.

When making a typical orb web, a spider first establishes the guideline by letting silk be pulled by the wind until it can feel vibrations suggesting it has attached to something. Then the spider gets busy building the radial spokes that will make a true orb web, and busies itself along the non-sticky spokes of its web while secreting sticky spiral filaments that will catch prey. It has to be careful not to move onto the sticky fibres and get caught itself.

Not only that, the sticky threads have an electrostatic charge that increases the chance that an insect will get caught if it gets too close.

Once the web is built, the spider waits for a signal that dinner has arrived.

How does a spider know it's caught something?

Most spiders, especially orb-weaving ones, have terrible eyesight: they rely on their incredible ability to sense vibrations in the web to know where to find dinner. Spiders feel the longitudinal waves that are sent along a thread when something hits it. 

When spiders miss that signal, they have been observed plucking certain areas of the web and feeling the waves along that piece of thread to determine whether something is caught there.

Spiders have been shown to modify the features and tension of thread to increase the chances of capturing prey — they literally tune their webs just like you'd tune a guitar or a violin. 

Why do they tune their webs?

Spider silk cannot be both stretchy and stiff at the same time.

The stretchy properties allow spiders to detect very small movements on the web, which is good for detecting prey, but not so great for the web's mechanical function. It still has to be strong enough to catch the insects. 

"If you imagine something like a windy day, you might want to prioritize mechanical stability so you don't lose the web in high wind," Beth Mortimer, a spider silk researcher from Oxford University's Department of Zoolology, said. "But that might be at the cost of a lower volume of vibrations." 

This is a daily occurrence: spiders change the stiffness of certain parts of their webs to adapt to shifts in temperature, humidity and wind speed. 

And when a spider tunes its web, each individual string has a huge range of different frequencies that can be transmitted — a wider range of notes, if you will. 

The frequencies that travel back to the spider communicate a lot of information, and allow the spider to differentiate between a tasty meal and a potential mate. This cute video from NPR explains the concept well:

Giraffe assassin bugs: a spider's worst enemy 

Spiders themselves make for very tricky prey. If they detect predators on their webs they can launch a counter-attack and either use their venom — or their silk. 

But giraffe assassin bugs have figured out a way around this. They're found in western Australia, feed on venomous spiders, and have the best name ever. 

These five-centimetre-long bugs can walk along a web without being detected because they carefully snap individual threads with their front feet, hold onto those ends loosely so they can't transmit vibrational information, then stay still while holding part of the web for minutes until gently releasing that fibre to the wind.

A giraffe assassin bug then slowly makes its way across the web until they can dine on the spider. 

So while you may have a new appreciation for that gorgeous spider web in the corner of your house, you might now be terrified of the giraffe assassin bug. For that, I apologize. But remember, it's only found in Australia.

Happy Halloween!