I did a little research and found the ant and the aphids have a symbiotic relationship. The ant "farms" and protects the aphid colony from other preditory insects and the aphids secret a little honeydew that the ant loves to eat. Some ants actually "milk" the aphids by stroking their little abdomens to get them to secret just a little more of that honeydew. I think that is what this ant was actually doing today in Charlestown Meadows. I watched him!! Wild.