On their rear is a smaller black tube. This is where they excrete excess sugar water, sometimes called honeydew. Bees, Ants, and Wasps love the sweet taste and will seek out aphids and feed on this liquid. Ants will actually take the time to protect them from predators in order to keep getting that 'sugar high'. In the animated movies Ants and A Bugs Life, the main characters are always shown drinking from aphid butts at the bar. This explains the joke so many didn't get watching those films.
Please visit Lisa Sells at her blog Zen Through A Lens to see a detailed post on this aphid species.