Dear Straight Dope:
I woke up one morning to find a spider web spanning my garden covered in morning dew. The web was 7 or 8 feet wide, leading me to wonder: how does a spider position the anchor points of a web so far apart without it breaking or getting caught?
SDStaff Doug replies:
The details are pretty widely known and easily found; that said, there are some little-appreciated points worth making.
Many spiders, including those that build orb webs such as you saw, release a strand of silk into the wind (it’s effectively a quick-drying liquid) and allow the wind to catch it. If the spider is small, the pull of the wind on the silk can exceed the weight of the spider, sending it aloft — a process known as “ballooning,” perhaps best known to most folks from the popular children’s book Charlotte’s Web.
Orb-weaving spiders use a variant of this technique when starting a web, releasing a long silk line and waiting until they feel the far end of the silk snag on something — a sort of sticky version of a grappling hook. They then anchor the near end and crawl back and forth on the grappling line, reinforcing it by releasing additional silk as they go.
Once this single line has been reinforced sufficiently, they drop a “Y”-shaped arrangement of silk, and build the orb using that as a starting point, adding and subtracting parts of the orb as they go. The process is well-documented and well-studied but remains a delicate feat. With no wind at all, it’s almost impossible to begin the process, but it’s equally impossible if there’s too much wind.
Many fragments of silk can get lost in the breeze, either during construction or following deterioration of an abandoned web, and these bits of loose silk (along with the pieces used by ballooning spiderlings) are an ubiquitous component of aerial flotsam. They can gather in crevices both outdoors and in to form cobwebs, even in places where no spiders live, although many cobwebs are simply old abandoned webs.
Also, despite the importance of gravity to the whole process, spiders can still make orb webs in zero-gravity chambers — though it’s not easy for them, and the results aren’t quite the same as a normal web.
SDStaff Doug, Straight Dope Science Advisory Board
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