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Similarities and differences between the 2 main kinds of compound eye (apposition and superposition) are briefly explained before several promising topics for research on compound eyes are being introduced. Research on the embryology and molecular control of the development of the insect clear-zone eye with superposition optics is one of the suggestions, because almost all of the developmental work on insect eyes in the past has focused on eyes with apposition optics. Age- and habitat-related ultrastructural studies of the retinal organization are another suggestion and the deer cad Lipoptena cervi, which has an aerial phase during which it is winged followed by a several months long parasitic phase during which it is wingless, is mentioned as a candidate species. Sexual dimorphism expressing itself in many species as a difference in eye structure and function provides another promising field for compound eye researchers and so is a focus on compound eye miniaturization in very small insects, especially those that are aquatic and belong to species, in which clear-zone eyes are diagnostic or are tiny insects that are not aquatic, but belong to taxa like the Diptera for instance, in which open rather than closed rhabdoms are the rule. Structures like interommatidial hairs and glands as well as corneal microridges are yet another field that could yield interesting results and in the past has received insufficient consideration. Finally, the dearth of information on distance vision and depth perception is mentioned and a plea is made to examine the photic environment inside the foam shelters of spittle bugs, chrysales of pupae and other structures shielding insects and crustaceans., (© 2014 Institute of Zoology, Chinese Academy of Sciences.)

The aims of this paper have been (a) to characterize marginal ommatidia from different eye regions through a detailed description of their distinct ultrastructural features in three different size-classes of L. exotica, and (b) to compare microanatomical characteristics of the marginal ommatidia with those of ommatidia of the same eye, but located further centrally. On the basis of transverse as well as longitudinal sections we conclude that new ommatidia are added from a crescentic dorso-anterio-ventral edge of the eye and that maturing ommatidia go through a sequence in which originally the nuclei of cone-, pigment-, and retinula cells are arranged in three separate layers. At the beginning of the microvillar development, the organization of the corresponding rhabdomeres is still quite different (much less regular) from that of those rhabdomeres that make up the mature rhabdom. Marginal ommatidia always possess smaller diameters than more centrally located ones and retinal screening pigment granules are most apparent in the retinula cells only after the first microvilli have appeared. The diameters of rhabdom microvilli (approx. 55 nm) do not differ in ommatidia from the five investigated eye regions in small specimens (< 1.5 cm body length), but show a tendency to be slightly wider in the anterior (= frontal or rostral) regions of the eye (approx. 65 nm) in larger specimens (> 2.0 cm body length).

Compound eyes, nauplius eyes, frontal organs, intracerebral ocelli, and caudal photoreceptors are the main light and darkness detectors in crustaceans, but they need not be present all at once in an individual and in some crustaceans no photoreceptors whatsoever are known. Compound eye designs reflect on their functions and have evolved to allow the eye to operate optimally under a variety of environmental conditions. Dark-light-adaptational changes manifest themselves in pigment granule translocations, cell movements, and optical adjustments which fine-tune an eye's performance to rapid and unpredictable fluctuations in ambient light intensities as well as to the slower and predictable light level changes associated with day and night oscillations. Recycling of photoreceptive membrane and light-induced membrane collapse are superficially similar events that involve the transduction cascade, intracellular calcium, and membrane fatty acid composition, but which differ in aetiology and longterm consequence. Responses to intermittant illumination and linearly polarized light evoke in the eye of many crustaceans characteristic responses that appear to be attuned to each species' special needs. How the visual responses are processed more centrally and to what extent a crustacean makes behavioural use of e-vector discrimination and flickering lights are questions, however, that still have not been satisfactorily answered for the vast majority of all crustacean species. The degree of light-induced photoreceptor damage depends on a large number of variables, but once manifest, it tends to be progressive and irreversible. Concomittant temperature stress aggravates the situation and there is evidence that free radicals and lipid hydroperoxides are involved.

