This caterpillar has chewed a Black Willow leaf into the desired shape. He is now spinning a web to wrap himself in the leaf nest, called a hibernaculum. The leaf on the left already contains a tiny caterpillar. The hibernacula are about a half inch long and very thin. When you look inside, you see the tiny caterpillar. The hibernaculum is very snug. The leaf falls from the tree, and the caterpillar spends the winter in leaf litter under the bare tree.