All immature animals undergo remarkable morphological and physiological changes to become mature adults. In winged insects, metamorphic changes either are limited to a few tissues (hemimetaboly) or involve a complete reorganization of most tissues and organs (holometaboly). Despite the differences, the genetic switch between immature and adult forms in both types of insects relies on the disappearance of the antimetamorphic juvenile hormone (JH) and the transcription factors Krüppel-homolog 1 (Kr-h1) and Broad-Complex (BR-C) during the last juvenile instar. Here, we show that the transcription factor E93 is the key determinant that promotes adult metamorphosis in both hemimetabolous and holometabolous insects, thus acting as the universal adult specifier. In the hemimetabolous insect Blattella germanica, BgE93 is highly expressed in metamorphic tissues, and RNA interference (RNAi)-mediated knockdown of BgE93 in the nymphal stage prevented the nymphal-adult transition, inducing endless reiteration of nymphal development, even in the absence of JH. We also find that BgE93 down-regulated BgKr-h1 and BgBR-C expression during the last nymphal instar of B. germanica, a key step necessary for proper adult differentiation. This essential role of E93 is conserved in holometabolous insects as TcE93 RNAi in Tribolium castaneum prevented pupal-adult transition and produced a supernumerary second pupa. In this beetle, TcE93 also represses expression of TcKr-h1 and TcBR-C during the pupal stage. Similar results were obtained in the more derived holometabolous insect Drosophila melanogaster, suggesting that winged insects use the same regulatory mechanism to promote adult metamorphosis. This study provides an important insight into the understanding of the molecular basis of adult metamorphosis.insect metamorphosis | insect hormone | evolution of metamorphosis T he transformation from immature stages to a fully reproductive adult form is a fundamental hormonally dependent process during the development of higher organisms. Insects have been very useful model organisms to uncover the mechanisms underlying this complex transformation as all winged insects undergo metamorphosis to reach adulthood. Particularly interesting is that this transformation can be completed through two different metamorphic strategies, either directly (hemimetaboly) or through an intermediate metamorphic-pupal stage (holometaboly). Hemimetabolous insects, such as true bugs, cockroaches, or locusts, hatch from the egg resembling miniature adults, with external wing pads that encase the wing primordia, and metamorphose into adults with functional wings and genitalia during the last juvenile (also called nymphal) instar. In contrast, the larvae of holometabolous insects, like beetles, butterflies, or flies, undergo a complete morphological transformation to form the adult. The reorganization is so dramatic in these insects that a two-stage metamorphic process bridged by the intermediate pupal stage is required to fully transform the crawling larva into a wi...