Geometer moths (click here for my images) are a large family of moths. They are called geometer moths because their larva were said, by ancient people, to be measuring the earth with their looping, inchworm like movements. There are more than 1,600 species of geometer moths in North America alone. So while I know this is a geometer larva, I don't know which one. This one was on the leaves of our day lilies in August.
Other common names for geometer larvae are measuring worms, inchworms, loopers and spanworms.