Tarantual Hawk, from Wikipedia:
http://en.wikipedia.org/wiki/Tarantula_hawk
We had these all over the mountains in Arizona where I grew up. They sounded like an angry B-17 to me as a kid. The "Usually not aggressive" blurb quoted above: Complete B.S. I've been chased by these things many, many times, both on foot and on a bicycle. You may not believe it, but a Murray 20" BMX bike will do the better part of mach 5 when powered by fear and adrenaline......
My worst sting was when I was about 7 or 8, from a hornet. I walked through a patch of clover in our front yard, barefoot, and the little sucker nailed me right in the arch of my foot. I just screamed and dropped to the ground like a sack of wet cement. I spent the rest of the summer patroling the yard wearing my home built bee suit (Paper sack over head, snow suit, gloves, and boots) killing every bee, wasp, or hornet I could find.