In the holometabolous larva, there are two cell populations:
(1) The larval cells that are used for the larval structures and
(2) The imaginal disc and the histoblasts that are present in clusters, awaiting the signal to differentiate.
Imaginal discs do not take place in the nymphs and larvae of hemimetabolous insects. The imaginal discs or buds are actually rudiments of future organs of the adult, like mouth parts, wings, antennae, walking legs, and internal organs etc. Figure displays the imaginal discs in the larva and pupa of a fly (Musca). These discs develop directly from the eggs and remain nonfunctional during the larval stages. During the pupal stage they grow in size and differentiate to form adult structures which remain collapsed and folded. While the reorganization is completed the pupa moults to set free the adult or imago. Once the adult or imago blood is pumped into this collapsed structure it causes them to unfold and inflate. Furthermore chitin is deposited on then to harden them.
Figure: Imaginal Discs