They are designed so that you can put sugar water or honey water onto the sponge, which attract the butterflies. The slot represents a crack in bark that the butterflies would naturally roost or hibernate, once they crawl in there is a branch fastened in there that has rough bark, and they can climb on to it and roost over night some species will hibernate through the winter.
For more information see Butterfly Roost Information Sheet
Please Note our stock turns over very quickly and at times because we are at shows we are unable to update the stock online. If you are unable to see what you see looking for or it says sold out, please contact us and we can send you pictures and prices.