Beech Blight Aphid

These small insects formed an undulating mass, waving their tails back and forth. These members of the order Hemiptera feed on the sap of beech trees. The undulations are a defensive behavior, and if you come across a group of them not dancing, gently blowing on them will get them started, no music required. They produce copious amounts of honeydew, upon which a specialized fungus grows, more about that here http://www.projectnoah.org/spottings/37532049 and here, http://www.projectnoah.org/spottings/17194206