One of my favorite theories is the theory of why metamorphosis exists in insects. After all, being immobile and having one's entire body get reshaped is a pretty labor intensive process, so it only makes sense to ask why.
First, some traits that hint to the evolution of metamorphosis are the existence of paedomorphic species (wherein only one sex, usually males, pupates while the other looks like a giant version of their younger self). The fact that molting can occur within an egg led some early entomologists to believe that pupae were actually another form of egg! The belief that a pupae is an egg has since been disproven, with pupae capable of movement.
My favorite theory explaining why metamorphosis evolved is the one wherein larval insects otherwise compete with adults of their species for food - this is a problem for every species, with cannibalism primarily targeting young. If larvae ate different foods than the adult insects did, then a parent would not be competing with their own children. Perhaps what makes this theory convincing to me is that some adult insects evolved reduced or no mouths altogether. Obviously larvae cannot compete with adults for resources if adults take up no resources whatsoever! 16Please respect copyright.ＰＥＮＡＮＡQ6zPpeJ2sK
Other evidence includes the specialization of the different forms, with larvae primarily having sensory organs dependent on their environment - if you live underground, you don't need eyesight, but your adult counterpart capable of flying who needs to find a mate will need eyesight, so eyes only occur on the adult insect. Larvae that need to actively search for food will have sight as larvae.
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