The caste system of social insects is the division of labor between genetically identical individuals. Polyphenism determines whether larvae develop into queens, workers, and in some cases soldiers. In the case of the ant, "P. morrisi", an embryo must develop under certain temperature and photoperiod conditions in order to become a reproductively-active queen. This allows for control of the mating season, but like sex determination, limits the spread of the species into certain climates.