Although the caste concept has been central to our understanding of the organization of work in social insect colonies, the concept has been the subject of considerable recent criticism. Theoretically, it has been suggested that temporal castes are too inflexible to allow a colony to rapidly reallocate labour in response to changing conditions. In addition, several authors have suggested that task switching is so prevalent that it precludes even the possibility of a rigidly controlled temporal caste system. This study addresses these two criticisms by presenting and testing a revision of the temporal caste concept that recognizes two categories of tasks: those that require a physiological specialization for their efficient performance, and those that all workers are equally able to perform. Only those tasks requiring a physiological specialization are relevant to the temporal caste concept. Two castes of honeybees were shown to vary in response to increased nectar influx, which requires a physiological specialization, but not to heat stress, which requires no specialization. This work suggests that the organization of work in social insect colonies reflects a compromise between selection for the benefits of division of labour and opposing selection for flexibility in task allocation.