This caterpillar turned up in a bag of hickory leaves in which I was trying to rear this sawfly larva:

A week later, it had developed into what looks to me like a good match for Ruby Quaker (Orthosia rubescens). Hickory is not a known host plant for this species, but it is a generalist so I don't think that rules it out necessarily. I wanted a second opinion before I move these photos into the guide. I don't remember what happened with this caterpillar... it's possible I still have the pupa somewhere...