Spider webs are a masterpiece of structural engineering. Whether you're looking at the strength, the architectural organization, or even the make-up of the webbing itself, it's hard not to be impressed by the construction abilities of spiders. But how do they decide where to build?

In response to seeing this genuinely impressive use by a spider of a rock as a counterweight pillar to anchor a web (seriously, just look at it), a discussion began about another seemingly clever engineering decision that commenter Sajanas1 had noticed among the spiders at her house.

Sajanas1

I have a healthy respect for spider ingenuity. One summer, these penny sized spiders kept making huge webs in the evening on the front porch that we kept walking into when we came out of the house. But, after about a week, the spiders moved their webs up and to the right about a foot, just enough to let a human pass by without hitting it.On the plus side, it was surprisingly considerate of them. On the minus side, the spiders are LEARNING.

OzzyOnIce

I have some spiders around my house that do this. They build their webs about 3 inches taller than I am, regularly. It can't be an accident that they all do it, every time. They're learning, and possibly communicating about it?

Sajanas1

Well, a lot of spiders rebuild their webs every day, so it makes sense that they'd have some way of adjusting its positioning to avoid continual problems. Still, pretty impressive work for things that barely weigh anything at all, especially with how precise they'd make it to avoid humans.

It turns out spiders do, indeed, have a method for determining the position of the next day's web, and (alas!) it has very little to do with polite spiders trying to keep their webbing (literally) out of your hair.

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We already know that there are many different kinds of spiders and, therefore, many different kinds of webs, but even within a single web there are different kinds of threads. One of the most important structural parts of a web is a thread called a bridge line. This thread in the web is not only thicker (due to its layers of extra silk), it also serves as a way to anchor the web to a more stable point (a wall, or a tree, for instance.)

While the spider may not leave its whole web up overnight, if the bridge thread has survived the day (i.e. by not being pulled down by a passing person), the spider will often leave just a bridge thread up to use as a guide for where to build the next day's web.