While raising red spotted purple caterpillars on willows, cottonwoods, or wild cherries, one option is to place a "bouquet" of host plant in a glass terrarium and then secure the terrarium with a glass lid.  This allows a nearly 100 percent humid environment which is appropriate for any admiral or viceory.  Rearing these caterpillars in a 5 or 6 gallon bucket also works fine as long as you raise them in a 100 percent humidity setup.