Flies are left a few hours free to feed in the dark on agar-filled wells of a plastic plate. Each well is either filled with agar with sucrose (3 %) and a blue dye, or with sucrose (3 %) + quinine and a red dye. After this period, flies are narcotized and the color of their abdomen is evaluated under a stereoscopic microscope. If quinine treated solutions are avoided, most flies present a blue-colored crop.