Doris photographed this caterpillar in Montreal, feeding on prickly ash, the only known larval food plant up this way. Although not photographed at the FWG, we decided to post this to show visitors to this site what the larva looks like. We have seen the adult butterflies at the garden in both 2012 and 2013, but I don't expect to see larvae as we have no prickly ash growing. However, it is a fairly common shrub around the Ottawa region, so knowing what the caterpillar looks like, might help with ID elsewhere in the area.