pairing is of the viceroy and the queen butterfly, whereas in Mexico the viceroy resembles the soldier butterfly. The viceroy is thus involved in three different Müllerian pairs. This example was long believed to be Batesian, with the viceroy mimicking the monarch, but the viceroy is actually more unpalatable than the Queen. The genus Morpho is palatable, but some species (such as M. amathonte) are strong fliers; birds – even species that specialize in catching butterflies on the wing – find it hard to catch them. The conspicuous blue coloration shared by most Morpho species may be Müllerian, or may be "pursuit aposematism". Since Morpho butterflies are sexually dimorphic, the males' iridescent coloration may also relate to sexual selection. The "orange complex" of distasteful butterfly species includes the heliconiines Agraulis vanillae, Dryadula phaetusa, and Dryas iulia. At least seven species of millipedes in the genera Apheloria and Brachoria (Xystodesmidae) form a Müllerian mimicry ring in the eastern United States, in which unrelated polymorphic species converge on similar colour patterns where their range overlaps. Emsleyan/Mertensian Emsleyan or Mertensian mimicry describes the unusual case where a deadly prey mimics a less dangerous species. It was first proposed by M. G. Emsley as a possible explanation for how a predator can learn to avoid a very dangerous aposematic animal, such as a coral snake, when the predator is very likely to die, making learning unlikely. The theory was developed by the German biologist Wolfgang Wickler who named it after the German herpetologist Robert Mertens. The scenario is unusual, as it is usually the most harmful species that is the model. But if a predator dies on its first encounter with a deadly snake, it has no occasion to learn to recognize the snake's warning signals. There would then be no advantage for an extremely deadly snake in being aposematic: any predator that attacked it would be killed before it could learn to avoid the deadly prey, so the snake would be better off being camouflaged, to avoid attacks altogether. But if the predator first learnt to avoid a less deadly snake that had warning colours, the deadly species could then profit (be attacked less often) by mimicking the less dangerous snake. Some harmless milk snake (Lampropeltis triangulum) subspecies, the moderately toxic false coral snakes (genus Erythrolamprus), and the deadly coral snakes (genus Micrurus) all have a red background color with black and white