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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).

1. The pattern of non-specific flight activity in Anopheles gambiae females appears to be bimodal, with an initial peak followed by about 12 h of secondary activity, with a maximum 6-10 h after the main peak. 2. The secondary activity is more apparent when the activity level is high, and can account for over 60% of the total activity. When D < 12 h activity is curtailed by the inhibitory effect of light, but there may be a minor peak in the light period, 8-9 h after light-off. 3. The main peak normally follows light-off, but is delayed in LD 3:21 and 1:23. 4. There may also be a peak at light-on if it is abrupt and occurs during the active half of the cycle. The peak is not observed if the light-intensity is changed slowly.

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