Adult survival is thought to be an important factor regulating the size of aquatic insect populations, yet very little is known about the factors that cause mortality during the adult stage. Percentage mortalities over varying time intervals and 96-h lethal temperature values (96-h_dmax LT₅₀) were calculated for the adults of the common New Zealand caddisfly Hydrobiosis parumbripennis McFarlane (Hydrobiosidae) and stonefly Zelandoperla decorata Tillyard (Gripopterygidae), and 48-h_dmax LT₅₀ values were calculated for the leptophlebiid mayfly Acanthophlebia cruentata (Hudson) using an amplitude of diurnally varying temperature regimens within and outside the insect’s normal environmental range. Maxima ranged from 18 to 40°C and amplitude varied from 6 to 18°C. Mortality of adult mayflies and caddisflies were consistently higher than that of stoneflies for all comparable diurnal temperature trials. Daily temperature maxima were more important than the diurnal range in regulating mortality, with a rapidly increasing mortality as temperature maxima exceeded 24 (Acanthophlebia) or 30°C (H. parumbripennis and Z. decorata). Interpolated LT₅₀ values for diurnally varying air temperature regimens were ∼31–33°C for adult H. parumbripennis and Z. decorata and ∼28–29°C for Acanthophlebia adults. The LT₅₀ value for Z. decorata based on diurnally varying air temperature regimens was ≈10°C higher than that for constant temperature regimens. These findings have potential implications for managing riparian zones alongside streams and also for predicting the impacts of global warming on aquatic insect distributions.