A friend recently asked me about what to do about the black widow that was spinning a web over her entire sliding glass back door. (When you are the naturalist among your friends, you tend to get a lot of extermination questions.) To be perfectly frank, I hate spiders. Whenever I make the conscious choice not to squash a spider on sight, I give myself a little mental cookie, much the way I do when I put an aluminum can in the recycling bin. Call it the Indiana Jones principle: You’re allowed to be creeped out by one type of animal. I don’t understand some people’s fear of snakes, but I can
respect tolerate it. I know some bad-ass people who are afraid of rats. I love most animals. Just not spiders.
But I do love a well-spun spiderweb. A dew-dappled spiderweb early in the morning is a thing of supreme beauty, and the silk itself is an awe-inspiring substance. Normal spider silk has the tensile strength of steel, while the silk of the Darwin’s Bark Spider is ten times stronger than Kevlar. Most silk lines are only a few microns across, but if a spider could weave a strand the width of a pencil, that strand could conceivably stop a Boeing 747 in mid-flight. What’s more, spiders are capable of weaving up to 8 different kinds of silk from its spinneret glands: silk for draglines, silk for wrapping egg sacs, silk for wrapping prey, silk for parachutes, etc. And not every web is the classic “spiral orb;” webs are also designed as tubes, funnels, tangles, sheets, and domes.
However, the spider I want to focus on today is an orb-weaver, the Australian St. Andrew’s Cross spider, pictured above. Members of the Argiope family, such as the St. Andrew’s Cross, are often called “garden spiders,” or “writing spiders,” on account of their habit of decorating their webs with flourishes that sometimes resemble language. The name for these decorative markings are stabilimenta.
Argiope aurantia. Don’t read too much into it.
At first glance, the “X” shaped stabilimentum of the St. Andrew’s Cross spider seems to have an obvious purpose: the make the spider’s silhouette less obvious to both predators and prey. But stabilimenta take many forms and shapes with spiders all over the world, and the reasons for them are legion, varied, and mysterious.
Theory 1: The stabilimentum exists to make the nest visible. If you’re an Argiope building her nearly-invisible web close to the ground, you need a way to prevent blundering megafauna from walking or flying into it. So you make designs that stand out with special silk that reflects UV light, making them extra-visible for birds. (This is the equivalent of those orange balls they put on power lines so low-flying planes don’t hit them.) Unfortunately, this seems to be in conflict with…
Theory 2: The stabilimentum exists to hide you. Placing yourself in the middle of a big fluorescent “X” is the equivalent of… well, putting yourself in the middle of a big fluorescent “X.” Not a great way to avoid drawing attention to yourself. That’s why many non-Argiope spiders actually avoid the center of their web, attached to the bulls-eye by a non-sticky fishing line that lets them know if anything bites. Research seems to show that the stabilimenta does, in fact, attract curious spider-eating birds.
Theory 3: The stabilimentum is an ultraviolet flourish that attracts prey. As anyone with a bug zapper knows, insects are unreasonably attracted to blacklights. Much like ravers. However, a Cambridge study I found described how starving spiders put less effort into their stabilimenta than well-fed spiders, suggesting that stabilimenta are not primarily used for prey capture. And new research shows that stabilimenta designs may actually be cryptic to insects. Once the insects evolve to figure out that “X” marks the spot for “certain death,” that shiny UV trick would be useless.
Theory 4: Stabilimenta stabilize the web. That is where the name comes from, but it’s been proven wrong. Webs without stabilimenta are not significantly structurally weaker than those with. But can it hurt?
Theory 5: Stabilimenta capture heat and allow the spider to control its thermoregulation. Neat idea. Unproven. But the concept of an animal creating its own microclimate isn’t without real-life precedent.
Theory 6: The spiders are just letting off extra silk. Maybe… but if so, why the geometric shapes? And why would they produce more than they need, anyway? Nature may reward excess when it comes to reproductivity, but when it comes to pretty much anything else, evolution abhors waste.
Theory 7: The pattern make the spider look bigger. Indeed, the St. Andrew’s Cross spider does seem to look bigger when it’s in the middle of the “X.” But what advantage would this confer? If it’s sexual selection, well, only the females make the cross shape. So are females advertising their worth to males? Do they use their webs as the equivalent of kissy-face Facebook profile pages?
The reason the spider “writes” on its web may be any number of the above reasons, or may vary from species to species. But Theory 7 may be the most interesting one. If the female spiders are using their webs to advertise themselves in a sexual way, it would not only be a daring gender role reversal (males are usually the show-offs), but would imply that an invertebrate arachnid can actually have a sense of aesthetics. Female St. Andrew’s Cross spiders and other Argiopes — and perhaps other spider genera — may use their architectural skills and interior design tastes to advertise their reproductive status in much the way a male bowerbird does. One study in Spain found that the presence of a male in the area had a positive correlation with the size of a female Argiope’s stabilimenta. So it may be that their web design is like our web design: functional yet fashionable, designed to catch the greatest number of “hits” while making us appear in the most attractive light to members of the opposite sex.
Me, I go for chicks that dig cockatoos.