How do spiders bridge a gap?

It's all a bit chancy really, it's not a determined thing. Once that first little thin line has stuck on they'll climb along it and reinforce that first strut. Then they actually use their own bodies and footsteps to measure along and count back to the middle of it to get to the central point. Then they drop a sort of plumb line using themselves as the weight. They'll go down like an absailer down a cliff. They absail down that line to make a Y-shape and that is the scaffolding they'll fill in with different patterns.