Ants and aphids have a symbiotic relationship. Aphids feed on plant sap from the phloem. They secrete honeydew which is rich in carbohydrates. The ants feed on this honeydew. Ants are known to protect aphids in their "farms".
More ant close-ups here:
Shoot details: Shot with a Tamron 90mm 1:1 macro. F/16 for depth of field. 1/200s. Hand held.
Lighting: Vivitar 285 through a plastic container on left top.