epithelial infoldings in the larvae of holometabolous insects (e.g. Lepidoptera, Diptera) that rapidly develop into adult appendages (legs, antennae, wings etc.) during metamorphosis from larval to adult form.
By implanting discs into the haemocoele of an adult insect their differentiation can be blocked, though their determination remains unchanged except occasionally (transdetermination). The hierarchy of transdetermination has been studied in great detail in drosophila.