You sound like some sort of an expert.
How did metamorphoses evolve?
For that matter, nobody seems to have any real proof of evolution but we know that metamorphoses works. Has anybody ever investigated the possibility that metamorphoses may have been the basic mechanism for the world's biological diversity?
Thanks, but I fake it.
How did metamorphoses evolve?
Think about what metamorphosis really does. It lets the growing insects and the adult insects live in different ways on different food--and thus not compete across generations. Or you can think of it as exploiting more than one niche at a time. Adult butterflies sip nectar while caterpillars chomp leaves and wait to spin their cocoons.
Most of the mechanism is already in place, since all the sexual multicellulars already start from a fertilized egg and tend to resemble, at various stages of their history, different primitive stages of their development. You were once a one-celled animal, then a simple colonial multi-cellular, then a chordate, then a vertebrate, etc.
So in any event, you have to go through the stages. There can be environmental pressures that say there's an advantage to be had in staying stuck in a certain stage for a while, just to grow to a certain size or whatever. So you have the stages, you have some pressures conveying an advantage if your species stays at a stage for a time and exploits a different niche from the adult. Stumble onto the right answer, beat out the competition.