The latest installment for of the caterpillar and moth for “The Little Book of Metamorphosis” is now completed. The caterpillar of Citheronia regalis is known as the Hickory Horned Devil. It is the largest caterpillar in North America, reaching about 5 inches long. The mature form of Citheronia regalis is the Regal or Walnut Moth. It is also quite large, with a wingspan of up to 6 inches.