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Larvae of the weakly blue-luminescent fungus gnat Keroplatus nipponicus possess on either side of their heads a small black stemmatal eye with a plano-convex lens approximately 25 μm in diameter. In total, 12-14 retinula cells give rise to a centrally fused rhabdom of up to 8 μm in diameter. The rhabdom's constituent microvilli, approximately 70 nm in width, are roughly orthogonally oriented, a requirement for polarization sensitivity. Screening pigment granules are abundant in the retinula cells and measure at least 1 μm in diameter. In comparison with the stemmatal eye of the brightly luminescent Arachnocampa luminosa, that of K. nipponicus is considerably smaller with a poorer developed lens and a rhabdom that is less voluminous, but possesses wider microvilli. Although the larval eye of K. nipponicus can be expected to be functional, as the larvae react to light with a behavioural response, the eyes are probably mainly involved in the detection of ambient light levels and not, as in A. luminosa, also in responding to the luminescence of nearby conspecifics., (Copyright © 2017 John Wiley & Sons, Ltd.)

Afferent and efferent neural elements of the retina and central ganglia in the freshwater snail Planorbarius corneus were labelled using retrograde transport of neurobiotin through the optic nerve. Axons of at least some photoreceptor cells become direct contributors to the optic nerve as no synaptic junctions could be detected. The processes enter the cerebral ganglion and form a dense bundle of thin afferent fibres, the so-called optical neuropil. Efferent neurons were revealed in all ganglia, except the buccal ones. Some of the ascending axons branch in the cerebral ganglia, cross the cerebro-cerebral commissure, reach the contralateral eye and form arborizations in the eye cup. Some efferent neurons send axons to different peripheral nerves as well: n.n. intestinalis, pallialis dexter, pallialis sinister internus et externus. Serotonin- and FMRF-amide-ergic fibres were revealed in the optic nerve. These fibres belong to those central neurons which send their axons to the ipsilateral eye only. They form abundant varicoses in the eye cup and nuclear layer of the retina, and possibly help to regulate retinal sensitivity to light.

We are describing a rhabdom organization of the eye of the chrysanthemum beetle Phytoecia rufiventris that to date has not been described from any other insect. In cerambycid beetles free rhabdomeres, forming a circular, open rhabdom, surround a central rhabdom made up of the rhabdomeres of one or two cells. In Phytoecia rufiventris the central rhabdomeres are missing throughout the eye and the microvilli of the outer 6 rhabdomeres are regularly oriented in three directions. Following the classification of rhabdom types suggested by Wachmann (1979), we suggest to name the rhabdom arrangement seen in the retina of Phytoecia rufiventris "Grundmuster 3". This pattern ought to facilitate polarization sensitivity and movement perception, features that agree with the behavioural repertoire of Phytoecia rufiventris.

The structural organization of the eyes belonging to 12 winged male and 12 wingless female Orgyia antiqua moths, exposed for 1 h to UV-radiation (lambda(max)=351 nm) of 1.4 kW/m2, was compared with that of 12 male and 12 female non-irradiated control specimens. Following the UV-exposure, the screening pigments were found in a position indicative of extreme light-adaptation. Extensive formations of vesicles along the perimeter of the cones as well as disintegrating ER in the cone cytoplasm were noticeable, especially in the eye of the female. On the retinal side of the clearzone, the microvilli of the rhabdoms had become affected by the UV in characteristic ways: in the male eye, retinal cell damage in the form of microvillar swellings and disintegrations were largely confined to just two cells per ommatidium, placed opposite to each other. The female eye, once again, exhibited greater vulnerability and more widespread microvillar disruptions that affected all of the ommatidial retinula cells. The greater resistance of the eye of the male to an exposure with UV makes sense, if we consider the consequences of the retinal damage, which would clearly be a more severe handicap for an actively flying individual than for an almost sedentary one like the wingless female.

The pollen-consuming beetle Xanthochroa luteipennis, which belongs to the family Oedemeridae, possesses a nearly spherical eye of approximately 400 microm in diameter. The eye contains 750-800, mostly hexagonal ommatidia, which are of the acone apposition type and have an open rhabdom. A well-developed pupil mechanism controls the light flux to the rhabdom. The pupil is formed with the help of screening pigment translocations, involving primary and secondary (accessory) pigment cells. Cross-sections of rhabdoms reveal that they are developed as ring-like structures, made up of the rhabdomeres of six retinula cells, surrounding a rod-like inner column of two fused rhabdomeres. Rhabdoms of ommatidia in the middle of the eye differ somewhat from those in more peripheral areas. In the former the central rhabdom is circular in cross-section, while in the latter it is spindle-shaped. The rhabdom organization in combination with the distal pupil mechanism is seen as an adaptation to maximize photon capture under a variety of ambient light intensities, for Oedemerid beetles are commonly active during the day as well as the night.

