An adult fruit fly emerges from the egg and pupa in about two weeks. As the eyes are developing in a fruit fly embryo, an amazing process unfolds. A wave of signals sweeps across stem cells, switching on certain progenitor cells to stop proliferating and signalling other cells to undergo programmed cell death (apoptosis). The result is an amazing organ—the compound eye—and it comes in matching left and right sides, like a pair of rubies.