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Murgantia histrionica
seems to be something in the Genus Podisus, maybe. But it's a Nymph that's for sure.
Cool find! Here is the species page for the Green Stink Bug. I agree with saramrall.
Green Stink Bug:
http://bugguide.net/node/view/9066
For comparison, here is the Harlequin Bug page:
http://bugguide.net/node/view/557
harlequin nymphs are similar, but lack the yellow shoulders, have a yellow spot right behind the head, and have two yellow stripes down the abdomen.