Our findings demonstrate that the first week following adult eclosion represents a coordinated phase of physiological remodelling rather than a simple continuation of metamorphosis. During the post-eclosion maturation, T. molitor adults undergo a tightly synchronized transition in which energetic reserves, structural maturation, and immune competence are progressively reorganized until a stable physiological state is reached by approximately seven days after emergence. Rather than developing independently, these processes appear to be integrated components of a common developmental programme that prepares newly emerged beetles for adult life in a window of 7 days. The dynamics observed throughout early adulthood strongly support this interpretation. However, early adulthood experienced a generalized proportional loss of body mass across both sexes during the first five days after eclosion. Body condition, estimated using the Scaled Mass Index (SMI), improved significantly by day 7. This indicates that the early reduction in biomass does not reflect a deterioration of physiological status but rather the strategic utilization of larval energetic reserves to complete adult maturation. The absence of sexual differences in body condition indicates that males and females maintain comparable energetic status during early adulthood despite differences in absolute size. Thus, the observed dimorphism is primarily allometric rather than physiological, reflecting differences in body size rather than unequal energetic allocation, as observed in the yellow dung fly Scathophaga stercoraria, where sex-specific energy reserve differences also vanish when standardizing for structural size [22]. These findings indicate that individuals reached peak structural quality upon maturity, potentially supported by the extensive internal tissue remodelling that occurs during the pupal stage to prepare the adult for its reproductive role [13]. Previous studies showed in T. molitor that larger beetles generate significantly heavier individual larvae, and selecting for larger size, particularly in fathers, can enhance overall farm productivity [46]. Future studies quantifying specific energy reserves (such as total lipids, carbohydrates, and glycogen storage) will provide deeper insights into metabolic costs during this phase.

The marked reduction in body mass coincided with the period of maximal cuticular tanning, emphasizing the substantial energetic investment required to transform the newly emerged exoskeleton into a mechanically resistant and physiologically functional structure. Cuticular sclerotization and melanization demand substantial amino acids and metabolic energy [47], driving this immediate mass loss followed by progressive stabilization until day 7. This synchronized endpoint in cuticle tanning suggests that reaching a highly melanized cuticle represents a strict evolutionary constraint shared equally by both sexes [48]. The adaptive significance of this investment extends well beyond cuticle pigmentation itself. A fully melanized exoskeleton is essential for multiple biological functions, including (i) resistance to desiccation, as the cuticle serves as a multifunctional device to protect insects from dehydration [25]; (ii) mechanical protection, as it provides a robust physical barrier that acts as a first line of defence against wounding and external stressors [49]; and (iii) and defence against pathogens, as the degree of cuticular melanization in T. molitor is a strong indicator of resistance to entomopathogenic fungi, with darker beetles possessing thicker, less porous cuticles that are more resistant to microbial penetration [50]. A fully tanned exoskeleton is also a fixed physiological priority for long-term survival [49]. The timing of this maturation also coincides with the peak expression of TmTH and TmDDC, key genes regulating the synthesis of melanin precursors required for cuticular hardening [23]. Furthermore, while cuticular darkness is a strong indicator of pathogen resistance in T. molitor, the degree of melanization ultimately reflects the underlying architecture and thickness of the exocuticle [24]. Crucially, the finding that cuticular tanning effort did not impose a detectable somatic drain on body condition for either sex points to highly efficient internal physiological buffering. Although cuticular melanization and sclerotization are energetically and biochemically expensive [51], T. molitor successfully fulfils these requirements without compromising relative body condition during this early ontogenetic window. This capacity to isolate the somatic costs of exoskeleton consolidation from overall condition highlights the species’ metabolic resilience and the strategic importance of its initial energetic reserves [52].

In parallel with exoskeleton maturation, the immune system underwent substantial reorganization, as highlighted by more than a two-fold increase in THC between days 2 and 7. This age-dependent expansion occurred similarly in both sexes, indicating that the establishment of cellular immune competence represents a conserved component of early adult maturation rather than a sex-specific trait. As granulocytes and plasmatocytes mediate key cellular responses, including phagocytosis and encapsulation [14,27], the increase in circulating haemocytes suggests that, as the exoskeleton hardens, the insect scales up immune defences for adult reproductive challenges. During metamorphosis, haemocytes are heavily sequestered for internal tissue remodelling and larval fat body degradation [53]; this could explain their low numbers in early adulthood. As internal organs stabilize and the insect approaches sexual maturity, these cells are recruited into the hemocoel [54], marking a programmed developmental transition into full cellular readiness. The expansion of THC in T. molitor mirrors the post-emergence maturation pattern of the invasive ladybird Harmonia axyridis, where haemocyte density increases six-fold during the first 8 days of adult life [55]. This “bolstered” adult state contrasts with the developmental trajectories of other orders, such as the cockroach Blattella germanica, which exhibits a continuous linear increase in THC throughout its life cycle [56]. This maturation process is likely coordinated by developmental hormones such as 20-hydroxyecdysone (20E), which acts as a dual regulator driving haemocyte differentiation while simultaneously controlling the expression of TmTH and TmDDC for cuticular tanning [23,34]. This shared endocrine upstream control explains why these distinct processes are so tightly synchronized. Future investigations should incorporate functional immune challenges (e.g., LPS injection or bacterial exposure) to verify whether this day-7 peak state directly translates into enhanced survival or pathogen resistance.

