How do spiders spin webs across open spaces?

Helen - This is particularly relevant because I think there's been a hatching of spiders in Cambridge very recently. If I sit in my garden for more than about 5 minutes, I get turned into a spider web. So, they're doing it at the moment.

Chris - Turned into a spider web?

Helen - Okay. One gets made around me. So they're definitely out there doing this but how are they doing it?

Chris - Hopefully, it's a money spider. Yes, a good question, isn't it? because you think, "I see this web. It goes from one tree over there to one tree over there. did the spider go all the way down, walk along the ground, up the other tree, and then string this piece of thread across the two?" The answer is no, of course. It's too small to know these places exist relative to each other.

The way the spiders do this actually is that they sit on the end of one twig or something, and they stream out this very long but very light thread of silk which gets picked up on air currents and it floats away from the spider, and the spider is continuously testing the tension in the thread. When it feels it goes taut, it realises it must have snagged on something. So it will then fix that end and go across counting steps - because the spiders measure distance by counting their steps, and it therefore knows how far away it is. It then counts back halfway, knows that it's halfway back across and then drops a perpendicular. So it goes down to the ground and that's the middle of its web in a sort of T-shape, and fixes the bottom thread, and then after that, it's got the three points it needs to start making the web. So that's how it does it, ingenious stuff.