Life cycle complexity drives variation in thermal tolerance and plasticity

Accumulating evidence suggests that heat tolerance varies substantially across insect development, yet patterns of variation remain difficult to generalise across species. We discuss how the diversity of insect developmental strategies shapes both the intensity and predictability of thermal environments across ontogeny, and how this likely generates variation in heat tolerance, plasticity and carry-over effects. We hypothesise that large developmental variation is expected in holometabolous insects and in species undergoing pronounced microhabitat or diel activity transitions. These transitions can modify heat exposure, behavioural thermoregulatory abilities and the physiological or genetic regulatory network underlying heat tolerance, weakening correlations among life stages. We discuss when carry-over effects are likely to be adaptive, highlighting the importance of environmental predictability, ecological similarity among stages and the balance between heat injury and repair. We argue that an ontogenetic perspective capturing the microenvironmental conditions experienced by each life stage is essential for predicting insect vulnerability to extreme heat.