The structural organization of the retinula cells in the eye of Ligia exotica changes diurnally. At night, the microvilli elongate, losing the regular and parallel alignment characteristic of the day condition. Crystalline cones and distal rhabdom tips are not pushed into each other during the day, but at night the rhabdoms protrude into the crystalline cones by up to 5 microm. Screening pigment granules in the retinula cells disperse during the night, but migrate radially towards the vicinity of the rhabdom during the day. No such displacements of the pigment granules of either distal or proximal screening pigment cells were observed. The sensitivity of the eye, monitored by electroretinogram (ERG) recordings, changes diurnally: values at midnight are, on average, 10 times those occurring during the day. However, intracellular recordings from single retinula cells (50 during the day and 50 at night) indicate that the difference between night and day sensitivities is only 2.5-fold. Two-dimensional angular sensitivity curves, indicative of a single unit's spatial sensitivity, had considerably less regular outlines at night than during the day. If based on the 50 % sensitivity level, day and night eyes possessed receptive fields of almost identical width (approximately 2 degrees), but if sensitivities below the 50 % limit were included, then receptive fields at night were significantly more extensive. We suggest that the morphological adaptations and diurnal changes in chromophore content seen in the apposition eye of L. exotica allow this animal to improve its photon capture at night while preserving at least some of the spatial resolving power characteristic of the light-adapted state. This would explain why this animal is capable of performing complex escape behaviours in the presence of predators both in bright and in very dim light.

The electroretinogram (ERG) waveform of Petrolisthes elongatus is biphasic and transient and does not change with age (monitored as carapace length and weight of the individual). Though there was a very slight decrease in sensitivity with age (R2 = 0.008), spectral peaks and curves remained virtually unchanged. A shift of 2.2 log units in threshold sensitivity, however, was observed in relation to the different adaptational states. The greatest difference, not unexpectedly, occurred between dark-adapted eyes, tested at night, and light-adapted eyes tested during the day at a background illumination of 250 1x. The gradient of the V/LogI curve (n) in light-adapted animals increased, which means a smaller increase in intensity is required to produce an increase in response compared to a dark-adapted animal, but there was no difference in n with regard to the different age-groups. Also, no significant difference was noticed between dark-adapted eyes tested during day and night. It is concluded that the intensity range over which vision in P. elongatus is maximally sensitive to contrast (the dynamic range) is 2.5-3.0 log units. Since there is apparently little or no alteration in the photic environment and the behaviour of P. elongatus as it ages, there is no advantage in fundamentally altering the parameters of the absolute and spectral sensitivity function with age. Our observations suggest that the eyes are able to adjust within 30 min to changing ambient light levels irrespective of whether or not a circadian activity rhythm operates.

The eye of Petrolisthes elongatus is of the reflecting superposition type. The facets are square and the dioptric apparatus consists of a crystalline cone and a conestalk, each made up of four identical radially arranged units. The rhabdom has an elonagated (2-3 microns diameter) distal segment and a much thicker (6-16 microns diameter) proximal section. The distribution of screening pigments alters dramatically in a migration associated with changes in adaptational state. The following trends with increasing age, monitored as carapace length, were found: (1) increasing number of facets; (2) increasing size of eye and eye-stalk; (3) increasing length of eye-stalk in proportion to eye length; (4) increasing breadth of eye-stalk in proportion to eye breadth; (5) increasing breadth of eye-stalk and eye in proportion to length of eye-stalk and eye; (6) increase in volume of both retinal and dioptric structures; (7) slight increase in rhabdom width; (8) increase in crystalline cone length and breadth and (9) increase in focal length. The results of these trends were presumed to be, firstly, an increase in absolute sensitivity of single ommatidia; secondly, an increase in sensitivity to point sources of light and, thirdly, an increase in angular visual field of the whole eye.

In Streetsia challengeri left and right eyes have fused and become a single cylindrical photoreceptor, which occupies the basal half of a forward directed head projection. This unusual compound eye consists of approximately 2500 ommatidia, which are arranged in such a way that the animal has almost circumferential vision, but cannot look ahead or behind. It is thought that the eye operates on light-guide principles, and that the crystalline cones are the major dioptric component. Ommatidia in anterior-posterior rows show a greater overlap of visual fields than dorso-ventrally arranged ommatidia. Cone layer and retinula are separated by a 4 micrometer thick screen-membrane, which contains tiny pigment granules of 0.15 micrometer diameter. Cells of unknown function and origin, containing unusual multitubular organelles, are regularly found near the proximal ends of the crystalline cone threads. The twisted rhabdoms measure 18--20 micrometer in diameter, and consist of microvilli 0.05 micrometer in width, which belong to five retinula cells and which show no trace of disintegration. The position of interommatidial screening pigment, the density of retinula cell vesicles and inclusions, and the narrowness of the perirhabdomal space all suggest that the eyes have been light-adapted at the time of fixation for electron microscopy. The retinula cell nuclei lie on the proximal side of the heavily pigmented basement membrane. A tapetum or basal retinula cells are not developed. It is concluded that the eye optimally combines acuity with sensitivity, and that for distance estimation parallax may be important.

