The Universe of Discourse
           
Fri, 14 Sep 2007

Why spiders hang with their heads down
Iris asked me last week why spiders hang in their webs with their heads downwards, and I said I would try to find out. After a cursory Google search, I was none the wiser, so I tried asking the Wikipedia "reference desk" page. I did not learn anything useful about the spiders, but I did learn that the reference desk page is full of people who know even less about spiders than I do who are nevertheless willing to post idle speculations.

Fortunately, I was at a meeting this week in Durham that was also attended by three of the world's foremost spider experts. I put the question to Jonathan A. Coddington, curator of arachnids for the Smithsonian Institution.

Professor Coddington told me that it was because the spider prefers (for obvious mechanical and dynamic reasons) to attack its prey from above, and so it waits the upper part of the web and constructs the web so that the principal prey-catching portion is below. When prey is caught in the web, the spider charges down and attacks it.

I had mistakenly thought that spiders in orb webs (which are the circular webs you imagine when you try to think of the canonical spiderweb) perched in the center. But it is only the topological center, and geometrically it is above the midline, as the adjacent picture should make clear. Note that more of the radial threads are below the center than are above it.


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