The temperature dependence of the developmental rate of ectothermal organisms may change under the influence of abiotic and biotic environmental factors modifying the thermal reaction norms for development - the temperature threshold and the regression coefficient, i. e. the degree of thermal sensitivity of development, and the sum of degree-days. A similar manifestation of the phenotypic plasticity of the thermal reaction norms for development is described in many species of insects. In this investigation, we studied the effect of photoperiodic conditions on the development of the bug Graphosoma lineatum (L.) from the St. Petersburg (59.9° N) and Bryansk (53.2° N) populations. With the bugs from each population, two experiments were performed, the experimental material for which differed in terms of the period of the adult collection in nature. Two photoperiodic regimes were used (12 and 22 hours of light per day, for the St. Petersburg population, and 12 and 18 hours of light per day, for the Bryansk population)