What are These Caterpillars?

These huge caterpillars were crawling across my lawn last fall. What are they?

—Mary Ann Morris, Haubstadt, Indiana

Tom: These interesting-looking caterpillars are some of my favorites. Commonly called hickory horned-devils or royal walnut moths, they transform into large, beautiful red and yellow moths in summer. The caterpillars feed on either hickory trees or black walnuts. Then they bore into the ground and form their pupa just under the surface, where they stay protected from winter weather. When they emerge, these fliers are attracted to lights at night. If you want to find one in your backyard, try looking in the morning. If you're lucky, you might spot one from the night before, clinging to trees or other objects near light sources.