1. In sugar-fed A. gambiae females, light may affect flight activity directly or by changing the phase of the circadian rhythm; both responses depend on the phase of the rhythm.
2. The phase-response curve (1 h, 70 lux, signals given in the first cycle in DD following LD 12:12) shows a sharp swing, at about 3 h after normal light-off, from a maximum phase-delay to a maximum phase-advance, each of about 2 h. When signals are given at this time, phase re-setting is very variable; cyclical activity continues but the individuals are out of phase.
3. Phase shifting appears to be a function of the energy of the signal. A 5 min, 70 lux signal has no apparent effect. The effect of a 1 h signal increases with intensity, up to at least 500 lux, but does not appear to be significant below 10 lux.
4. Light normally inhibits flight activity, but there is a burst of activity at light-on (light-on response) if it occurs during the active half of the cycle following the initial activity peak. A vigorous light-on response occurs even at the lowest intensity used (0.3 lux).