The Stabilimentum
And some notions concerning its function

Web construction by spiders is a fascinating study. Somewhere in the process of that study one eventually has to confront the structural mystery known as the stabilimentum.

Obviously, the name given to this structure derives from someone's early presumption that it served to stabilize the web in some way. That theory is not given much credence today. On the other hand, stabilimenta may serve as camouflage, an idea that is bolstered by the fact that they are constructed only by spiders that typically sit in the hubs of their webs, out in the open. Camouflage would be helpful in concealing the spider's presence from visitors who would like to avoid them, as well as in preventing predators from spotting an easy meal.

Some authorities discount the camouflage theory, because they have concluded that spiders like the colorful Argiope aurantia are simply not camouflaged very well by that structure. My own observations run counter to this, especially when the spider is very young. The stabilimentum seems very effective as camouflage at this stage, even for Argiope aurantia. A hungry spider who is not catching much prey will often skip the construction of a stabilimentum.

An Argiope species ...

The stabilimentum may also serve as a warning to birds, to keep them from flying into the web. This would reduce the need to rebuild or repair the web. On the other hand, it also makes the web more obvious to those birds who are fond of eating spiders...

The silk used to make the stabilimentum has been shown to reflect ultraviolet light very efficiently. As a result, this part of the web would be highly attractive to insects, luring them to its threads.

Some other possibilities include its use as a molting platform, a protective shield from the searing rays of the sun, and a depository for surplus silk. The same silk used in the stabilimentum is also used to wrap prey, and the spider may find it expedient and useful to exercise these glands by producing stabilimenta during periods of low insect activity.

More than likely, the stabilimentum is simply a multi-functional structure that has proven useful for all of the reasons described above. As such, a spider who creates one in an early web quickly finds it to be worth the investment in time, energy, and silk. Later, when a new web is needed, the spider is led to build a stabilimentum into it again.


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