The cells that will form the adult are sequestered in 19 imaginal discs and a set of histoblast nests. The larva undergoes two molts, to give second and third instars, followed by pupation and metamorphosis . The adult body arises from imaginal discs and histoblast nests during metamorphosis. This determination is heritable through cell generations and can be demonstrated by serial transplantation, then metamorphosis. This early finding suggested that the differences between discs might be under the control of genetic switches.
Discs can be fate-mapped by subjecting fragments to metamorphosis.
For example, if a third instar wing imaginal disc is transplanted into the abdomen of another third instar larva the disc will metamorphose along with the host and form an extra wing in the abdomen of the adult. By transplanting only specific portions of the disc it is possible to demonstrate that different portions will always form different characteristic wing structures. The cells that will form the imaginal structures are segmentally determined at cellular blastoderm. Damage or removal of cells from the cellular blastoderm results in structures missing in the larva or adult, allowing the construction of blastoderm fate maps. Cells from the blastoderm, when transplanted, develop autonomously into adult structures characteristic of the cell's position of origin.