It looks like what the book says is a Tarantula hawk, 1 1/2 inches long (I say 10 inches...he was HUGE). They provision their nests with spiders, including tarantulas, as food for their immatures. They sting the tarantulas and drag them into burrows in the ground. It says females can sting but are not aggressive, but males can't sting and are aggressive. I DIDN'T HANG AROUND LONG ENOUGH TO FIND OUT! It was eating on a rotten banana I put out for the butterflies.
They were around today: Gulf Fritillary: http://www.pbase.com/image/32979263
Queen: http://www.pbase.com/image/32979472