I found these wasps nectaring on wild parsnip. Cedar Creek says they are often collected on water hemlock, which I found out is a member of the parsnip family. These wasps are almost as slender as a sewing needle, very hard to follow, but the payoff can be huge. I had no idea what they really looked like until I d/l'd them onto the big screen. Here is a page with various large hi-res as well as shots of the male, too:
http://www.cirrusimage.com/hymenoptera_wasp_Gasteruption.htm