[polyfractal]

A lot of insects have a fully intact, if immature, nervous system during their adolescence. For example, fruit flies (drosophilla) have brain features called "mushroom bodies". These grow during larval development then hit a state of hibernation until the larva changes into an adult fly, at which point growth continues and the mushroom body fully develops.

I'm not familiar with a lot of other insects, but I believe it is fairly common for the bulk of structures to form, hibernate, and then finish growing during metamorphosis.