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Arilus cristatus
6-Legged, long antennae, black with red butt, reminds me of cross between praying mantis and ant.
Back yard
Found 3 of them on my deck exploring an old mattress we set out.
I agree with Gerardo, an Assassin Bug for sure. It looks most like a Wheel Bug nymph. Here are some images to compare to:
http://bugguide.net/node/view/21070
http://bugguide.net/node/view/193784
http://bugguide.net/node/view/290711
This one looks like a Assassin Bug but not 100% sure check this link http://en.wikipedia.org/wiki/Reduviidae