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Micrathena gracilis
Coolest spider I've ever seen in Iowa. Found at Maquoketa Caves State Park.
They are an interesting spider. The Missouri Department of Conservation has an article about these spiders. You might find it interesting.
http://mdc.mo.gov/discover-nature/field-...
Thanks for the ID suggestion, gatorfellows! I kept trying to search for spiny orb weavers, but I had never heard of Spined Micrathena. I think you're right on target.
Lot of variability with this one, but Bugguide had several examples. See what you think.