A Trap in Plain View: Orb web spiders broadcast their webs

St. Andrew's Cross spider (By Andre Walter)

Have you ever walked through the woods or even a doorway and received a face full of spider web? It’s not a pleasant experience. The invisible threads of the web stick to your skin and because they’re incredibly thin and delicate, removal doesn’t happen nearly as fast as you’d like.

While a Charlotte’s Web-esque sign of “Danger!” would be nice, the spider’s web is a trap for unsuspecting insects, so it’s typically inconspicuous. Some orb web spiders, though, do something unusual: They decorate their webs. One of these spiders, the St. Andrew’s Cross spider, weaves thicker bands of zigzagging silk that form easily identifiable crosses. The crosses broadcast the location of the web as well as that of the spider, which sits in the web’s center. But what’s the purpose of the decorations since they seem to defeat the very definition of a trap?

The decorations tell animals the spiders don’t eat to “Stay Away!” say researchers from the University of Melbourne. When unaware animals, such as mammals and birds, run into a web, its owner must spend both time and energy in reconstructing it. Spiders that need to rebuild lose opportunities to catch meals.

If its web is heavily damaged, then the St. Andrew’s Cross spider will make the crosses larger in the next round, found the recently published Behavioral Ecology and Sociobiology study. Over the course of two weeks, the researchers caused damage to the webs of 44 spiders. In one group, they inflicted mild damage, which would be similar to the spider’s prey landing in the web. In the other group, the scientists made the webs collapse by cutting two crucial corner tethers. The group that sustained the heavy damage and had major remodeling to do made their future webs smaller with larger decorations than the group that sustained mild damage, which didn’t change its building habits.

In nature, decorated webs stay around longer than undecorated ones. Although the decorations may shoo non-prey animals away, scientists debate whether they also serve as advertisements to the spider’s own predators that flash “I’m here!”

Also see: Spinning Their Spell (A Photo Gallery)