This ant is protecting its "herd" of aphids. However, there are two interlopers. The small green and brown caterpillar-like critters, are the larvae of hover flies (Syrphids)which are eating the aphids. Adults of some hover fly species lay their eggs amongst aphid colonies. When the eggs hatch, the larvae have a ready made supply of food. Hover flies can therefore be good predators of aphids.

The ants protect these colonies for their own purpose: They like the sugary excretions that the aphids exude!