Gray Comma is a woodland butterfly, found at Fermilab in the "Big Woods" (ELM-24) on the western part of the site.  They seem particularly fond of the semi-open Hawthorne grove at the western edge of the woods, where I have reliably found them every year for four years now.  Gray Comma does not seem to be a common butterfly in the Chicago area.  The only other place I regularly find them is at Waterfall Glen Forest Preserve in DuPage County, another high-quality woods. 

See Gray Comma metamorphosis for some photos of a captive caterpillar, its chrysalis, and the emerged adult.