Caste evolution is a central process in the adaptive diversification of social organisms. Specialized defense traits play a crucial role in surviving enemy attacks and securing resources. In some social insects, major workers called soldiers are morphologically specialized as defenses. Among them, the soldier that blocks nest entrances with her plug-shaped head is one of the most specialized case (i.e. plugging behavior). It has been assumed that the soldiers defend their nest all day long because there is a possibility of being attacked at any time. However, the empirical evidence of time investment for plugging behavior is lacking. In this study, to reveal whether a soldier defends nest during entire day or several soldiers defend nest, namely work in shift.
　We observed who was blocking a nest entrance by paint markers for identifying individuals in the lab using the arboreal ant Camponotus nipponicus. We show that a subset of soldiers defended the nest entrance all day long. This result showed that soldiers worked in shift when there were several soldiers in the colony. We discuss chronological task allocation among soldiers.