Some butterfly caterpillars protect themselves by mimicking bird lime.  Since birds are natural enemies to butterflies, there is a certain advantage for a butterfly caterpillar to mimic bird lime.  Afterall, what bird would want to eat its own frass?  This is a photo of a giant swallowtail (Papilio cresphontes) third instar caterpillar on ash (Ruta graveolens).