DID NEURAL POOLING FOR NIGHT-VISION LEAD TO THE EVOLUTION OF NEURAL SUPERPOSITION EYES

Observations of the infrared deep pseudopupil, optical determinations of the corneal nodal point, and histological methods were used to rela te the visual fields of individual rhabdomeres to the array of ommatid ial optical axes in four insects with open rhabdoms: the tenebrionid b eetle Zophobas morio, the earwig Forficula auricularia, the crane fly Tipula pruinosa, and the back-swimmer Notonecta glauca. The open rhabd oms of all four species have a central pair of rhabdomeres surrounded by six peripheral rhabdomeres. At night, a distal pigment aperture is fully open and the rhabdom receives light over an angle approximately six times the interommatidial angle. Different rhabdomeres within the same ommatidium do not share the same visual axis, and the visual fiel ds of the peripheral rhabdomeres overlap the optical axes of several n ear-by ommatidia. During the day, the pigment aperture is considerably smaller, and all rhabdomeres share the same visual field of about two interommatidial angles, or less, depending on the degree of light ada ptation. The pigment aperture serves two functions: (1) it allows the circadian rhythm to switch between the night and day sampling patterns , and (2) it works as a light driven pupil during the day. Theoretical considerations suggest that, in the night eye, the peripheral retinul a cells are involved in neural pooling in the lamina, with asymmetric pooling fields matching the visual fields of the rhabdomeres. Such a s ystem provides high sensitivity for nocturnal vision, and the open rha bdom has the potential of feeding information into parallel spatial ch annels with different tradeoffs between resolution and sensitivity. Mo dification of this operational principle to suit a strictly diurnal li fe, makes the contractile pigment aperture superfluous, and decreasing angular sensitivities together with decreasing pooling fields lead to a neural superposition eye.