Aglais milberti

Milbert's Tortoiseshell

Several somewhat similar nymphalid caterpillars feed on Stinging Nettle, but this one can be easily recognized because it occurs in huge masses. When in the final instar, individuals may wander somewhat, but others can nearly always be found nearby. The species closely resembles the European N. urticae and it wouldn't surprise me if they were conspecific. As each new butterfly book is published, the butterfly nimbly hops from the genus Nymphalis to Aglais and back.