Experiments with ectothermic animals usually involve the use of constant temperatures, although the ambient temperature does fluctuate in nature. It is well known that diurnal temperature changes often accelerate development and growth in the lower part of the thermal range as compared with the corresponding constant temperatures, while the reverse is true for thermoperiods that extend to the upper thermal extremes. Thus, extrapolation of experimental results to field conditions may be problematic. However, there exists a commonly neglected artifact that may exaggerate the effect of fluctuating or alternating temperatures on biological rates. The problem arises when rearing temperature cycles beyond upper and lower thresholds for development such that the mean temperature is not equivalent physiologically to a corresponding constant regime. We reanalyze published data on growth and development in a number of invertebrate species by calculating threshold-adjusted, physiologically comparable constant temperatures for variable regimes. In most cases, the effect of daily temperature variation on the biological rates is driven more by the portion of development spent outside the thermal thresholds than by the non-linearity of the developmental response to temperature. Our findings suggest that measuring biological rates at constant temperatures is often justifiable, provided that the results are interpreted properly. Abbreviations: DTR: diurnal temperature range; LTT: lower temperature threshold; SDD: sum of degree-days.