Maryland, US. Many dozens massed on terminal growth of grapevine. Extremely small. Appear to be sucking, no discernable movement. Noticeable outcurving horn like structures (2) at tail end. Legs appear banded. Black or possible dark green. Possible light spot between eyes. Numerous ants crawling all over these guys. Cellphone pictures sorry.
These are aphids (aka 'plant lice'); they are sap feeders, and some species can vector viral diseases of plants. Ants often 'attend' some some aphid species for the sweet secretion (honeydew) given off by the aphids. See http://ipm.ucanr.edu/PMG/PESTNOTES/pn7404.html
