The life cycle of the Drosophila melanogaster—a fruit fly—has four phases: embryo, larva, pupa, and adult. During the larval stage, the fly prepares for pupation by creeping into dark crevices, concealing themselves from predatory eyes. Until now, scientists haven’t been able to understand how the larvae’s aversion to light is synchronized with its development. Naoki Yamanaka et al. found the timed trick is due to a hormone called PTTH, which also promotes the production of steroids for growth. The single biochemical messenger’s duties are two-fold: flee from the light and grow larger. In doing so, the hormone plays a crucial role in determining where and when the fly undergoes metamorphosis.