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Thursday, Dec. 11, 2003

ANIMAL TRACKER

Writing spider


* Japanese name: Koganegumo
* Scientific name:Argiope amoena
* Description: Writing spiders, also known as yellow garden spiders, have an egg-shaped abdomen with yellow or orange markings on a black background. The legs have red or yellow sections. While insect bodies are generally comprised of three sections -- head, thorax and abdomen -- spiders have two: an abdomen and a cephalothorax, which in writing spiders is covered in silvery hairs. There is sexual dimorphism in size: females range from 19-25 mm, males reach only 5-9 mm.
* Where to find them:All over Japan (except Hokkaido) in rice fields, open land and gardens. Webs are constructed over several hours each day, and consumed at night. The next morning, the spider usually builds a web in the same place. Males build smaller webs on the edges of the female's web. After mating, females produce one to three egg sacs, each containing 300 to 1,400 eggs, and attach them to the center of the web. She guards the sacs until she dies at the beginning of winter. The spiderlings emerge from the sac in spring.
* Food: Aphids, flies, moths, dragonflies, crickets, bees, wasps -- in short, any small winged insect that is caught in its web. Writing spiders may bite humans if handled, but the venom is not dangerous to us.
* Special features: Writing spiders are so named for the extraordinary zig-zag patterns they often weave into their webs. These are called stabilimenta, because they were once thought to provide structural stability. But their real function is not well understood. Some biologists think the patterns attract flying insects, luring them to their death, but other studies have shown that webs with more stabilimenta attract fewer insects. Alternatively, they might function like the Xs of duct tape humans stick on windows to discourage birds from flying into them. They might also help the spider detect the various prey stuck in the web, since stabilimenta vary the tension of the silk threads. Whatever the function, it is probably visual rather than structural, because only diurnal spiders construct stabilimenta.



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