This is the Brown Recluse spider, a non-descript, little brown spider that is one of the most venomous spiders in the US. While I don't think there are any recorded deaths from its bite, it does cause an ulcerous wound that is very slow to heal. The one bite I've seen looked like the aftermath of a copperhead bite. The violin-shaped mark on its back is what to look for. I suppose it's to play Hearts and Flowers for you after you're bitten. It's about 3/4 of an inch across.
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