Insect metamorphosis is triggered by a decrease in juvenile hormone (JH) in the final juvenile instar. What induces this decrease is therefore a relevant question. Working with the cockroach Blattella germanica, we found that myoglianin (Myo), a ligand in the TGF‐β signaling pathway, is highly expressed in the corpora allata (CA, the JH‐producing glands) and the prothoracic gland [(PG), which produce ecdysone] during the penultimate (fifth) nymphal instar (N5). In the CA, high Myo levels during N5 repress the expression of juvenile hormone acid methyl transferase, a JH biosynthesis gene. In the PG, decreasing JH levels trigger gland degeneration, regulated by the factors Krüppel homolog 1, FTZ‐F1, E93, and inhibitor of apoptosis‐1. Also in the PG, a peak of myo expression in N5 indirectly stimulates the expression of ecdysone biosynthesis genes, such as neverland, enhancing the production of the metamorphic ecdysone pulse in N6. The Myo expression peak in N5 also represses cell proliferation, which can enhance ecdysone production. The data indicate that Myo triggers the premetamorphic nymphal instar in B. germanica and possibly in other hemimetabolan insects.—Kamsoi, O., Belles, X. Myoglianin triggers the premetamorphosis stage in hemimetabolan insects. FASEB J. 33, 3659–3669 (2019). http://www.fasebj.org

Significance Mayflies are the only extant insects that molt after having formed wings, in a stage called subimago. Numerous authors have wondered whether this stage is a nymph, an adult, or a kind of intermediate. Another question is why mayflies have a subimago stage, when molting a wing is risky. Working with Cloeon dipterum , we found that metamorphosis is regulated as in neopteran insects and that it is determined prior to the formation of the subimago. Thus, it should be considered an instar of the adult stage. We also found that the forelegs grow dramatically between the last nymphal instar, the subimago, and the adult. That necessary growth may help to explain the functional sense of the subimago.