Sensitivity and dynamics of the pupil mechanism in two tenebrionid beetles

Using infrared reflectometry of the deep pseudopupil, we have measured the absolute sensitivity, the dynamic range and the speed of the pupil mechanism in the acone apposition eye of two tenebrionid beetles: Zophobas morio F. and Tenebrio molitor L. The following conclusions are made from the results:

1. There is a substantial difference in sensitivity of the pupil mechanism between the two beetle species. The pupil is about 5.3 log units more sensitive in Zophobas than in Tenebrio.
2. There is also a difference in sensitivity between day and night. Surprisingly, the sensitivity is higher at day-time, and the difference is about 0.5 log units in both Zophobas and Tenebrio.
3. Light adaptation is completed faster during daytime than at night in both Zophobas and Tenebrio, whereas dark adaptation is completed about equally fast both day and night in both species. The speed of the pupil response, however, is dependent on the preceding adaptation history.
4. The pupil mechanism in both species is under the influence of a circadian rhythm, which determines the size of the pupil aperture in such a way that the pupil is maximally open when dark-adapted at night, but only partially open when dark-adapted at daytime.

The differences in sensitivity and dynamics of the pupil mechanism between day and night are mainly due to the circadian rhythm setting the control range of the pupil aperture in both Zophobas and Tenebrio. The pupil differences between the two beetles are discussed regarding behavioural differences between the two species.