The Stabilimentum of the Silver Argiope Spider

Last month I invited you to examine the webs of the Silver Argiopes in the Prickly Pear around the Nature Center. You will have noticed that some of the spiders, particularly the older females, had 1 to four zig-zag bands of silk (collectively called a linear stabilimentum) in their webs while some of the younger spiders reinforced the hubs of their webs (this hub reinforcement is called a disc stabilimentum). Disc stabilimenta appear to provide some protection for immature spiders, but males switch to the linear form before similarly sized and colored females do.

Since stabilimentum construction costs time and energy, one might expect that it offers some advantage for the individual. For the Silver Argiope, various observers have suggested the linear stabilimentum:

  1. has a mechanical function, stabilizing or strengthening the web
  2. has a protective function, helping to camouflage the spider
  3. makes the spider appear too large for common predators (like lizards)
  4. warns of an obstruction (like the marks we use on glass doors), thereby protecting the web from destruction
  5. helps attract prey (The bodies and stabilimenta of Silver Argiopes reflect ultraviolet light making the central part of the web ultraviolet bright. Various insects use ultraviolet to orient towards openings in vegetation. Some flowers also use ultraviolet patterns to attract insects. )
  6. is a response to stress
  7. is a decoration (One study reported that females built more stabilimenta when males were present.).

The main problem with all of these explanations is inconsistency. An individual spider will not construct a stabilimentum each time it builds a web (reportedly each day). When it does construct a linear stabilimentum, the number of zig-zag bands may not always be the same. If a stabilimentum is useful one day, why not the next?

Most of the proposed explanations seem to have some merit. It is conceivable that several operate together and that the local environment affects the relative importance. The studies that I examined were done in the tropics. Since Silver Argiopes have a preferred habitat around here, the Prickly Pear cactus, studies of local populations might prove enlightening and perhaps easier to carry out.

I should point out that Silver Argiopes are not the only species that build stabilimenta. One of these, found among the cactus pads and elsewhere at the Center, is the much smaller Cyclosa spider. Its orb web has lumpy, vertical stabilimenta. See if you can find the spider.

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201