Friday, May 02, 2014

Traplines in the air

It's going to be a good summer for watching spiders. The rhododendrons and cedars on the sunny side of our lawn are festooned already with big webs. The spiders, female Araneus diadematus, aka cross spiders, are still tiny, barely an orange speck in the centre of each web. They'll grow; by the end of the summer some will be up to an inch across, fat bellies showing their success as trappers.

They hang, belly out, upside-down, in the centre of the web. One back leg holds a drag line attached outside the web. This spider has caught and wrapped some sort of fly. An early supper!

"It is common for a web to be about 20 times the size of the spider building it." Wikipedia

Another spider, still waiting for her prey.

These are seriously smart critters. Building a web isn't just a rote operation; every site has its special requirements, and the webs are more elaborate than the simple spiral and ray arrangement shown in children's books.

The spider launches a thread from the top of her chosen location, waits until it makes contact with another branch, then runs down it to glue it down well and reinforce it. She picks a centre and builds another ray out from there, then more until she's filled her space. Then she makes a small spiral in the centre, using non-adhesive silk, glued together where they cross the radials. This is her resting place and launch pad.

8 rounds in this spiral. Note the glue spots at the nodes only.

Then there's a gap, about twice the diameter of the inner spiral. What is function is, I don't know. Maybe it keeps the struggles of her prey out of her private space. Only she really knows.

Then comes the business part of the web. She fills most of the available space with more spirals, built first with non-adhesive silk, then replaced with the sticky stuff. (She eats the first lines; spiders recycle!) She leaves more dots of glue here, spaced randomly, not usually on the nodes.

Outer web. Note the glue spots. The rays are not sticky; the rest is.

And here's where her web differs from the standard drawing; every so often, along those regularly-spaced spirals, she breaks the pattern to make an X, sometimes a Y, sometimes a knot of angled threads. The spider at the top here has a large area like this near the inner edge of her trap; the second spider is a bit more restrained, sticking to a few simple Xs and offset sections.

I was inclined to think of these, at first, as mistakes, the spider losing her way briefly, getting confused. But every cross spider does this; it probably has some function. Maybe it's like the trusses in bridges and roofs, using the triangular shape to add more strength.

What went on in the spider's head? (Or belly, or legs, since her brain is too big to fit in that little cephalothorax, and she's outsourced it to several parts of her body, including the legs. Up to 80% of that little body is brain.) How does she decide it's time to change direction? Does she do the math? Or just sense some instability in the web and X it out?

Questions, questions.

As I sit here typing, a pinhead spider has been busy building a web on a sparrow feather beside my desk. She came down the wall, jumped the gap, and made a beeline for that feather, climbed it and dropped her anchor. How did she know the feather was there? How does she figure that's a good hunting spot?

And what will she be catching. She's so small I barely see her; does she see my desk crawling with little beasties that I can't see? Now I'm itchy!


  1. Fascinating! I'm going to have to take a much closer look at the spider webs I see!

  2. Furry; the best part of that is that every species of spider does it slightly differently. Or majorly differently, as the case may be.


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