Spiders Decorate Webs with Ornaments

Ribbons, shimmery fluff, silk tufts and hints of red and green might sound like Christmas tree ornaments, but these decorative touches have all been spotted in the webs of orb-weaving spiders, according to a new study.

However, the festive scene takes on sinister new meaning. Researchers have just discovered that, similar to how human eyes are drawn to holiday ornaments, spider prey are drawn to their death by colorful, shimmering decorations.

"This is the first study to show that plant-detritus decorations serve as luring signals to attract prey," co-author Daiqin Li, associate professor of biological sciences at the National University of Singapore, told Discovery News.

The "detritus" in this case consists of bits of leaves, flowers, twigs, bark and any other plant materials that the spider web decorator chooses.

Along with silk-made ornaments, gory-looking prey remains and egg sacks, the plant materials provide the finishing touches to spider webs created by species like Cyclosa ginnaga, which was the focus of this particular study.

Li and his colleagues conducted both lab and field experiments, the latter in the forests of Xishuangbanna, China, to study how spider victims reacted to ornamented versus plain webs.

Bottom line: It pays for spiders to decorate.

The researchers discovered that webs with the ornament-like additions lured anywhere from 200 to 233 percent more insect victims than webs without the augmentations.

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"The silk items are made by spiders after the completion of the web and just loosely attached to the web spiral, radial or frame silk threads, depending on the species," explained Li.

As for the plant materials, he said that "the spider usually uses silk to 'glue' the debris to the web silk."

One theory as to why spiders with fancy webs might do better in the food department holds that the spiders can hide behind the bits of plant material and other handiwork to sneak up on prey as well as to escape their own bird predators.

Li and his team, however, found that wasn't the secret to the spider's success.

Further studies on the visual systems of birds and typical spider prey, such as bees, determined that both groups could easily spot the spider and its decorations.

"This is a crucial piece in the puzzle, as only then do behavioral data make sense," commented Mariella Herberstein, a senior lecturer in the Department of Biological Sciences at Macquarie University. She did not work on the new study, which has been accepted for publication in the journal Animal Behavior, but she has conducted many other studies on spiders and is a noted arthropod expert.

"It might be that the ultraviolet component of the color signal is responsible," she said. "Insects love UV and just can't help themselves from flying towards UV."

Li and his colleagues even found that the spider's silvery abdomen adds to the UV mix and heightens the color contrast between the web ornaments and the surrounding environment.

The scientists had hoped to test whether the decorated webs protected spiders against bird predators, but no spiders -- with or without fancy webs -- wound up in a bird's mouth during the study. Herberstein therefore hopes Li and his team will revisit this possible extra benefit of the ornaments in future.