The film canisters provide an ideal lay spot for many of the the thumbnail species. We assume that they remind the frogs of their natural laying sites such as the funnel of a bromeliad, the overlapping of leaves or any other waterfilled structure they find in the wild. The film canisters make it very easy to remove the eggs compared to removing them from a bromeliad. Film canisters also provide great places for them to deposit their young which makes it easy to collect them for artificial rearing (aside from obligate egg-feeders). Alot of times thumbnails will lay eggs which are very well hidden from their keepers or in a spot that makes removing the eggs nearly impossible without damaging them. When this occurs, it is best to just let the parents do the work of transporting them in hopes that they will deposit them in something removable such as film canisters or baby food jars.