Ommatidial adaptations for vision in nocturnal insects
Abstract: Nocturnal vision is a demanding task for insects with small eyes. As it gets dimmer the noise imposed by the stochastic nature of photon arrival makes vision unreliable. Despite this, there are quite a number of animal species that are active at night and apparently see well. In this thesis I show that the compound eyes of nocturnal insects are sufficiently flexible to adapt to the particular window of intensities in which the species is active. These ommatidial adaptations for vision in nocturnal insects are found in the optics and morphology of the eye (Chapters I - III) and in the physiology of the photoreceptors (Chapters IV & V). I have also discovered that the various visual adaptations that optimise sensitivity in a compound eye, and thus adapt the eye for a crepuscular or nocturnal lifestyle, do not contribute equally in different groups of insects. The crepuscular Caligo memnon has evolved large eyes with large facets but retained a reasonably high spatial and temporal resolution. In the nocturnal bee Megalopta genalis, the eyes and facets are enlarged, the rhabdoms are wide and the spatial and temporal acuity very poor compared to diurnal insects. In addition to this, M. genalis has a high gain of transduction present in its photoreceptors. The nocturnal dung beetle Onitis aygulus has refracting superposition eyes and increases optical sensitivity by possessing a wide superposition aperture. In addition, this beetle has slow photoreceptors with high gain. Thus, there are a variety of solutions to improve visual sensitivity and reliability in dim light. Exactly which solution evolved in which group most likely depends on constraints imposed by different phylogenetic histories, developmental constraints and different selection pressures due to different life styles.
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