I looked for a good explanation of "juvenile" elsewhere on the web to link you to, and strangely, I couldn't find one! So here it is. In the context of organisms undergoing metamorphosis, a "juvenile" is not simply a "youngster". The distinction between a larva and a juvenile is this: a juvenile is what a larva is transformed into during metamorphosis. It looks like an adult, radically different from the larva, but it is not an adult because it has not reached reproductive maturity, or adult size.