For today's Macro Monday post, I want to talk about stabilimenta, the somewhat mysterious structures that some spiders build into their webs. Back in July I posted some images of a female yellow garden spider (Argiope aurantia) with a dragonfly trapped in its web. At the time of that post, I'd planned a follow up post on the stabilimenta that yellow garden spiders build into their webs, but various distractions ensued, and now here I am finally getting to it in late September.
I'll start off by saying that most yellow garden spider webs that you see are the webs of the female. Adult males are much smaller and roam around in search of a female. Once they find a suitable mate they build relatively small webs in or near the female's web. Here is an image of male and female yellow garden spiders together in a web. When you first see one, you might be tempted to think that the male is a young spider.
Stabilimenta (stabilimentum, singular) are conspicuous silk structures built into the webs of some spiders. Many species of spiders build stabilimenta and it is like to have evolved multiple times within the arachnids, but it only occurs in diurnal (active during the day) spiders build stabilimenta. Below is the typical stabilimentum of an adult yellow garden spider.
The function of a stabilimentum is still controversial, although most scientists can agreee that the orignal explanation that it stabilizes the web (and hence the name) is incorrect. Here are some of the common hypotheses for the behavior:
1. Makes web visible to birds and thus prevents damage to web
2. Attracts prey by reflecting UV light
3. Protects spider by camouflaging it or making it look larger
4. Helps the spider regulate excess silk
5. Serves in mate attraction when the female is ready to breed
There are other hypotheses, but these are the major ones. It is of course possible that several of these explanations may be true at the same time or that it is a different reason for different species or groups. Some studies by Todd Blackledge (and colleagues) showed that spiders on a restricted diet did not build stabilimenta and that webs with a stabilimentum catch less prey. However, some other studies have showed that the stabilimenta attract prey.
Here are some more images of the yellow garden spider and its web.
Hope you enjoyed the post! Remember to look for stabilimenta next time you are out in the field.