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A. aurantia  (writing spider)

 

The largest orb-weaving spiders (family Araneidae) in Wisconsin are two species in the genus Argiope (pronounced ar-GUY-uh-pee), A. trifasciata and A. aurantia (shown here in ventral view). Females of the two species are easy to separate by looking at the top of their abdomens. A. aurantia displays an elaborate pattern of yellow spots on black, and A. trifasciata’s abdomen is crossed with numerous black, silver, and yellow stripes.

Both species are daytime predators that usually hang head down in the center of their webs, which they often spin in fields between tall grasses or forbs. Their web is distinctive not only for its great size—dragonflies are often snared by them—but because of the zigzag silk banding, called the stabilimentum, that the spider decorates the center of the web with. Scientists argue about the function of the stabilimentum (some evidence indicates it attracts insects), but this gives the spiders one of their common names, “writing spiders”. These species are the basis of the popular children’s story about a literate spider, Charlotte’s Web. It was once held in the southern United States that seeing one’s name written by the spider presaged one’s death. In any case, the writing spider’s web does indeed spell death for many flying and jumping insects of Wisconsin’s fields, such as the flower fly (family Syrphidae) wrapped up in the upper left corner of this photo (note the large red eyes).

Just before dying in the autumn, the females will produce a single brown leathery egg sac, filling it with hundreds of eggs. The spiderlings hatch, then spend the winter inside the sac. They emerge in the spring and make small webs near the ground. As they grow through the summer, the webs get larger and higher, and the web/spider combination becomes conspicuous in August and September, before the first hard freeze kills most of those who remain. The males are rarely seen except in the vicinity of the female’s web; they are about 100 times smaller than the females.

 

Contributed by Michael Draney, UWGB assistant professor of biology in the dept of Natural & Applied Sciences

© 2001-2004 The Cofrin Center for Biodiversity and the University of Wisconsin Green Bay, All Rights Reserved
Last updated on April 15, 2014