For ectotherms such as insects, their low- and/or high-temperature tolerance is one of the most important traits not only for their physiological as well as ecological and evolutional processes. Here, we review the temperature tolerance of insects in relation to their development and suggest a novel method of specifying low and high threshold temperatures. To date, the upper and lower critical thermal threshold for development as T_min and T_max, respectively, which are derived from nonlinear empirical models, has been extensively used. These indicators, which originated from the artificial empirical models, however, may not be reliable. Consequently, the Sharpe–Schoolfield–Ikemoto (SSI) model which is a nonlinear theoretical model based on thermodynamics, was implemented as an alternative tool to express tolerance temperatures. In the model equation constructed with subunits, when the reversed denominator (P₂ function) is maximum (nearly 100% in general), it denotes the probability of an enzyme being in the almost fully-active state of the intrinsic optimum temperature (T_Φ). The profile of the P₂ function shows a peak at the T_Φ temperature and the two temperatures of T_L50 and T_H50 which indicate the 50% active and 50% inactive state (P₂ = 50%), respectively, are given as parameters in the P₂ function of the SSI model. Even so, it is possible to select any values within the range of 0<P₂<100% to determine the lower and upper threshold. Namely, the two temperatures associated with a specified P₂ value could be determined also within the range of real-medium temperature in which ectotherms is living in nature.