Caterpillars of some species of butterflies prefer to rest off the plant.  Although natural in nature, this poses a problem with using an open terrarium or open bucket technique for these larvae because they keep crawling off the plant and can't crawl back because of the obstacle of the bottle holding the host plant cuttings.  An alternative to this is the twin cup method.  This fairly simple setup is explained in these two movies as well as the photos provided below.