First, thank you to Samantha for capturing both sides of this large and brightly colored female orb weaver! Like other female orb weavers (family Araneidae), Banded Garden Spiders construct intricate, circular webs to ensnare their insect prey. Adult male orb weavers are typically smaller and are not seen as often. They generally do not spin webs, but wander in the search for potential mates. For this species, “Adult males are found at the edge of the female’s web” (National Wildlife Federation Field Guide to Insects and Spiders of North America).
Some genera of orb weavers (specifically Argiopes) also spin a thick zig-zag pattern through the center of their webs (as seen in the photos), called the stabilimentum, reflecting the first, and now-dismissed hypothesis that they are constructed to stabilize the web. There are several theories as to what purpose the stabilimentum serves and it’s likely that different species use them for different purposes. Some possible theories include: providing protection by camouflaging the spider — breaking up its outline or making it appear larger by extending its outline, making the web visible so animals like birds don’t damage it, attracting prey by reflecting UV light, attracting a male mate, thermoregulation, regulation of excess silk …and more.
Banded Garden Spiders can be found in open places with tall grasses or shrubs in southern Canada and throughout the U.S. (and many other countries around the world). While the females will soon die, the next generation will overwinter as eggs in a kettledrum-shaped sac that may contain more than 1000 eggs.
Photos by Samantha Erlenbusch Davis on 9/28/20 (top) and 9/29/20 (bottom) in Lolo, MT