Spiderweb stabilimenta: To see, or not to see?

I go to a behavioral ecology/evolutionary biology discussion group every Friday, and this the topic of discussion was the role of stabilimenta in spider webs. What is their function, and when is it advantageous to have them?

Just as some background information, stabilimenta are "web decorations," patterns of extra silk that a spider weaves into its web (and you thought Charlotte's scripting skills were purely from E.B. White's imagination!*). The "orb weaver" genus Argiope is the most common example of elaborate stabilimenta (although Cyclosa turbinata and Uloboras glomosus also utilize them), but the exact function of these "decorations" has been debated over the years. As the name implies, it was originally thought to be a way of providing extra structural support to the web, but this theory has not been supported by research.

Another theory was that stabilimenta help to attract prey into the web, which is an enticing idea since the silk reflects UV wavelengths, which, although beyond our limits of perception, is visible to insects. It appears that the silk is reflective at a broad range of wavelengths, however, encompassing most of the visible range for most species, so a specialization for signaling to insects seems unlikely.

Orb weavers do not put stabilimenta into every web, indicating that there must be some situations where the structures are more or less advantageous/useful. A study by Todd Blackledge and John Wenzel showed that stabilimenta actually decrease the number of prey captured by almost 30%. Why in the world would spiders expend so much extra energy creating intricate patterns that result in a drastic decline in hunting success?

The Blackledge and Wenzel study suggests that the stabilimenta do function as "signposts", but not for insects. The researchers showed that webs with stabilimenta were less likely to be destroyed by flying birds. This is apparently a behavioral defense mechanism: a spider will go out of its way to make the web conspicuous so that birds will see it in time to avoid flying through it. I do a lot of field work tromping around in the forest, and I hate the feeling of walking into/through a big spider web, apparently songbirds tend to agree: the study showed that webs with stabilimenta were 45% less likely to be damaged by birds.

There is obviously a cost-benefit compromise going on here. Stabilimenta mean less food, but also less web destruction. If a spider has to rebuild her web after being bombed by a klutzy sparrow, she could lose much more than the 30% of its prey yield that the stabilimenta would have cost her. (I use female pronouns on purpose, only females build webs, males are very small and are pretty much just breeding machines).

Something else that that supports the idea of a cost-benefit analysis at work here: There is also evidence that spiders are much more likely to build stabilimenta into their webs just before laying eggs or molting: at times when they cease to feed. Since catching less prey at those times doesn't matter, the cost plummets and the spiders are much more likely to advertise their webs to protect them from potential damage. One other note of interest, Argiope spiders are diurnal, just like the birds that they seem to be advertising to.

I found this to be extremely fascinating. It reminded me of Richard Dawkin's idea of the extended phenotype: the spiders have a gene for a behavioral with a physical manifestation that also affects the behavior of other species. This also shows why it is important to consider interspecific interactions and ecological context when studying animal behavior, as opposed to just bringing something into a sterile lab and thinking you can a) get it to behave naturally and b) understand why it does what it does.

I highly recommend the Blackledge and Wenzel paper (there are other fascinating studies on stabilimenta in the literature as well, if you're interested!), the citation is below and if you aren't able to access it just leave a comment and I can send you the file!

Blackledge, Todd A. and Wenzel, John W. 1999. Do stabilimenta in orb webs attract prey or defend spiders? Behavioral Ecology 10(4): 372-376.


*I loved the book/movie of Charlotte's Web, but I have had one major beef with it ever since I was about eight: in the cartoon, Charlotte is drawn with antennae. Spiders do not have antennae!!!



(Photo credits to Teshia Bilecki and Todd Blackledge)