Stabilmenta are conspicuous patterns or decorations made by spiders – particularly orb-web spiders – in their webs. Google ‘stabilmenta’ (singular: stabilmentum) and you will see many wonderful examples of these structures, including crosses, spirals, zigzags and so on.

There are a number of different theories as to why spiders make these structures, including: to attract prey; as camouflage; as a moulting platform to stand on; as a way of warming up the web; and as a warning signal for any potential predators which might want to, or just inadvertently, destroy the web (1, 2, 3). It is possible that they have more than one function, although camouflage seems to be the most popular, or agreed upon, theory (2, 4). Nevertheless, some researchers have shown that more flying insects (apart from grasshoppers) are caught, or intercepted, on webs decorated with stabilimenta (5). Which suggest that they might enhance the efficiency of the web; although other researchers came up with a completely different result (see below).

Spiders in the Araneid spider-genus Argiope often adorn their webs with these structures. I photographed this stabilmentum (below) made by Argiope pulchella in Thailand. [N.B. this species is very similar to Argiope versicolor and are indistinguishable in the field.] The spider positioned itself over the X-shaped stabilmentum, but moved off it to wrap-up any prey caught in the web.

Some experiments have shown that stabilimentum building is a defensive behavior (3), in effect advertising the presence of the spider’s web and preventing birds from flying through the webs. There is no question that they are highly visible and in some situations, actually reduce the number of prey that are caught (3). This ‘cost’ to the spider can presumably be set against the benefit of not having to rebuild the nest every time a bird flies through it by mistake! Unfortunately for the spider making the stabilmentum, other predatory spiders – such as web-invading jumping spiders – have learnt to recognise the patterns and use them to find their prey (6). Perhaps this is why some spider species make silk replicas of themselves! (7). To fool would-be predators! (8)

As many people may have noticed, spiders webs can be highly visible when covered in dew in the morning, or after a rain shower. I photographed this spider’s web in Spain, after a passing shower.

I agree with another blogger (9), that stabilimenta are probably multi-functional structures, and the fact that they are so common in certain species, must mean that they are being selected by evolution. So the overall benefits must out-weigh the costs.

1. http://www.mol-ecol.uni-halle.de/research/former_topics/stabilimenta/
2. Cloudsley-Thompson, J. L. (1995). A review of the anti-predator devices of spiders. Bulletin of the british arachnological society, 10(3), 81-96.
3. Blackledge, T. A., & Wenzel, J. W. (1999). Do stabilimenta in orb webs attract prey or defend spiders?. Behavioral Ecology, 10(4), 372-376.
4. https://en.wikipedia.org/wiki/Web_decoration
5. TSO, I. M. (1996). Stabilimentum of the garden spider Argiope trifasciata: a possible prey attractant. Animal Behaviour, 52(1), 183-191.
6. Seah, W. K., & Li, D. (2001). Stabilimenta attract unwelcome predators to orb–webs. Proceedings of the Royal Society of London B: Biological Sciences, 268(1476), 1553-1558.
7. https://www.wired.com/2012/12/spider-building-spider/
8. http://www.popsci.com/article/science/what-i-learned-hunting-amazonian-spiders-weave-fake-spiders
9. http://www.bugsinthenews.com/stabilimentum_and_some_notions_on%20function.htm