Many species of fly become "adults" only for a single day and lack feeding parts altogether. They are maggots for essentially their entire lives, sometimes years, and metamorphose into their final stage only to fly around, look for a mate, hopefully find one and then die after running out of energy. It seems like these organisms are surely more maggot than fly and yet we insist on calling them flies. They don't really fly all that much, only for a day out of their whole existence.
A similar situation are the "cyclical" cicadas. Many species of cicada live underground as nymphs for thirteen to seventeen years before emerging as "adults." The "adult" phase only lasts about a month, and the main purpose is basically the same: grow wings so you can find a mate more easily. I'm not sure what the ancestral form of the insect world may have been like but it seems that wings may have been an adaptation specifically for finding mates and nothing else. Many of these animals seem perfectly content to spend the vast majority of their lives burrowed underground or in a rotting carcass or up a deer's nose.
So are flies maggots or are maggots flies? Are beetles grubs or is it the other way around? Is it a monarch caterpillar? It's odd to imagine but I wonder how we would look at things if vertebrates spent the bulk of our time on earth in one form and then in our twilight years, just before our cell replication was ready to give out we changed shape into something completely different and flitted off to find a partner with which to reproduce.
UC Claremont: http://www.biology.clc.uc.edu/steincarter/cicadas.htm