Slavery was outlawed in 1865 with the Thirteenth Amendment to the United States Constitution, but apparently ants did not get the memo.
Ants will often farm (AKA keep them as slaves) and protect aphids. In return for their guard duty the ants relish the honey dew which is excreted by the aphids. The ants stroke the rear of the aphid with its antennae and this causes the honey due to be expelled. This is then eaten and regurgitated in the nest for the developing larvae and queen.
If that is not enough, here is a video for your enjoyment:
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