The two-spotted cricket, Gryllus bimaculatus, a hemimetabolous insect, is a valuable model for investigating the hormonal and molecular mechanisms that regulate growth and metamorphosis. Recent studies have shown that the transforming growth factor-β (TGF-β) family ligand myoglianin (Myo) modulates juvenile hormone (JH) biosynthesis in the corpus allatum, thereby helping coordinate somatic growth with endocrine regulation. Functional analyses using RNA interference (RNAi) and CRISPR/Cas9-mediated gene knockout demonstrated that Myo is required for normal nymphal growth and suppression of JH, which enables the onset of metamorphosis. Based on these findings, we propose a threshold-based regulatory model in which Myo levels, potentially reflecting the progression of somatic growth, reach a critical physiological level at which JH synthesis is downregulated, and additional growth is terminated. This Myo–JH regulatory system provides a homeostatic mechanism that links growth progression to the determination of the final body size. Comparative considerations based on findings from insects and vertebrates further suggest that vertebrate homologs of Myo, such as myostatin (growth differentiation factor 8 (GDF8)) and GDF11, may retain evolutionarily conserved functions related to body size regulation. Collectively, these insights support an integrative framework in which Myo-mediated signaling contributes to the coordination of somatic growth, developmental endocrinology, and organism size determination. Manipulation of this pathway may offer a promising strategy for engineering crickets with optimized body sizes for sustainable food production and biomaterial applications.