Why Are Orb-Weaving Spider Webs So Sticky?

Orb-weaving spider webs are some of the toughest around. You can see this yourself if you find a typical garden spider‘s web and try to break it. It will break – but the web strands will then stick to your finger. The only way to remove them is to use a tissue and throw it away. From anecdotal observation, you can see how this stickiness is very advantageous: besides being excellent for catching prey, even larger and heavier ones that would break lesser webs, the stickiness of the web provides an interesting defence mechanism. Go near one of these spiders and poke around it to disturb it into thinking there’s a predator nearby. It will shake the web back and forth very, very violently, and the spider can no longer be seen – it gets blurred out because of the very fast movement. While my eyes are below average in their efficiency, I can imagine that the same effect is seen by other animals. This defence behaviour would not be possible if the web was not so strong and sticky.

Naturally, the question that first comes to mind is how this extreme stickiness is achieved. The silk itself is regular spider silk, a relatively elastic material by itself. The key to the stickiness is that the spider pours glue on the silk strands. Each glue droplet consists of a ~20┬Ám glycoprotein granule surrounded by a coating of salts that controls water regulation. The whole thing is pictured below (Sahni et al., 2010).

The glycoprotein granule is what acts as the gluing material. When it gets disturbed, for example by an insect, the granule deforms and stretches. The faster its expansion rate, the more viscous (i.e. sticky) the granule gets – this is how an orb web spider can capture prey that can usually escape webs of other spiders (for example, birds!).

Reference:

Sahni, V., Blackledge, T. A. & Dhinojwala, A. 2010. Viscoelastic solids explain spider web stickiness. Nature Communications 1, 19.

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