Regulation of metamorphosis in hemimetabolous insects

Insect metamorphosis depends on the precise coordination of developmental timing, growth, and morphogenesis. Hemimetabolous insects, which undergo gradual postembryonic development culminating in a single terminal molt, provide a critical framework for understanding how these processes are integrated and how holometabolous metamorphosis evolved. Here, we synthesize recent advances in the regulation of hemimetabolous metamorphosis, focusing on how stage-specific transcription factors - E93, Kr-h1, Chinmo, and Broad - constituting the Metamorphic Gene Network, control temporal identity and determine the irreversible transition to adulthood ("the when problem"). We then highlight a parallel regulatory layer governing the growth and morphogenesis of metamorphic traits ("the how much problem"), involving Myoglianin-dependent endocrine gating, local growth-control pathways, and transcriptional modulation of growth mode. Together, these findings reveal a modular yet integrated regulatory architecture and highlight key unresolved questions regarding how hormonal, transcriptional, and tissue-intrinsic growth pathways are mechanistically coupled to coordinate developmental timing, trait growth, and metamorphic commitment.