Hylaeus anthracinus nest in coral rock, Puuhonua O Honaunau NHP. While bees have often been seen entering holes in coral rubble and seem to be most abundant where there is a lot of it, this is the first definite evidence that they're nesting in it. The cellophane-like substance is a secretion that the female bee produces to line the nest cell and make it waterproof; when filled up, she seals it up like this until the new adults emerge.