Ants take on a role of either helping or farming aphids, I haven't
quite made up my mind which it is. They will protect the aphids and I
really think it's to fatten them up for the munch.
Got to be seen large!
UPDATE: Cool! From Wikepedia:
provide from Flickr Member Agrinberg
Aphids secrete a sweet liquid called honeydew which they exude in the process of feeding from plants. The sugars can provide a high-energy food source, which many ant species collect. In some cases the aphids secrete the honeydew specifically in response to the ants tapping them with their antennas. The ants in turn keep predators away and will move the aphids around to better feeding locations. Upon migrating to a new area, many colonies will take new aphids with them, to ensure that they have a supply of honeydew in the new area.