Nightly increase in visual sensitivity correlated with bioluminescent flashing activity in the firefly Photuris...

Changes in visual sensitivity as a function of time-of-day were studied in the firefly Photuris versicolor. The electroretinograms (ERGs) elicited by photic stimuli of constant intensity and duration were recorded over long periods (18-52 h) while the insect was in constant darkness. The amplitudes of the ERGs elicited by the standard flash were small and remained constant during the daytime hours and increased on average sixfold during the evening hours, reaching a maximum at the onset of darkness and remaining at that high level until the morning hours. The amplitude of the standard response again was reduced in the morning hours to a daytime low value which again persisted until the following evening (Fig. 1). In addition, a series of flashes of varying intensity were administered periodically and logV/logI curves as a function of time-of-day were obtained. Compared to the daytime logV/logI curves, the nighttime curves were shifted about 4 log units towards the left on the intensity axis (Fig. 2). This period of high visual sensitivity was correlated with the species' flashing activity period, which occurs during the night hours (Fig. 1).