Entrainment of flies by spectral light. Shown are representative actograms from wild-type flies (WT), so¹ flies, andso¹;gl^60j flies subjected to 12 hr light/dark cycles (LD) of different wavelengths, the phase of which was advanced twice by 6 hr. One-half of the flies subsequently were recorded under constant darkness (DD). Light was blue-green (top left panel, 460–540 nm), red (top right panel, >600 nm), blue (middle and bottom left panels, 420 nm), or green (middle and bottom right panels, 486 nm). Irradiances are indicated in themargin in photons · cm⁻² · sec⁻¹. Wild-type flies re-entrained quickly to the 6 hr phase advances, showing bimodal activity patterns with morning and evening peaks. Activity was restricted mainly to the light phase of the LD cycle. Mutants extended activity into the dark phase and needed several days to re-entrain to phase advances. Some flies failed to re-entrain to green light (so¹;gl^60j and so¹ after second phase advance). The morning peak was visible in so¹ flies only during the blue LD cycle after the first phase shift and faintly during the green LD cycle before the first phase shift. Theso¹ fly in green light showed antidromic phase shifting.