Mosquito circadian flight rhythms: differential effects of constant light

The flight activity of Culiseta incidens was automatically recorded in constant darkness and constant light after entrainment to light:dark 12:12 to determine the usefulness of a formal circadian pacemaker model in which the periods of two oscillators respond reciprocally to light intensity. Period at activity onset lengthened approximately 0.4 h from darkness to 0.1 lx, whereas period at offset shortened approximately 0.4 h. Thus within this range, the onset of the nocturnal active phase obeyed Aschoff's rule for nocturnal animals, and the offset (or onset of diurnal rest phase) obeyed the rule for diurnal animals. These data supported a model in which, as light intensity increases, the period of one oscillator (evening) increases while that of another (morning) decreases. This model, with additional assumptions, also provided a framework for mutually consistent explanations of other features among the data. These included long periods at 3.5 lx, short periods at 35 lx, a progressively earlier occurrence of inactivity with increasing light intensity, and a clear example of circa-bi-dian (approximately 2 days) rhythmicity.