Abstract: Seasonal variation of the duration and temperature dependence of development was discovered for the first time using pupae of two ant species of the genus Myrmica. Worker pupae produced at the beginning of summer from overwintered larvae (so-called winter brood) developed significantly slower and their development rate was less dependent on temperature in comparison with worker pupae produced later in the season from summer brood from newly laid eggs. The differences between mean development times of winter and summer brood pupae amounted to 7 - 12 % in M. ruginodis from St. Petersburg (Russia), 9 - 11 % in M. rubra from St. Petersburg and 2 - 5 % in M. rubra from Belgorod (Russia). The lines of regression of development rate on temperature had greater slopes in summer brood pupae as compared to winter brood pupae. The differences between winter and summer brood pupae appeared greater in northern populations of both species (St. Petersburg region) compared to the southern population of M. rubra (Belgorod region), both in development times and in temperature dependence of development. Summer brood pupae were found to be smaller and developed significantly faster compared to winter brood pupae. The mean head width of M. rubra (Belgorod region) workers produced from summer brood was almost the same at 20 - 24 °C but substantially decreased at 18 and 16 °C. Thus, larvae developing at low temperatures pupate at smaller size in order to finish their development in time before the autumn cold weather. Summer brood workers from St. Petersburg M. rubra population were significantly smaller than those from Belgorod population. Size differences between summer brood workers were greater in St. Petersburg population compared to the Belgorod region. We suggest that the smaller size of summer brood workers is an adaptation which allows northern ant populations to rear summer brood faster and to produce some adults from summer brood in spite of cooler and shorter summer season.