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how to understand reason for intense colors of animals(e.g. butterflies)?

[...], while the intense colour of the dorsal sides of the wings is a warning to predators that the Red Lacewing has a bad taste, deriving from the poisonous host plants of the caterpillars.


poisonous animals don't have necessarily intense colors. E.g. Sydney funnel web spider is very toxic, but doesn't have intense colors, while there are also butterflies which are colorful but not poisonous like mentioned here http://www.kidsbutterfly.org/faq/general/5 .

==> How can i understand that "warning reason" from a biological perspective, as nature doesn't "think and reason"? ==> I mean, i doubt a simple "survival of the scar", because of the colorful, but not poisonous animals.