Thermal extremes can affect insects at any developmental stage, but vulnerability is often inferred from a single exposure period, endpoint, or observation time. Such inference is problematic because mortality, developmental disruption, reproductive impairment, physiological injury, carry-over effects, and post-stress recovery can have different thermal sensitivities. This systematic review evaluates evidence published from 2017–2025 on stage-specific responses of insects to extreme or stressful heat and cold, with particular attention to whether effects remain confined to the exposed stage, persist into later stages, or change during recovery and repeated exposure. Across the evidence base, no developmental stage emerges as universally most vulnerable. Instead, apparent vulnerability depends jointly on thermal intensity, duration, fluctuation or repetition, the biological endpoint measured, developmental timing, sex, genotype or population, and environmental context. Embryonic and juvenile exposures can produce delayed consequences after metamorphosis, whereas pupal exposures may generate adult traits with contrasting reversibility. Adults frequently show reproductive effects at thermal exposures that do not cause equivalent mortality, and repeated events can produce cumulative rather than compensatory responses. Cold acclimation and post-heat recovery further demonstrate that tolerance measured immediately after stress is not equivalent to long-term restoration of performance. We therefore propose that insect thermal vulnerability should be interpreted as a stage–exposure–response–time problem rather than as a fixed property of a life stage. This framework preserves distinctions among immediate injury, cross-stage carry-over, recovery, and repeated-exposure history while identifying major taxonomic, laboratory-to-field, and mechanistic limitations in the current evidence.