In contrast to the rapid cellular expansion, the humoral immune compartment remains remarkably stable throughout early adult development. The maintenance of a constant PO zymogen reserve suggests that this defence is already fully functional at adult emergence. The PO cascade is a fundamental component of the constitutive, activatable immune response [29], ensuring rapid activation of melanization responses while the soft, newly emerged cuticle remains vulnerable to injury and infection [49]. The stability of PO activity in T. molitor during the first week after emergence is distinct from that in other beetles. The ground beetle Carabus lefebvrei exhibits high PO activity in tanned larvae and adults but very low levels in the pupal stage [57]. In the Lepidopteran Spodoptera littoralis, PO activity is dynamic during maturation, reaching a maximum at pharate stages before dropping after ecdysis [58]. Other beetles, like the burying beetle Nicrophorus vespilloides, have more dramatic changes in their immune traits [53]. However, our analysis reveals a substrate-level trade-off exclusive to males. While tyrosine is required for both exoskeleton hardening and internal immunity [24], males allocating greater resources towards extensive cuticular melanization suffer a significant depletion in their final potential immune defence. This sex-specific bottleneck demonstrates that early maturation in male T. molitor is strictly constrained by a metabolic trade-off between exoskeleton consolidation and humoral competency. This pattern was not detected in females, which suggests that the distribution of resources during the early stages of maturation differs between the sexes. Our observations display early adulthood as a period of meticulously orchestrated up-regulation, wherein both external armour and internal defences undergo synchronized fortification. The physiological changes observed in T. molitor during the first week after emergence are important for the development of many insects. This is also seen in the fruit fly Drosophila montana, where life-history traits are “reset” and adjusted after this period to establish the baseline for future fitness [2].

In evaluating the cross-talk between initial energetic reserves and mature humoral capacity, we observed descriptive, non-significant trends suggesting divergent trajectories between the sexes. Although these statistical values reflect an underpowered subset preventing definitive conclusions, they suggest potentially divergent allocation strategies. Females tended to show a positive association between initial reserves and subsequent PO capacity, whereas males hint at a potential constraint. This tentative divergence warrants targeted, high-power validation in future resource-allocation designs. Such sex-specific trajectories are biologically plausible, as female insects frequently face trade-offs between immune investment and egg production during vitellogenesis [26], whereas male reproductive success may depend more strongly on early reproductive activity [16]. Previous studies suggest T. molitor males reach reproductive potential as early as two weeks after emerging, whereas females require a more gradual maturation process [59]. Furthermore, Krams et al. [60,61] suggest that males may adopt a terminal investment strategy, maintaining high reproductive effort or sexual attractiveness at the expense of future immune competence. This supports our interpretation that males prioritize rapid exoskeleton stabilization and immediate mating effort, even if immune defences are depleted. Although beetles were reared on a standardized diet, the pronounced physiological plasticity of T. molitor [62] suggests that dietary manipulation [1,14,24], particularly supplementation with limiting precursors, such as tyrosine, could substantially alter these developmental trajectories. Moreover, because survival under nutritional stress is primarily determined by absolute body mass rather than immune expenditure [36], the absence of a relationship between tanning effort and body condition further supports efficient energetic buffering during early adulthood. Future studies should also account for environmental variables such as seasonal conditions and barometric pressure, which have recently been shown to influence developmental timing and body mass [62].

Finally, the developmental trajectories described here provide an essential physiological baseline for interpreting responses to environmental stressors. Anthropogenic factors such as microplastics [44] and pesticides [43] are known to disrupt age- and sex-specific physiological processes, frequently reducing or eliminating natural sexual dimorphisms [63]. Establishing normative developmental profiles therefore represents an important prerequisite for identifying stress-induced deviations. Nevertheless, the physiological equilibrium observed at day 7 should be considered a transient developmental state rather than a permanent endpoint. Ageing in T. molitor is accompanied by progressive immunosenescence and reproductive decline [38,59], including a reduction in circulating haemocyte abundance and cellular function by approximately day 30 [53]. Extending longitudinal studies beyond the first week of adult life will therefore be essential to determine how long this mature physiological state is maintained and how energetic allocation, immune competence, and reproductive investment continue to interact throughout the adult lifespan.