Müller's great discovery concerned the resemblance between two or more unpalatable species which are protected from predators capable of learning. The protection is often a noxious chemical, perhaps gained from the larva eating a particulat plant; or it may be a sting or other defence. It is an advantage for such potential prey to advertise their status in a way clearly perceptible to their predators; this is called "aposematic" or warning colouration. In Muller's case the prey were butterflies, and the predators usually birds or reptiles.