After monarch butterflies mate and eggs are laid onto the leave of milkweed plants, eggs hatch after four days. The caterpillars consume their egg cases, and then feed on milkweed. During the caterpillar stage, the monarch stores energy to carry them through the non-feeding pupa stage. The caterpillar stage lasts about two weeks. In the pupa state, the caterpillar spins a silk pad on a twig or leaf and hangs from this pad by its last pair of prolegs. It hangs upside in the shape of a J and then molts, leading to the development of a butterfly.