This question popped up in my learning community The SciHive and I thought to myself … what a wonderful question. So here we go.

We run #SpiderSunday where we post pictures of spiders and generally just learn about their biology when one of my members posted this cool shot of a zipper spider. Well … I’ve always called them Zipper Spiders. Apparently they’re also called “Writing spiders, black and yellow garden spiders, golden garden spiders, zigzag spiders ….” you get the idea.

A rant about common names another day…

To which Jared asked … “why do they make those little zippers?”

Well … the short of it is “we don’t know” and the long of it is “we don’t know but we have some good guesses.”

I wanted to take the approach today of looking at some of the various hypothesises that have been put forward and look at them with a critical eye. Is this likely? What evidence do we have to support these? What evidence do we have not to? I think – especially – in the age of Covid people think that scientists come up with an answer and then it’s fact. Forever. The end.

When in reality we propose a bunch of different ideas, test them, retest them, use new evidence, use more logic, and settle upon an idea. Usually after a decent amount of back and forth. Sometimes the idea that’s settled upon finally isn’t the initial hypothesis. And that’s okay. Science is all about fact checking and then re-fact checking.

So here we go.

What is the “Zipper” or “Writing” anyway?

The “zipper” or “zigzag” in question is called the ‘stabilimentum”. The Stabilimenta can be a variety of shapes ranging in form from big X shapes, to long zipper patterns, to circles of zig-zag spun silk.

Spiders that have the Stabilimenta are active during the day and tend to have their webs just hanging out in the open. Furthermore – they don’t build their web anew every day like other spiders. They tend to just leave them up for a few days on end airing them out like their dirty laundry. The “zipper” is a trait that spiders in the families Araneidae and Uloboridae have. The spiders in these families that have stabilimenta are diurnal. There are other diurnal spiders not in these families but they don’t have this structure which is curious. There are also nocturnal spiders that don’t have stabilimenta either. More on this in a few …

Hypothesis 1: It Stabilizes the Web

This would be a good theory … and its name certainly suggests as such … but you can just cut the stabilimentum straight out the web and it continues to function as normal. And plenty of webs from diurnal spiders of other species don’t have a stabilimentum. So if it was really stabilizing anything you’d expect the web to come crashing down when removed and you’d also expect most webs from various species of spiders to also have them. But that’s not the case. So …. probably not stabilizing anything.

Furthermore – spiders already spin stabilizing strands of silk to anchor the web in place. These are structural silk and dragline silk. Spiders that spin webs to capture prey create these as the anchors to spin the spiraling sticky web on.

Hypothesis 2: It Attracts Insects by Reflecting UV Light

Scientists have gone back and forth about the reasons why webs might reflect UV light. There is some evidence that some webs may attract pollinators by reflecting UV light and is a secondary mechanism for spiders to trap prey. Some suggest that it may just be a random effect based on the proteins the silk is made of an it just reflect UV for random reasons and not because it’s particularly useful.

But the problem with this hypothesis specifically in this case is that the stabilimentum isn’t sticky. And it does reflect UV light. So Insects can plow right into it and kind of just … bounce off? because that’s not where the sticky stuff is. So the insects attracted to that part of the web wouldn’t even be captured.

Hypothesis 3: Billboard for Birds

These spiders that build their web out in the open, during the day, and don’t rebuild their web every day, are the ones that have the stabilimenta.

They need a signal to tell birds like “Hey! Don’t crash into this pls!” Nocturnal spiders, and spiders that make webs in more secluded areas seem not to have stabilimenta. And diurnal spiders that remake their web every day don’t have them either.

Leaving a web up for a few days at a time saves you, the builder, a lot of energy. No wasted proteins, no wasted time constructing and deconstructing, and no wasted energy either with constant rebuilding from scratch. This would obviously have its advantages but there is a high chance that your web will get destroyed by any bird who just happened to not notice that a big net-thing was in the way and rudely destroy all your hard work in its oblivion.

Thomas Eisner conducted an experiment to test this putting fake stabilimenta in webs and found that webs with them were evaded by birds. Another bit of evidence that can point to this being true is that in Guam – invasive snakes have now eaten many of the common forest birds and the diurnal spiders have been noted to produce fewer and smaller stabilimenta.

Producing stabilimenta isnt without their risks though. Specific predators may hone in on them and eat the resident spiders. You win some … you lose some.

TL;DR

The “zipper” or “zigzag” shape in webs is the stabilimentum and is most likely used as flashy advertisement to birds to prevent the spider’s web from being accidentally destroyed.

---

Let’s Be Friends ^.^

• Facebook
• Instagram
• YouTube
• Twitter

REFERENCES

1. Craig CL & Bernard GD. 1990. Insect Attraction to Ultraviolet-Reflecting Spider Webs and Web Decorations. Ecology 71(2): 616-623
   
2. Eisner T & Nowicki S. 1983. Spider web protection through visual advertisement: Role of the “STabilimentum.” Science 219: 185-187
   
3. Heberstien ME, Craig CL, Coddington JA, Elgar MA. 2000. The functional significance of silk decorations of orb-web spiders: a critical review of the empirical evidence. Biological Review 75: 649-669
   
4. Kerr AM. 1993. Low frequency of stabilimenta in orb webs of Argiope appensa (Araneae: Araeidae) from Guam: an indirect effect of an introduced avian predator? Pacific Science 47: 328-337.
   
5. Römer L & Scheibel T. 2008. The elaborate structure of spider silk: Structure and function of a natural high performance fiber. Prion 2(4): 154-161
6. Seah WK & Li D. 2001. Stabilimenta attract unwelcome predators to orb-webs. Proceedings of the Royal Society B: Biological Sciences 268(1476):1 553-8