Early Post-Eclosion Physiological Remodelling in Tenebrio molitor: Body Condition, Cuticle Tanning and Immune Maturation During the First Week of Adult Life

Early adult life represents a critical developmental window during which insects undergo profound physiological reorganization. Despite the economic importance of the yellow mealworm, Tenebrio molitor L., this transition remains poorly characterized/understood. This study provides an integrated characterization of the energetic, cuticular and immunological remodelling that occurs during the first week post-emergence, a critical developmental window with important implications for experimental standardization and mass-rearing optimization. Body mass, Scaled Mass Index (SMI), and cuticular tanning trajectories were monitored at 0, 1, 5, and 7 days, along with cellular (tsotal haemocyte count, THC) and humoral (phenoloxidase activity, PO) immunocompetence at 2 and 7 days in both sexes. Newly emerged adults underwent an energetically demanding early phase characterized by significant proportional mass loss, followed by stabilization and improved SMI by day 7. In parallel, THC increased more than twofold, while the humoral defences remained stable. Cuticle melanization followed a synchronized age-dependent trajectory in both sexes, reaching structural stabilization by the end of the first week. Correlation analysis revealed sex-specific patterns of resource allocation. Females showed a positive association between initial body condition and mature PO activity. Conversely, males exhibited a negative trend between cuticle tanning effort and PO activity, suggesting a clear competition for shared physiological resources. Overall, our findings identify early adulthood as a phase of coordinated physiological remodelling, during which energetic reorganization, cuticle maturation, and cellular immune expansion are synchronized to establish adult functional competence.