I found this caterpillar hanging by a silk thread several feet below a tree.
I tried to get a different angle for the photograph by blowing on the caterpillar a little. That was a mistake, as it caused it to drop to the ground.
I tried to get a better view of it by digging in the grass. That only caused the caterpillar to curl up like a ball.
I think this is the caterpillar of the sycamore tussock moth, Halysidota harrisii.