There is a group of moths known as underwings. At rest, the forewings which are usually brown or grayish in color, cover the hindwings. The hindwing is much more colorful, often yellow, red, orange, or pink. The name underwing comes from the fact that the hindwings, which are the more colorful of the two pair of wings of these moths are hidden under the forewings.
The caterpillar in the photo above is an Ilia Underwing (Catocala Ilia). Ilia was a daughter of King Priam, the last king of ancient Troy. She was hauled off to Crete after the defeat of Troy and is the subject of an opera by Wolfgang Amadeus Mozart, Idomeneo. Catocala (pronounced, kah-TOCK-uh-lah, at least by some) is supposedly from the Greek kato meaning down, under or below and kalos meaning attractively good or beautiful.
This particular underwing caterpillar is a master of disguise. It’s found on oak trees, and can be the color of the bark of the tree or, less frequently, the color of the lichen that grows on the tree. The one in the photo looks very much like lichen.
This caterpillar was found at the base of a Willow Oak. It’s one of the more common species of underwings so you should have little problem finding one, if you care to look. That is, if you can spot it on the bark, the lichen growing on the bark, or in the leaf litter below the tree. They are there. I actually found two of them, quite serendipitously on two separate oaks in my yard while picking up downed branches.
Some of the best discoveries are made while involved in entirely unrelated endeavors. So, keep an eye out next time your outside doing manual labor, you never know what might show up.