There's probably no orb weaver more famous than the fictional Charlotte, the clever spider that saved a pig's life in E. B. White's beloved story, Charlotte's Web. As the story goes, White wrote Charlotte's Web after marveling at the intricate patterns in a spider's web in the barn on his Maine farm. While we've yet to discover a real spider capable of weaving "some pig" or "terrific" in silk, we do know of many spiders that decorate their webs with zigzags, circles, and other fancy shapes and patterns.
These elaborate web decorations are known as stabilimenta. A stabilimentum (singular) may be a single zigzag line, a combination of lines, or even a spiral whorl in the web's center. A number of spiders weave stabilimenta into their webs, most notably orb weavers in the genus Argiope. Long-jawed spiders, golden silk orb weavers, and cribellate orb weavers also make web decorations.
But why do spiders decorate their webs? Silk production is a costly endeavor for a spider. Silk is made from protein molecules, and the spider invests a lot of metabolic energy in synthesizing amino acids to produce it. It seems unlikely that any spider would waste such precious resources on web decorations for purely aesthetic reasons. The stabilimentum must serve some purpose.
Arachnologists have long debated the purpose of the stabilimentum. The stabilimentum may, in truth, be a multi-purpose structure that serves several functions. These are some of the most commonly accepted theories on why spiders decorate their webs.
The term stabilimentum itself reflects the first hypothesis about web decorations. When scientists first observed these structures in spider webs, they believed they helped stabilize the web. Of the theories listed here, this is now the one considered least plausible by most arachnologists.
VisibilityBuilding the web consumes time, energy, and resources, so the spider has an interest in protecting it from damage. Have you ever seen those stickers people put on windows to keep birds from flying kamikaze missions into the glass? Web decorations may serve a similar purpose. Some scientists suspect the stabilimentum serves as a visual warning to prevent other animals from walking or flying into it.
Other arachnologists believe the opposite may be true, and that the web decorations are a disguise of sorts. Most spiders that build stabilimenta also sit and wait for prey in the center of a rather large web, which could make them vulnerable to predators. Perhaps, some speculate, the web decoration makes the spider less visible by drawing a predator's eye away from the spider.
Prey attractionSpider silk is an excellent reflector of ultraviolet light, leading some scientists to hypothesize the stabilimentum may function to lure prey. Just as insects will fly toward lights, they may unwittingly fly toward a web that reflects light, where they would meet their death when the hungry spider moves eats it. The metabolic cost of constructing the flashy web decoration might be less than the savings from having your next meal come right to you.
Some arachnologists wonder if the stabilimentum is simply a creative way for the spider to expend excess silk. Some spiders that decorate their webs use the same kind of silk to wrap and kill prey. Research shows when these silk supplies are depleted, it stimulates the silk glands to begin producing silk again. The spider may construct the stabilimentum in order to deplete its silk supply and recharge the silk glands in preparation for subduing prey.
Mate attractionNature provides plenty of examples of organisms showing off to attract a mate. Maybe the stabilimentum is a female spider's way of advertising for a partner. Though this theory doesn't seem that popular with most arachnologists, there's at least one study that suggests mate attraction plays a role in the use of web decorations. The research showed a correlation between the presence of a stabilimentum in a female's web, and the likelihood that a male would present himself for mating.