Inside the chrysalis, the larva undergoes histolysis — larval tissues dissolve into a nutrient-rich cellular soup via autophagy and programmed cell death. Imaginal discs (small clusters of undifferentiated cells present since embryogenesis) then proliferate and differentiate into adult structures via histogenesis.
Drosophila has 19 imaginal discs, each fated for specific structures (wing, leg, eye). Each disc is a monolayer epithelium patterned by morphogen gradients (Dpp, Wingless, Hedgehog). The wing disc uses compartment boundaries as organizing axes — anterior/posterior boundary is the Dpp source.
Remarkably, conditioned behavioral responses learned as larvae can persist through metamorphosis into the adult fly — suggesting certain neural circuits survive histolysis. This challenges the view that the pupal brain is fully remodeled and implies circuit-level continuity across radical morphological change.