These are large leaf feeding caterpillars with numerous spines over the body. The body is protected with branched reddish spines that may have red or black tips. These can be quite common on oak or willow from spring to mid-summer. There is a single generation per year. The caterpillar is overall light green, but along each side there is a narrow reddish line bordered below by a white line.
Grown caterpillars are two inches long and covered with branched, black-pointed green spines.
They feed on a variety of plants including corn, roses, willow, linden, elm, oak, locust, apple, beech, ash, currant, and clover. There are several species of slug-like caterpillars that have poisonous spines. Slug caterpillars are short and stocky and creep about on leaves in a slug-like manner. The head and legs are not visible. Typically, there is one generation per year with the grown larvae found in the late summer.