Nymphalis antiopa

AJ and I were out in the meadow last week and saw a squirming mass of something on our pussy willow... a little internet research suggested that these are spiny elm caterpillars, and will eventually become mourning cloak butterflies.

These caterpillars feed "gregariously" - which apparently is the scientific term for en masse, on top of each other. I don't know if anyone else ever read Hope for the Flowers, but this scene immediately made me think of the caterpillar tower.