Study leader I.-Min Tso, an entomologist at National Chung Hsing University in Taiwan, first made the discovery walking through a research station in central Taiwan.
Tso noticed Jackson Pollack-style splotches of white bird dung, which stood out in stark contrast to the lush green foliage. But when he looked more closely, Tso realized that not all of the blobs were bird droppings: A few were spiders in their webs. (Read about a spider that weaves a mysterious picket fence.)
Tso recognized the spiders as C. ginnaga, a species found in Taiwan, China, Japan, and South Korea.
As the “architects of the spider world,” spiders in the genus Cyclosa are known to create elaborate webs, using their silk to make concentric circles like Saturn’s rings and adding debris such as twigs and leaves that hide young spiders from predators. (Related: “New Spider Weaves Spider-Shaped Web.”)
Now, Tso and colleagues have discovered their defense strategy is even more sophisticated than thought.
Cornell University arachnologist Linda Rayor applauded the study, pointing out that C. ginnaga is not alone in masquerading as bird droppings.
“It’s really not all that uncommon. Several other spiders, like Bolas spiders, also use this disguise,” she said.
Web of Disguises
To find out if the bird dropping-like web confused predators, Tso and colleagues measured the webs that resembled bird feces.
At approximately a 0.4 inch (0.6 centimeter) wide, the webs’ shapes and sizes are nearly identical to many of the actual bird droppings seen at the research station, the team discovered. (Also see “Photos: World’s Biggest, Strongest Spider Webs Found.”)
The scientists then measured the contrast in color between the spider—whose back is white—and its web decorations, and found that the differences in color were too small to be distinguished by its wasp predators. This bolstered the hypothesis that the bird droppings were helping to disguise the spider on its web. (See National Geographic’s spider videos.)
Lastly, the team set up an experiment: They caught wild spiders and darkened either the arachnid or its bird-dropping structure and set up video cameras outside to watch the spiders over several days. They also observed spiders that were not manipulated in any way.
Compared with the white-on-white control spiders, the spiders that had blackened webs or bodies were much more likely to be attacked by predators, according to the study, published March 29 in the journal Scientific Reports.
Even so, Tso and colleagues still can’t say whether predators see these spiders as bird droppings or whether they just blend in with the environment.