Basically, three things can happen to any given neuron in the central nervous system. It's a really complex proccess! A) some neurons are born very early in the caterpillar's life (embryonically) but are quiescent until adulthood - during metamorphosis, these neurons put on their game face and start to do real work in adulthood. B) Some neurons are useful in larval life and not in adult life, and basically die during metamorphosis. C) Some neurons are useful in both larval and adult life, but do different things - so they basically retract their projections during metamorphosis and make new ones for adulthood. All of these things are specified on a genetic level, but because of the variation in what happens to any given neuron it's a pretty complex process. The really crazy thing is that for each neuron, there is basically one genetic specification for its larval life and another for its adult life! (Although most neurons probably do something pretty similar in both.)