Thermal plasticity is often evaluated within a single insect life stage and then generalized to whole-organism performance or climatic vulnerability. That approach is problematic because temperature acts on a developmental sequence in which sensitivity, physiological capacity, resource demand, mobility, and reproductive function change over time. This integrative review examines how thermal responses arising during embryonic, larval or nymphal, metamorphic, pupal, and adult stages can persist or change across ontogeny. The evidence indicates that stage-specific thermal responses are frequently endpoint dependent: hardening capacity can differ from basal tolerance, metabolic adjustment need not preserve adult phenotype, and developmental thermal history can affect reproduction even after adult conditions change. Conversely, context-specific thermal matching can improve gametic performance, showing that plasticity is not intrinsically harmful. We propose that cross-stage thermal effects be interpreted through four separable questions: where the thermal exposure occurs, which trait responds at that stage, whether the response persists or is transformed across developmental transitions, and whether the eventual endpoint is performance, reproduction, or a defensible fitness component. This framing prevents physiological response, phenotypic carry-over, and adaptive value from being treated as interchangeable. It also highlights the need for stage-resolved, fluctuating-temperature, resource-aware, and sex-specific experiments that connect laboratory phenotypes to ecologically relevant thermal sequences.
