Silvery Checkerspots are common in West Chicago Prairie, just a few miles north of Fermilab.  So I have always been on the lookout for them here but have never found them at Fermilab.  It was Bob Lootens who not only spotted this one but photographed it, a new species record for the Fermilab site.  These look much like a large, female Pearl Crescent, but a key identifcation feature is that they have the white center in the middle black spot near the margin of the hind wing.