I love seeing garden spiders (Argiope aurantia), aka orb-weavers, spinning their webs in fall. They’ve been around since spring, but it is never until autumn when I start noticing them. These spiders are large and scary when first encountered, but they are considered beneficial because they eat aphids, flies, grasshoppers, mosquitoes, moths, and wasps. Of course, they will also eat almost anything caught in their webs, including bees, butterflies, and dragonflies. Still, most gardeners appreciate these spiders and like seeing them in the garden.
If you look at an orb-weaver’s web, you will notice a thick thread placed in the middle woven in a zig-zag pattern. This zipper-looking piece is called the stabilimentum. The stabilimentum is a puzzle to scientists, and there are several hypotheses why orb-weavers include them in their web. Some believe it strengthens the web. Others think it helps camouflage the spider. Some guess it is to warn birds of the web’s existence, so they will not fly through it. But the newest hypothesis is that it is used to attract insects because only the stabilimentum can be seen in ultraviolet light, a wavelength some flowers also emit.
For the most part, orb-weavers die in the winter, leaving behind one of more egg sacs which hold hundreds of eggs, but the chances of these eggs hatching are small. In one study over a three-year period, more than 97% of the egg sacs were damaged by birds. And over half of the sacs observed held parasitic wasps or parasitic flies. If the orb-weaver eggs survive, small spiders will hatch in the spring, when the weather warms, biting a tiny hole through the sac for them all to crawl out of. Even after emerging, many will not live to adulthood. No wonder these spiders are such a marvel to see.
The large spiders you see on the web are the females. The males are much smaller and most die after mating. They are not aggressive, and bites are rare. It eats by wrapping its prey in silk and injecting it with a special enzyme which liquifies it. The spider can then suck the juices out though the webbing.
Orb-weavers shed their skin as they grow, will regenerate limbs if necessary, and can rebuild their webs nightly. If you get too close, you may see it vibrate its web, which is to make the spider look larger to animals which might do it harm. Sometimes orb-weavers sit away from the web with a line on the end of one leg. When the line pulls, it knows something has gotten caught.
When you encounter an orb-weaver in your garden, rejoice. These gentle giants are good to have, and a wonder to see.