Geometer Caterpillar

Notes:

Saw this Geometer/Geometridae caterpillar (confirmation please!) today. It was poised straight as an arrow, and it had this ant running up and down it continuously. When the ant would run back down and around the bottom part the caterpillar would lash at it, and then the ant would run back up again. Couldn't tell if the ant was trying to eat it, or was cleaning it. Anyone know if these two have some sort of relationship?