Sunday, July 6, 2014

The Black-and-yellow Garden Spider, Argiope aurantia

If you have an overgrown garden you have benignly neglected for many years, if you never use pesticides, if you have fervently absorbed Oliver Rackham's (in his great book, The Illustrated History of the [English] Countryside) sternest and most important dictum about having a garden, "Learn to detest neatness," then by mid summer you are bound to have at least a few Argiopes behind bushes or in ignored corners, those enormous golden garden spiders sitting in the center of their big webs with the zig-zag stabilimentum going down through the middle. I have been observing them for years, always learning new things.

Now, these big brightly colored spiders seem to appear out of nowhere. They aren't there one day, and the next day suddenly there they are. Well, if you start really looking you find they are there all the time, only in smaller, less recognizable instars. It might take you a couple of seasons, but eventually you push them back to the beginning. Maybe the first day of summer, June 20th, is a good day to start looking out for them.

What you'll see first, down low in thick vegetation, is a white thing about the size of a postage stamp.

Take a close-up with your digital camera and you will see a tiny spider. The only similarity it has with a full-grown Argiope is that it is sitting head down with two pairs of legs forward and two back. The stabilimentum, instead of going up and down in a zig zag, is a tightly woven squarish circle just the size to hold the spider's body and legs.

Nobody knows precisely the purpose of a stabilimentum, though recently it has been shown that they reflect ultra-violet light, and thus might act as an attractant to insects. But this baby stabilimentum has its own neat little function. Now you see the spider, but at the least disturbance, the spider vanishes quicker than you can see.

After another instar the silk mat is a bit larger, to hold a spider that is beginning to look more like an Argiope.

By another instar the spider is still too small to be noticed unless you are deliberately looking for it, but now the baby mat is beginning to extend into the up-and-down zig zag of the adult.

It can still perform its disappearing act.

With the next instar it has its adult stabilimentum.

And then, its adult color and pattern, though its abdomen still has its long narrow shape, not yet its rounded adult shape.

Interestingly, I saw all these different stages on the same day last week. The progress is very irregular. I think the tiniest spider must set up shop, and just wait for insects to come. If they don't, it stays the same size, if it makes a couple of lucky catches, it molts up to the next stage, and the more insects it catches, the quicker it moves from stage to stage, while the unlucky spider with a poor spot sits waiting. Even quite late in the summer I have seen that first postage stamp size, while the huge fat adult females are laying egg nest after egg nest.

At any rate, with decent luck, at last the spider reaches the penultimate molt, and males, battling one another, begin hanging around the edges of the web, to be the first one there when she sheds into full adulthood ready to mate. Here is that next to last molt, and she still has her girlish waistline, before it starts ballooning out with eggs.

If you want to follow the story, I've done an album of the life history of Argiope aurantia which includes a life history of Argyrodes, the tiny spiders that live parasitically in Argiope's web.

Click here

1 comment:

  1. I don't think I'll ever forget seeing my first adult black and yellow when I was around 6, in Montgomery, AL. Really startled me, I had no idea spiders could get that big! Great post. It solved some mysteries for me. Only wish I could have read it back when we were in Ashley County, it would have given me a lot of things to look for in the yard besides the adults, which were hard to miss.