Illinois Natural History Survey - University of Illinois

Black and Yellow Garden Spider

 

Susan Post

 

Almost anyone who has ventured into an oldfield or even a garden in late summer has undoubtedly had a close encounter with an imposing yellow and black female garden spider.  Her large size, bold colors, and habit of sitting rather menacingly in the center of a large web, nearly always built in open, sunny locations, make the garden spider easily recognizable. Such encounters are unforgettable, if not downright startling.

The black and yellow garden spider, Argiope aurantia, is one of two species of garden spider found in Illinois. A. aurantia are found from southern Canada, south through the lower 48 United States, Mexico, and Central America. They are found as far south as Costa Rica. These spiders have egg-shaped abdomens with yellow or orange markings on black backgrounds. Their legs are black with the upper portions orange. Like most spiders, the females are larger 19–28 mm, while the males are 5–8 mm.

Garden spider webs are of the classic orb design—a central hub with a geometric arrangement of spirals and strengthening threads radiating from the center like the spokes of a wheel.  This design forms a strong, yet flexible structure up to two feet in diameter, making it a highly efficient interceptor of flying or jumping insects as large as grasshoppers.  Adding to this efficiency is the unique zigzag pattern of shiny white silk extending from the center of the web.  This was once thought to be a warning device to prevent birds or other animals, perhaps even the inattentive gardener, from blundering into the painstakingly constructed web. However, recent research suggests an even more ingenious purpose.  Because it reflects ultraviolet light much like flowers that use these signals to attract insects for pollination, the structure may actually be sending an irresistible invitation to insects. They may fly into the web thinking they are headed to a flower rather than onto a dinner table!

Once snared in the web, the insect’s struggles are sensed by the female, who has been lying in wait, with her head downward and legs outstretched to better detect even the faintest hint of a meal.  She deftly moves to the source of vibration and quickly restrains the captured prey by wrapping it in silk before biting and injecting it with paralyzing venom.  This ready-to-eat package is then moved to an out-of-the-way portion of the web so it will not interfere with later catches. The hapless insect can then be sucked dry at the spider’s leisure.

In spite of their impressive sizes and seemingly ruthless demeanor, garden spiders are harmless to humans and should be seen as allies in the garden, even when they cause us heart palpitations in the tomato patch! This Illinois resident will perish with the first hard freeze, leaving only a brown, fuzzy egg cocoon filled with tiny spiderlings that must await the coming spring.

 

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