Spiny oak slug ( Euclea delphinii ), Newark DE. September 2017.
As the name implies, this caterpillar is one of the “slug” caterpillars (Family Limacodidae), a group characterized by their slug-like appearance: the head projected downward and hidden by its body, and the suction-cup-like prolegs normally found on many caterpillars are reduced to tiny hooks or spines, making it appear as though the caterpillar crawls like a slug might. The caterpillar also possesses clusters of poisonous spines that sting when touched.
Caterpillars of this group feed on more than just oak, but can be found on many woody plants in the Mid-Atlantic, commonly found in forests and woodlands (although difficult to find due to their size). At least one generation in the Mid-Atlantic, with caterpillars by June. Caterpillars overwinter as pupae.