The lateral lens eye of adult Craterostigmus tasmanianus Pocock, 1902 (a centipede from Australia and New Zealand) was examined by light and electron microscopy. An elliptical, bipartite eye is located frontolaterally on either side of the head. The nearly circular posterior part of the eye is characterized by a plano-convex cornea, whereas no corneal elevation is visible in the crescentic anterior part. The so-called lateral ocellus appears cup-shaped in longitudinal section and includes a flattened corneal lens comprising a homogeneous and pigmentless epithelium of cornea-secreting cells. The retinula consists of two kinds of photoreceptive cells. The distribution of the distal retinula cells is highly irregular. Variable numbers of cells are grouped together in multilayered, thread-like unions extending from the ventral and dorsal margins into the center of the eye. Around their knob-like or bilobed apices the distal retinula cells give rise to fused polymorphic rhabdomeres. Both everse and inverse cells occur in the distal retinula. Smaller, club-shaped proximal retinula cells are present in the second (limited to the peripheral region) and proximal third of the eye, where they are arranged in dual cell units. In its apical region each unit produces a small, unidirectional rhabdom of interdigitating microvilli. All retinula cells are surrounded by numerous sheath cells. A thin basal lamina covers the whole eye cup, which, together with the distal part of the optic nerve, is wrapped by external pigment cells filled with granules of varying osmiophily. The eye of C. tasmanianus seemingly displays very high complexity compared to many other hitherto studied euarthropod eyes. Besides the complex arrangement of the entire retinula, the presence of a bipartite eye cup, intraocellar exocrine glands, inverse retinula cells, distal retinula cells with bilobed apices, separated pairs of proximal retinula cells, medio-retinal axon bundles, and the formation of a vertically partitioned, antler-like distal rhabdom represent apomorphies of the craterostigmomorph eye. These characters therefore collectively underline the separate position of the Craterostigmomorpha among pleurostigmophoran centipedes. The remaining retinal features of C. tasmanianus agree with those known from other chilopod eyes and, thus, may be considered plesiomorphies. Characters like the unicorneal eye cup, sheath cells, and proximal rhabdomeres with interdigitating microvilli were already present in the ground pattern of the Pleurostigmophora. Other retinal features were developed in the ancestral lineage of the Phylactometria (e.g., large elliptical eyes, external pigment cells, polygonal sculpturations on the corneal surface). The homology of all chilopod eyes (including Notostigmophora) is based principally on the possession of a dual type retinula., ((c) 2006 Wiley-Liss, Inc.)

Only one insect (the scale insect Eriococcus sp.) is known, in which photoreceptive lamellae appear to have replaced the usual arthropod rhabdom microvilli. We are now reporting the presence of photoreceptive membranes, which also appear to resemble lamellae rather than microvilli, but they are in the ocellus of the tiny wasp Centrodora sp., which parasitizes scale insect eggs. The apparently optically homogenous lens of the Centrodora ocellus measures approximately 10 microm in diameter and, thus, operates at the limits of diffraction. We calculated that the lens is capable of focusing a parallel bundle of rays on the retina of the ocellus.

An essential feature of the superposition type of compound eye is the presence of a wide zone, which is transparent and devoid of pigment and interposed between the distal array of dioptric elements and the proximally placed photoreceptive layer. Parallel rays, collected by many lenses, must (through reflection or refraction) cross this transparent clear-zone in such a way that they become focused on one receptor. Superposition depends mostly on diameter and curvature of the cornea, size and shape of the crystalline cone, lens cylinder properties of cornea and cone, dimensions of the receptor cells, and width of the clear-zone. We examined the role of the latter by geometrical, geometric-optical, and anatomical measurements and concluded that a minimal size exists, below which effective superposition can no longer occur. For an eye of a given size, it is not possible to increase the width of the clear-zone cz=dcz/R1 and decrease R2 (i.e., the radius of curvature of the distal retinal surface) and/or c=dc/R1 without reaching a limit. In the equations 'cz' is the width of the clear-zone dcz relative to the radius R1 of the eye and c is the length of the cornea-cone unit relative to R1. Our results provide one explanation as to why apposition eyes exist in very small scarabaeid beetles, when generally the taxon Scarabaeoidea is characterized by the presence of superposition eyes. The results may also provide the answer for the puzzle why juveniles or the young of species, in which the adults possess superposition (=clear-zone) eyes, frequently bear eyes that do not contain a clear zone, but resemble apposition eyes. The eyes of the young and immature specimens may simply be too small to permit superposition to occur.

