Many of us have shared the experience of wandering along a path through a meadow and bumbling into an enormous spider web ruled by a fearsome yellow and black spider. Argiope aurantia, the so-called black and yellow garden spider, is extraordinarily common this year in some locations. Garden spiders belong to family known as Araneidae, the orb weavers, made famous by E. B. White in his classic tale called Charlotte's Web. Like Charlotte, the garden spider performs a ritual of building a web only to take it down each day. Webs of the black and yellow garden spider can be extraordinarily large and often span several feet. One such web I recently bumped into was home to an entertaining female garden spider.
Webs
A stabilimentum of heavy silk adorns the center of this web.
You may recall that Charlotte used her web to write eloquent praises of her friend Wilbur in an attempt to rescue him from a dinner date. To my disappointment, the garden spider I met had only somewhat mastered the letter “W” which it copied repeatedly to create a conspicuously large band of zigzagging “W’s” in the center of its web. Spider aficionados call this conspicuous band of heavy silk the stabilimentum. The function of the stabilimentum is a topic of debate among arachnologists. Some suggest that the band helps disguise the spider from its predators by providing a form of camouflage as the spider rests in the center of its web. Others believe that the silk may act as a tiny parasol shielding the spider from intense rays of the sun. One fascinating study revealed that the conspicuous bands of silk acted as a visual warning to low flying birds thereby reducing the likelihood of devastating web-destroying crashes much the same way an image of an owl on a large plate glass window dissuades misguided birds from crashing and breaking their necks. Of course, the spider cares not for the welfare of the bird, but repairing bird-damaged webs takes time away from important projects like capturing and eating insects and other prey. Although I tried to resist poking the large spider, my curiosity got the better of me and my digital inquiry provoked a remarkable behavioral display. The garden spider retreated slightly from the stabilimentum and began to rhythmically flex and extend its legs. These gyrations set the entire web rocking back and forth in ever-increasing waves. Swaying the web in rhythmic motion is called web flexing and it is often observed in orb weavers. Web flexing has been reported as a way to dislodge potential predators or causing prey to become entangled in sticky capture-threads in the web. Web flexing may serve other defensive purposes.
Predators
Enemies of the orb weaver include predatory lizards, toads, and other spiders that rely on keen eyesight to locate and capture prey. In an interesting treatise on orb weavers, Wayne Tolbert suggested that web flexing might be a clever way for the spider to conceal its exact location, thereby confounding hungry predators. Photographing the swaying garden spider was challenging and watching some of the video clips made me seasick. Perhaps the cliché, “it’s harder to hit a moving target”, applies well to our friend the garden spider so rock-on spider, rock-on!
References:
We thank Jeff Shultz for providing insights into the curious behavior of spiders. Two great articles “Predator avoidance behaviors and web defense structures in the orb weavers Argiope aurantia and Argiope trifasciata” by Wayne Tolbert and “Do stabilimenta in orb webs attract prey or defend spiders?” by T.A. Blackledge and J.W. Wenzel were consulted for this episode of Bug of the Week. To learn more about these awesome predators, please visit the following web sites.
http://www.uark.edu/ua/arthmuse//argiope.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Argiope_aurantia.html