Insect stress physiology is often interpreted upward across biological scales: molecular induction is treated as protection, protection as improved performance, and improved performance as evidence of fitness benefit or adaptation. The available evidence does not justify that sequence as a default. This critical review evaluates how inference changes from molecular stress signaling through cellular, tissue and organ responses to whole-animal performance, survival and reproduction. Functional genetic studies show that stress-response genes can contribute to tolerance while producing context-dependent or opposing effects across stress regimes. Mitochondrial and redox responses likewise range from signaling and compensation to injury, and plausible energetic markers can fail to explain phenotypic differences. At tissue and organ scales, ion-transport capacity, neural function and epithelial integrity provide stronger mechanistic bridges when linked directly to systemic homeostasis, yet assay context and candidate-mechanism nulls remain consequential. Whole-animal behavior and hardening can reduce exposure or increase tolerance while simultaneously imposing costs or degrading other performance components. The review therefore argues that molecular response, physiological protection, organismal performance and fitness consequence are distinct evidential claims. A proposed cross-scale interpretation framework developed later in the review requires explicit linkage, endpoint matching, exposure realism, and validation before adaptive meaning is assigned.
