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.