I'm always a bit tentative about insect identification, but I'm fairly certain of this one. Hey it's got a distinctive look! And they were on the ground under Pterocarya and near Platycarya. FIDL 41, Forest Insect and Disease Leaflet 41, from the USDA Forest Service suggests that they feed exclusively on the foliage of Juglandaceous plants. These guys surely won't go hungry where they were.