Orb-weaving spiders are those spiders that build the spiral wheel-shaped webs that we often tend to think of as the Platonic ideal of spider webs. The ones you find draped between two dewy branches in a sun-dappled meadow, spider sitting in the dead center lying in wait for hapless flys and other insects to collide with the nearly invisible, impossibly sticky threads and get trapped.
While this technique keeps the spiders’ bellies full, it’s not without its problems. Those of us who’ve walked through a doorway or between two tree branches only to come away covered in thin, sticky web threads know that it’s not pleasant (for me, it is the stuff of nightmares). For the spider, it’s absolutely disastrous. Severe damage to the web by humans and other animals that the spider has no intention, or hope, of devouring costs them the production of more silk for a new web, exposure to predators, lost hunting opportunities and missed meals, and ultimately plays with the odds of their survival.
What’s a spider to do, then, when getting through the day requires a web that’s inconspicuous enough that prey don’t notice it, but has enough presence to warn animals that would just wreck it?
The species of the genus Argiope pull off a contradictory, but seemingly necessary, signaling paradox by constructing “decorations” of zigzagging bands of silk on their webs. They make the web very conspicuous to the naked eye, even from a distance, but don’t seem to tip off prey insects that they’re about to stumble into a trap.
Previous studies showed that decorated webs are damaged and destroyed less frequently than undecorated ones, and André Walter and Mark A. Elgar from the University of Melbourne wondered if this protective function was what motivated the spiders to build the decorations. It’s one of three explanations for the decorations that entomologists have tossed around for year, the others being that the decorations provide a signal for prey attraction, or conceal the spiders’ outlines and camouflage them from predators.
Walter and Elgar collected females of the species A. keyserlingi (above, also known as the St. Andrew’s Cross spider because the decoration they build at the center of their web resembles the X-shaped cross the Christian St. Andrew was crucified on) near their lab at the University of Melbourne and left them to build their webs on plastic frames set up in the lab.
Once the webs were built, Walter and Elgar rained destruction down upon them. They divided the spiders into three groups, left one group’s webs alone, “lightly damaged” another group’s by cutting web threads to simulate damage caused by prey impact and “heavily damaged” the third group’s by cutting ¼ of the web threads plus two diagonally opposite anchor threads to collapse the web. When the webs were rebuilt, they cut them down again and again for what might have been the most stressful 14 days of these spiders’ lives.
Throughout their reign of terror, Walter and Elgar kept tabs on two key characteristics of the webs: the size of the web’s “capture area” (the central sticky part where insects get stuck), and the pattern and the size of web decoration bands.
Capture area size didn’t change in the webs that experienced no damage or mild damage, but shrank by about 13% over the course of the experiment in the webs that experienced heavy damage. In all three groups, the proportion of spiders that built web decorations increased. At the start of the experiment, about a quarter of the spiders in the no damage group decorated their webs. At the end, a little more than half of them were building decorations. The decoration increase in the mild damage group wasn’t much greater than that in the no damage control, but the heavy damage spiders really got in touch with their inner Martha Stewarts, and the proportion of decorators shot up from 28% to 81%.
The post-damage decorating craze led Walter and Elgar to think that the spiders are using the decorations tactically to make their webs more obvious to passing animals that might unintentionally tear them down. But why the increase in decorating among spiders that didn’t get their webs wrecked too badly or even touched at all? Walter and Elgar think that the slight uptick there was because the captive spiders were well fed and could afford to invest a little more in their webs, whether their homes needed sprucing up or not.
Reference: André Walter, & Mark A. Elgar (2011). Signals for damage control: web decorations in Argiope keyserlingi (Araneae: Araneidae) BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY : 10.1007/s00265-011-1200-8
Image: “Argiope aetherea - St Andrew’s Cross spider” by Amos T Fairchild. Used under a Creative Commons License