This is the egg of a weidemeyer's admiral (Limenitis weidemeyeri latifascia).  Eggs of admirals, viceroys, and even sisters are best kept on the original leaf they were laid on until they hatch.  If the leaf they were laid on dries up, the egg can collapse and perish as well.  Simply take the sprig the leaf is on, place in bottled water and place that in a tub with 99 percent humidity.  This technique is discussed in another section.