Aphids exude a sweet liquid which is a good food source for the ants, who look after the aphids and ‘milk’ them with their antennae to get the honeydew. The ants protect the aphids from other predators, and will even herd them around, moving them from one colony to another.
The relationship is called mutualistic, as both species benefit. The ants get food, the aphids get protection.
Canon 60mm f/2.8 USM Macro lens