Imaginal discs primarily form:
Drosophila larva contains sacs of diploid epithelial cells called imaginal discs invaginated from larval epidermis during embryogenesis, specified by homeotic gene expression and held quiescent during larval feeding stages. Upon metamorphosis triggered by ecdysone pulse, discs evert, elongate and differentiate into adult structures: wing discs form wings and thorax, leg discs legs, eye-antenna discs eyes and antennae, genital discs external genitalia. Larval organs like salivary glands histolyze. Thus imaginal discs represent primordia for adult body plan, not larval organs, embryonic segments
Ref: Cohen, Annu Rev Cell Dev Biol 1993: Imaginal discs specification and patterning for adult Drosophila structures during metamorphosis.
