Friday, October 03, 2008

A Bold, Beautiful, Benign Brand of Spider

As I was trying to learn the exact species name for "Charlotte," I came across some other interesting and fun facts that I thought I'd share.

As a starting point, I'd recognized her as a black and gold garden spider (the common name I knew) in the genus Argiope. As I understood it, Argiope spiders were the ones who wove the zigzag pattern into their webs. I was also quite sure that she was female, based on her size and the relative rotundity of her abdomen.

As is often the case, things were a little more complicated than that.

Charlotte is indeed female. She is an Argiope aurantia, also known commonly as the black and yellow garden spider, the writing spider, the yellow garden spider, the corn spider, and a lot of other similar names. These spiders are common around gardens, homes, and open, sunny areas in much of the U.S.

Males of this species (and many other spider species) are much smaller and more slender than the females. In this species, the male finds his way to a female's web at about this time of year. There he lurks around the perimeter, sometimes building a smaller web on the outskirts of the female's web or just nearby. He carefully plucks and vibrates the female's web, ready to make a run for it if necessary, to let her know he's there. After mating, the male usually dies or sometimes is eaten by the female for the extra protein to supply her eggs.
The female then constructs from 1-3 egg sacs of brown silk, each about 1" in diamater with a narrow neck and containing from 300-1400 eggs, which she suspends near the center of her web. She protects the egg sac(s) as long as she is alive, but she will die with the first frost. The eggs hatch that same fall, but the spiderlings remain dormant in the sac until spring, when they exit their sac and disperse. There is one generation per year.

Believe it or not, the bright yellow and black coloring of these spiders actually serves as camouflage in the open, sunlit areas where they usually build their webs.

Black and yellow garden spiders are neat housekeepers, rebuilding the center of their web every night. Actually, when I saw her, Charlotte was working on her web in the early morning hours, just as the sun was coming up. It was fascinating to watch the reconstruction work. As you can see in the photo to the left, first she ate the old web strands, then she would spin the new. Just in the short time I watched, I noted her producing 3 different kinds of silk: the sticky single strand used to make the spiral, a fine strand (presumably not sticky) that makes the cloudy center of the web (the part she's eating in the photo), and a heavy multi-strand silk used to make the zigzag pattern. Add to those the heavier nonsticky strands used to construct the radials and the strands used to make the egg sacs, and Argiope aurantia produces at least 5 distinctly separate types of silk.

Each web has a "free zone" between the central hub and the sticky catching spiral. With only the non-sticky radial lines in this area, the spider is free to move back and forth from one side of the web to the other through this area.

The zigzag pattern woven into the web is called the stabilimentum and is a common feature in the webs of spiders who leave their webs up during the day. At least 78 species of spiders build similar structures in their webs. Since webs with a stabilimentum catch as many as 34% fewer insects than those without, it is thought to be a warning to birds to help them avoid flying through the web and ruining it. Another theory is that it serves as further camouflage for the waiting spider.

Black and yellow garden spiders are important predators on grasshoppers, among other flying insect species. They can catch and eat insects up to twice their own size in their webs.

At the same time, they themselves are eaten by wasps, birds, lizards, and shrews. Their egg sacs are also preyed upon by many other animals, including birds. In fact, one source reported a study showing that 19 species of insects and 11 species of spiders besides A. aurantia emerged from A. aurantia egg sacs.

Last, but certainly not least, black and yellow garden spiders are considered generally harmless to humans. They are very gentle, only biting upon extreme provocation. Their bite usually produces an area of sore, itching swelling that subsides after a couple days. (Note: Individual reactions to any insect or arachnid bite may vary due to individual sensitivity. I would hardly recommending provoking any spider into biting, if it all possible!)

This is a fun type of spider to watch during late summer and fall, when they are the most easily noticed, although not everybody is lucky enough to have their own Charlotte hanging just outside their kitchen door!

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