Mimicry evolves when organisms derive protection from attack by resembling different species. In Batesian mimicry, named after the 19th century naturalist H E Bates, an edible species is protected by mimicking an aposematic species. I showed slides of a spider that resembled an ant and a moth that resembled a wasp. For example, the head of the moth still looked like a moth's head, not a wasp's head. The antennae of the fake ant were the front legs of the spider and if you looked closely you could see that they were attached in the wrong place to be real antennae.
Mullerian mimicry involves resemblances between different distasteful species.
It is less obvious why this should evolve, but it has done so repeatedly. I'll explain it in class today. I also showed examples of insects with fake heads or fake eyes that deflected predator attack and misled predators about the direction in which the prey would be likely to flee. Lastly, I showed several Fulgorid bugs with air-filled sacs protruding from their bodies. However, a second species had a sac on the front of its head that resembled a miniature crocodile in minute detail, complete with teeth, eyes and nostrils.