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).

The eye of Ligia exotica is of the apposition type and has open rhabdoms. The facets are hexagonal, and the dioptric apparatus consists of a flat cornea and a spherical crystalline cone placed in the center of two large cone cells. Each ommatidium has seven regular retinula cells and one eccentric cell; a basement membrane forms the proximal boundary of the retina. With increases in body size from 0.6 to almost 4.0 cm, facet numbers and ommatidial diameters increased from 800 to 1500 and 35 microm to 100 microm, respectively; eye length and width grew from 1.2 to 3.2 and 0.9 to 2.5 mm, respectively; and length of dioptric apparatus and width of retinal layer changed from 70 microm to 180 microm and about 70 microm to 120 microm. Visual angles and interommatidial angles of centrally located ommatidia remained constant at about 30 and 6.9 degrees, respectively. An almost perfect linear relationship was found when eye length was plotted against the product between the square root of the total number of ommatidia and the ommatidial diameter. No difference between males and females was observed in any of the relationships, but the results suggest that, compared with smaller specimens, larger ones possess increased absolute sensitivity in single ommatidia, increased sensitivity to point sources, and overall larger angular visual fields for the eye in its totality. This means that larger individuals of L. exotica (which are also faster) have an advantage over smaller individuals at night, but that smaller individuals may cope better with bright lights. Vision in L. exotica seems useful not only in detecting potential danger, but also in locating and approaching cliffs from a distance of 2-4 m when swimming in seawater.

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.

Physical and biological characteristics of solar UV-radiation wavebands A, B, and C are explained and information is provided on UV-levels in particular environments and ocular tissues. The question whether or not the rise in circumpolar UV of the last 20 years or so can be regarded as a threat is briefly addressed and it is concluded that even if no threat to photosynthetic productivity of crops and vegetation exists, there is a danger regarding the status of health of human skin and eyes (in particular the lens). The nature of the UV-induced damage to cornea, lens, and even the retina with its photo-receptive cells and pigment epithelium is assessed and a word of caution is sounded with regard to possible injury-potentiating effects of certain chemicals as seen, for instance, in the recent and alarming rise of cataract in Scottish salmon. Finally, because of the multifaceted effects of UV (e.g. at molecular, cellular, tissue, individual, population, and ecosystem level), a plea is made for a concerted, well-funded, international effort to tackle the many remaining problems at all fronts and from all possible angles.

The effect of temperature on the metabolism of 10 isolated eyes taken from 5 lake-dwelling specimens of the crayfish Paranephrops planifrons was examined by measuring oxygen uptake and carbon dioxide production at 3 temperatures. The mean values for oxygen utilization at 3 degrees C, 13 degrees C, and 23 degrees C, expressed in microM O2 g-1 min-1, were 15.40 +/- 1.36, 21.91 +/- 1.20, and 22.52 +/- 2.47, respectively. The resulting Q10 values for respiration rate were 1.43 +/- 0.05 for the 3-13 degrees C range and 1.03 +/- 0.07 for the environmental range of 13-23 degrees C, indicating immediate compensation within the definition presented by Hazel and Prosser (16). The ability of this tissue to undergo immediate thermal compensation reflects the physiological adaptations of intertidal invertebrates, and is significant in terms of the feeding strategy that this animal adopts. Calculations of respiratory quotients for the 3 measurement temperatures indicate a switching in substrate utilization: at 13 degrees C lipid or lipid/protein oxidation and, at the extremes of 3 degrees C and 23 degrees C, carbohydrates or a carbohydrate/protein mixture.

