Ruggero Morimando wrote:I hope someone else will help me...
But not everything has something to do with mimetism in genetic evolution: sexual selection plays surely a role too, as well, I suppose, random events in Dna molecules and "simmetry" of living organisms... "Why a middle dorsal line?" could have the same meaning as the question "why tails grow always in the center of the dorsal spine?" I turn the question to other forumers with an elegant "I don't know!"
dorsal line can be indeed a by-product, not an adaptation. it can play role in sexual selection since dorsal line is present in females (and juveniles) only but i do not think that males perceive dorsal line of a famale