(BG) Slaty Skimmer (Libellula incesta) 6/28/14 D&M, MD

http://bugguide.net/node/view/946950/bgimage

Slaty Skimmer (Libellula incesta) fem/imm fem
http://bugguide.net/node/view/943628/bgimage
and
Great Blue Skimmer (Libellula vibrans)
http://bugguide.net/node/view/182934

They are similar but studying the pictures should make the differences clear. One of the differences, I've read, is that in incesta the legs are black except for the most extreme little bit near the base where they are brown.