Hawkmoth lamina monopolar cells act as dynamic spatial filters to optimize vision at different light levels

How neural form and function are connected is a central question of neuroscience. One prominent functional hypothesis, from the beginnings of neuroanatomical study, states that laterally extending dendrites of insect lamina monopolar cells (LMCs) spatially integrate visual information. We provide the first direct functional evidence for this hypothesis using intracellular recordings from type II LMCs in the hawkmoth Macroglossum stellatarum. We show that their spatial receptive fields broaden with decreasing light intensities, thus trading spatial resolution for higher sensitivity. These dynamic changes in LMC spatial properties can be explained by the density and lateral extent of their dendritic arborizations. Our results thus provide the first physiological evidence for a century-old hypothesis, directly correlating physiological response properties with distinctive dendritic morphology.Clemence Duea, Anna Zierscha, Moira Walsha and Emily Duivestey

The optical sensitivity of compound eyes: theory and experiment compared

The Land sensitivity equation is a well-known tool for comparing optical performance between eyes. Despite this, the equation has never been experimentally tested. Here, we present, to our knowledge, the first experimental validation of the equation. We have investigated different insect species active at different intensities, and possessing different types of compound eyes, to compare ratios of calculated sensitivities to ratios determined experimentally. Experimental optical sensitivities were measured by adjusting the intensity of an external light source until photoreceptors in the different eyes produced roughly equal numbers of photon responses (‘bumps’) per second. The sensitivity ratios obtained in this manner agree well with those obtained using the equation. We conclude that the Land equation remains an excellent tool for comparing sensitivities between different eyes

Light on the moth-eye corneal nipple array of butterflies

The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids

Vision in the deep sea

The deep sea is the largest habitat on earth. Its three great faunal environments - the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat- are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 in), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly, from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirel, dominated by point-like biolumincscence. This changing nature of visual scenes with depth - from extended source to point source - has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes - particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans - reveal that ocular designs are well matched to the nature of the visual scene at any criven depth. This match between eye design and visual scene is the subject of this review. The greatest variation eye design is found in the mesopelagic zone, where dim down-welling daylight and bioluminescent point Sources may be visible simultaneously. Some ruesopelagic eyes rely on spatial and temporal Summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivty, combined with surprisingly high spatial resolution, is very well adapted to the detection and locallsation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion bells are also frequently arranged in a horizontal visual streak, an adaptation for., the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light viewing and vision in the deep sea are renewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision