Yesterday I came across some ants tending aphids.
I am pretty sure the ant is Formica exsectoides, the Allegheny mound ant. I don’t know the aphid–I am pretty sure microscopic examination would be required to identify it.
There is a mutualistic relationship between the ants and the aphids–aphids providing food and ants providing protection. I also found some “ant mummies”–the remains of aphids that are involuntarily providing food for some wasp larva.