|MadSci Network: Zoology|
I suspect that you have a specimen of the orb-weaving spider Gasteracantha cancriformis (sometimes called a "crab spider", although it is not related to the true crab spiders in the family Thomisidae). You are a little far north for this spider, but with the warm weather we have been having in the United States this year it is not impossible. See http://www.ufsia.ac.be/Arachnology/Pages/Pictures/gastera1.jpg for a drawing of a similar species. All orb-weaving spiders in North America, including Gasteracantha, are apparently harmless.
Reference: Kaston, B. J. 1978. How to Know the Spiders. McGraw-Hill, Boston, Massachusetts.
David Richman later provided the following further information:
I made one mistake on this- I read Iowa instead of Louisiana as I think the person is from. In Louisiana it would probably not be unusual to find Gasteracantha.
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