The amphipod, Orchomene plebs, and the isopod, Glyptonotus antarcticus, both adapted to live in seawater of a temperature of -2 degree to 0 degree C, were kept for 7h at the unphysiologically high temperature of +10 degree C. Temperature elevation appeared to mimic light adaptation with regard to the position of the screening pigment granules within the visual cells, but not with respect to ultrastructural changes in the microvillar array of the rhabdom, i.e. the visual membranes. Cellular metabolism, membranous fatty acid composition, and ion fluxes, all known to be readily affected by an increase in temperature, are thought to be responsible for the observed effects. Pigment granules could possibly cause an elevation of intracellular temperatures due to the fact that they are dark and dissipate absorbed energy as heat.

The structural organization of the compound eye of the largest known isopod, Bathynomus giganteus, is described from four specimens maintained in the laboratory for as long as two months. Living specimens have not previously been available for study. The two triangular compound eyes measure about 18 mm on the dorsal edge and are separated by an interocular distance of 25 mm. They face forward and slightly downward and may have significant overlap in visual fields. Each eye contains about 3,500 ommatidia in animals of body lengths from 22.5 cm to 37.5 cm. The packing of ommatidia is not uniform across the retina, but is nearly hexagonal in the dorsal central region and nearly square in the ventral and lateral periphery. The dioptric elements in each ommatidium consist of a laminar cornea, which is flat externally and convex internally, and a bipartite crystalline cone. Sometimes seven and sometimes eight retinular cells closely appose the proximal tip of the cone and bear the microvilli of the rhabdom. Proximal to the rhabdom the retinular cells form thin pillars near the periphery of the ommatidium, and the central portion along the optic axis at this level is occupied by interstitial cells that contain massive arrays of clear vesicles thought to serve as reflective elements. The arhabdomeral segments of the retinular cells and the interstitial cells rest on a basement membrane. Within each ommatidium the basement membrane has two extensions with cylindrical cores and thin sheets of dense material and collagen-like filaments. These sheets occupy spaces between adjacent interstitial cells up to the level of the rhabdomeral segments of the retinular cells. Arrays of pigment cells with relatively weak light-screening properties separate adjacent ommatidia. Animals were fixed both in light within a week of being brought from depth into daylight, and after 2 months of maintenance in constant darkness following such daylight exposure. In both cases, microvilli of the rhabdom were severely disrupted and the retinular cytoplasm contained numerous multivesicular bodies. Exposure to natural daylight appears to cause irreversible structural damage to the photoreceptors of these animals.

The compound eyes of the mesopelagic eupausiid Thysanopoda tricuspidata were investigated by light-, scanning-, and transmission electron microscopy. The eyes are spherical and have a diameter that corresponds to 1/6 of the carapace length. The hexagonal facets have strongly curved outer surfaces. Although there are four crystalline cone cells, only two participate in the formation of the cone, which is 90-120 micrometer long and appears to have a radial gradient of refractive index. The clear zone, separating dioptric structures and retinula, is only 90-120 micrometer wide. In it lie the very large oval nuclei of the seven retinula cells. Directly in front of the 70 micrometer long and 15 micrometer thick rhabdom a lens-like structure of 12 micrometer diameter is developed. This structure, known in only a very few arthropods, seems to be present in all species of Euphausiacea studied to date. It is believed that the rhabdom lens improves near-field vision and absolute light sensitivity. Rod-shaped pigment grains and mitochondria of the tubular type are found in the plasma of retinula cells. The position of the proximal screening pigment as well as the microvillar organization in the rhabdom are indicative of light-adapted material. The orthogonal alignment of rhabdovilli suggests polarization sensitivity. Behind each rabdom there is a cup-shaped homogenous structure of unknown, but possibly optical function. Finally, the structure and the function of the euphysiid eye are reviewed and the functional implications of individual components are discussed.

When individuals of the crustacean Mysis relicta are fully dark-adapted and preparations for electroretinogramme recordings are performed in infra-red light of wavelength greater than 730 nm, responses to 300 ms flashes of light can be recorded. Though this is the first time that responses to such long wavelengths have ever been reported for any invertebrate photoreceptor, the authors suggest that near infra-red sensitivity in arthropods could be more widespread, and that because of very long adaptation and sensitivity regeneration times preparations should not be carried out in red light, hitherto deemed invisible to most arthropods, but with the aid of an infra-red image converter and a filter at wavelengths greater than 800 nm.