Found in my mixed-species dermestid rearing container where I dump any dermestid larva I find. It looks like it might be the pupa of the (or one of the) rusty-rumped larva species 1, 2.

The larva was collected in southern New Hampshire sometime in 2006. Many of the smaller dermestids apparently pupate inside their larval skins like this. If this pupa follows the rest of pattern, I expect the larval skin to split longitudinally down the back as it continues to assume the shorter, wider, domed shape of the adult.