Coragyps

Today's combined entomology and etymology lesson: That word comes from the Greek, and means something like "animals that throw off their skin." H L Mencken, back in the 1920's, coined the related word "ecdysiast" for a girl that worked in burlesque.
<back to your regularly sceduled program>

Quetzal

Let's not forget my favorite vertebrate metamorphosizers (is that a word?): Bufonidae, Hylidae, and Ranidae. Or those all-time champions of metamorphosis the Ambystomatidae - some of whom like the intermediate stage soooo much they simply don't change (Ambystoma mexicanis). Axolotls Rule!

Appears to me, anyway, that metamorphosis is the rule, rather than the exception.

Now back to your regularly scheduled creationist rant...

Grumpy

This statement, combined with OrderedChaos' observation about silverfish, adds an interesting twist to the discussion. Caterpillars didn't one day decide to mutate (for lack of a better word) into butterflies. Rather, the adult form shows greater continuity to the shared insect heritage. Those which undergo complete metamorphosis developed, sometimes separately, a larval phase that more closely resembles annelids. The different stages in the life cycle divide the "labor" -- and are therefore able to specialize independently.