Spider Web Builder

How an orb web catches insects, and why it beads with dew

An orb web looks delicate, but it is a carefully engineered trap. It must stop an insect flying at a metre or two per second without tearing, hold on to it, and tell the spider exactly where it is, all with silk thinner than a human hair.

Building in order

A garden spider starts by letting a thread drift on the breeze until it snags on something: the bridge line at the top of this web. It strengthens that line, drops down to make a Y-shaped frame, and adds more frame threads. Then it lays the spokes, or radii, from a central hub out to the frame. It walks a temporary spiral outwards to hold everything steady, and finally spins the sticky capture spiral from the outside in, crossing each spoke once per turn.

Two kinds of silk

The frame and spokes are made of dragline silk, which is stiff and very strong. The capture spiral is a different silk: soft and so stretchy that it can extend to several times its length, and coated with droplets of glue. When an insect hits, the stretchy spiral and the slightly springy spokes give way together, turning its energy of motion into stretch and then into heat, so the insect is stopped gently enough that it bounces back into the glue rather than snapping straight through.

Feeling the catch

A struggling insect shakes the threads, and the tight spokes carry those vibrations to the hub like strings on an instrument. The spider waits there with a leg on several spokes and can tell which way the shaking comes from, so it runs straight out to the right place and wraps its prey in silk.

Beads of dew

On a cool morning, water coats the threads. A thin cylinder of liquid is unstable: surface tension squeezes it more where it is narrow, so tiny ripples grow until the film breaks into a row of evenly spaced beads, with smaller satellite drops between them. This is the Plateau–Rayleigh instability, the same effect that breaks a thin stream from a tap into drops, and it is also how the glue on a capture thread gathers into droplets. The dew adds weight, so a beaded web sags.

For the curious

Each thread here pulls with F = EA·ε, where ε is its strain, with EA = 0.06 N for spokes and frame and 0.004 N for the spiral; threads go slack instead of pushing. Waves travel along a thread under tension T with mass per length μ at c = √(T/μ). A fly of mass m_f arriving at speed v hands the web its momentum, setting the nearby threads moving at about m_fv/(m_f + m_web). A liquid cylinder of radius r is unstable to ripples longer than its circumference, 2πr, and breaks up fastest at a wavelength of about 9r.