An additional cool detail that can help understand that this is not a coordinated mechanism but a matter of selection, is that sometimes the non-poisonous/venomous species will evolve to look exactly like a poisonous/venomous one. Plenty of insects that take the color scheme of wasps, for example. This is just because the individuals in a population that most closely resemble a wasp (due to mutations that happen over several generations) are less likely to be eaten by predators that fear wasps and so are more likely to reproduce