Flight is central to beetle foraging, dispersal and mate search, yet for ectotherms the temperature restricts when and where they can fly. We introduce a new flight model in the R package NicheMapR that predicts body-temperature dynamics during flight by combining metabolic heat production, convective heat transfer, radiative exchange (including emissivity), and microclimate conditions. We parameterised the model for two Australian jewel beetles that occupy contrasting thermal environments: a small species in cool climate and a large species in hot climate. Species-specific thermal limits were then used to translate predicted body temperatures into conditions under which flight should be safe or unsafe. We validated these predictions with field measurements of operative and body temperatures and the associated flight activity of beetles in nature. We found opposing constraints on flight. In hotter habitats, large beetles quickly overheat because high ambient temperatures interact with high metabolic heat production during flight. In cooler habitats, small beetles lose heat too rapidly to sustain flight because both metabolic heat production and air temperatures are low. These size-dependent limits predict when flight becomes unsafe or unsustainable and explain observed activity patterns in the field. By linking physiology, behaviour and microclimate, our framework provides a mechanistic basis for forecasting how thermal landscapes shape ectotherm flight, with implications for dispersal, foraging and mate search under current and future climates.