Social insect queens and workers can engage in conflict over reproductive allocation when they have different fitness optima. Here, we show that queens have control over queen-worker caste allocation in the ant Cardiocondyla obscurior , a species in which workers lack reproductive organs. We describe crystalline deposits that distinguish castes from the egg stage onwards, providing the first report of a discrete trait that can be used to identify ant caste throughout pre-imaginal development. The comparison of queen and worker-destined eggs and larvae revealed size and weight differences in late development, but no discernible differences in traits that may be used in social interactions, including hair morphology and cuticular odours. In line with a lack of caste-specific traits, adult workers treated developing queens and workers indiscriminately. Together with previous studies demonstrating queen control over sex allocation, these results show that queens control reproductive allocation in C. obscurior and suggest that the fitness interests of colony members are aligned to optimize resource allocation in this ant.

Social insects are interesting models for the study of anticipatory developmental plasticity because of the striking differentiation into reproductive queens and functionally sterile workers. A few ant genera, including Cardiocondyla, represent the pinnacle of social evolution in the Hymenoptera, where workers have completely lost their reproductive organs, minimizing reproductive conflicts between queens and workers. Here we show that late embryos and larvae of queens of the ant C. obscurior can be identified by the appearance of urate deposits around the forming ovaries. The discovery of caste-specific urate patterns in C. obscurior and three additional Cardiocondyla species will facilitate future studies of developmental plasticity in ants.