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Xylophanes tersa tersa
A moth, 2-3 cm's big.
That absolutely looks right to me. This pretty much looks like what I saw: http://eol.org/pages/896388/overview
Thanks!
Sphingidae/Hawk Moths, Xylophanes tersa. Unfortunately the most important dorsal shot of the whole moth is missing. Nevertheless I think it is Xylophanes tersa, also found in Uruguay (the link is from Florida).