An immature form of many creatures that develops into a different adult form by metamorphosis (5)
I believe the answer is:
larva
(Other definitions for larva that I've seen before include "Immature form of an insect before metamorphosis" , "E.g. caterpillar form" , "Caterpillar perhaps" , "Grub of e.g. beetle" , "Worm-like insect before metamorphosis" .)