Abstract
Eye pupil‐size changes, in response to repetitive ON‐OFF light stimulation, of 3 species of noctuid moths have been observed by infrared reflection photometry. For moderate ON stimulus intensities (2.5·10^9^ quanta·cm^−2^·s^−1^), eye reflectance changed exponentially with time, in both the ON and OFF phases. For low ON stimuli (2.5·10^8^ quanta·cm^−2^·s^−1^), however, the dark adaptation response in the OFF phase was composed of an initial exponential and a subsequent linear term. For repetitive low‐intensity ON‐OFF stimuli, the response amplitude changed in a diurnal way. The shape of the response to each ON‐OFF stimulus was, however, independent of diurnal time. Finally, in eyes that were spontaneously entering a light‐adapted state, pure exponential dark adaptation was elicited by intense light pulses. It is suggested that the substances controlling light and dark adaptation are interrelated. The substances constitute a chemical cycle running at constant speed, in the absence of light, and the substance causing pigment dispersion (light adaptation) is a photoproduct of the other substance. The concentration of the substance causing pigment contraction (dark adaptation) is controlled by the rate of the back reaction, which is composed of an exponential term, related to accumulation of the photoproduct during light stimulation, and a constant term, due to the speed of the cycle. The cycle speed is subject to diurnal changes. © 1992 Wiley‐Liss, Inc.