Tawny Emperor - Asterocampa clyton
Habitat: Hackberry woods, mountain canyons in Southwest, elsewhere dense fiver woods, dry woods, cities.
Nectar: Hackberries, and occasionally at milkweeds and rabbitbush.
Habits: Young caterpillars feed and rest in large groups; older ones are solitary.
As identified by other sources, this is probably a Tawny Emperor - Asterocampa clyton (see http://www.npwrc.usgs.gov/resource/distr/lepid/bflyusa/tx/39.htm, USGS).
Identifier: Mike Quinn (Invertebrate Biologist, Wildlife Science, Reserch & Diversity of the Texas Parks & Wildlife)
Left: Tawny Emperor on Hackberry. Courtesy of S. Peregrine Johnson
Below: Allen from Caterpillars in the Field and Garden.