The literature dealing with the lateral eye in lampreys is briefly reviewed here. While there appears to be no longer much doubt that the short and long photoreceptor cells in the lamprey eye correspond to rods and cones, questions of dark/light adaptational changes, the nature of visual pigments, and the roles of retinal serotonin and melatonin need to be re-addressed. Eyes of the larval and postlarval ("macrophthalmia") stages of the lamprey Geotria australis were examined by electron microscopy and it was found that the larval retina is largely undifferentiated except for a small central zone surrounding the optic nerve head. The retina of the postlarval stage is fully differentiated and the photoreceptor outer segments undergo renewal, which appears to involve the retinal pigment epithelium (RPE). The distribution of larval RPE and choroidal pigments, postlarval ganglion cells, and the orientation of scleral collagen are unusual for vertebrates. No obvious positional or size differences of any retinal cell type were apparent between day- and night-fixed specimens.

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.

Curis caloptera is a buprestid beetle, which is active in bright sunlight. Its eye, like that of many other diurnal arthropods, is of the apposition type, in which dioptric apparatus and receptor layer are not separated by a region devoid of pigment. Perhaps to prevent damage by U. V.-radiation, the cornea is relatively thick (approximately 90 micron) and crystalline cones are of the "eucone-type". In each ommatidium the cone cell extensions occupy regular positions between the 8 retinula cells and reach down to the basement membrane where they end in bulbous swellings and contain grains of screening pigment. Pigment grains, slightly smaller than those present in the primary pigment cells, are also found within the retinula cells. Although the rhabdom possesses a uniform diameter of approximately 2 micron over its entire length of almost 300 micron, the number of rhabdomeres contributing to the rhabdom varies and depends on the level at which the rhabdom is sectioned. At the distal end, only one retinula cell possesses a rhabdomere; the same holds true for the proximal end, where a different rhabdomere (with microvilli at right angles to those of the distal cell) dominates. One retinula cell, of darker appearance in electron micrographs, occupies a distal position in each ommatidium and remains preferentially oriented within a sector of 60 degrees irrespective of the ommatidial axis. The ommatidial axis itself was found to vary 235 degrees. We provide circumstantial evidence for the view that the cell in question could be a U. V.-receptor with a role to play in an unambiguous determination of the E-vector. Separate bundles, each containing 8 axons, pass through the basement membrane together with 1 or 2 tracheoles. A traceheal tapetum is not developed.

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.

A number of differences exists between the compound eyes of larval and adult rock lobsters, Panulirus longipes. The larval eye more closely resembles the apposition type of compound eye, in which retinula cells and rhabdom lie immediately below the cone cells. The adult eye, on the other hand, is a typical clear-zone photoreceptor in which cones and retinula cell layers are separated by a wide transparent region. The rhabdom of the larval eye, if cut longitudinally, exhibits a "banded" structure over its entire length; in the adult the banded part is confined to the distal end, and the rhabdom is tiered. Both eyes have in common an eighth, distally-located retinula cell, which possesses orthogonally-oriented microvilli, and a peculiar lens-shaped "crystal", which appears to focus light onto the narrow column of the distal rhabdom. Migration of screening pigment on dark-light adaptation is accompanied by changes in sensitivity and resolution of the eye. Retinula cells belonging to one ommatidium do not arrange into one single bundle of axons, but interweave with axons of four neighbouring facets in an extraordinarily regular fashion.

1. This is the first transmission electron microscope description of the retina of a parasitic fish. 2. The retina is well supplied with capillaries, some of which occur on the inner surface of the retina. An unusual and possibly primitive type of capillary junction is described. 3. The outer segments of the retina of Carapus mourlani, which measure approximately 2 micron in diameter, are of one type only. A certain degree of banking was observed. Outer segments are not isolated from each other by screening pigments; they are, however, surrounded by about 15 calycal processes. 4. Typical spheroids or conoids were not seen in the outer plexiform layer. The inner nuclear layer consists of only one or two layers of nuclei. The inner plexiform layer is unusually wide and some amacrine synapses can be identified. There are few ganglion cells and the ratio of nuclei in the outer nuclear layer to those of the inner nuclear layer to ganglion cells is approximately 100:10:1. 5. The eye of C. mourlani combines features of degenerated photoreceptors, characteristic of cave-organisms, with adaptations which are commonly found in nocturnal and deep-sea forms. 6. The visual behavior of the animal indicates that the eye is fully functional in spite of its corneal keratosis and small number of optic nerve fibers.

The eye of the deep-sea penaeid shrimp Gennadas consists of approximately 700 square ommatidia with a side length of 15 micrometer. It is hemispherical in shape and is located at the end of a 1.5 mm long eye stalk. The cornea is extremely thin, but the crystalline cone is well-developed. A clear zone between dioptric structures and the rhabdom layer is absent. A few pigment granules are found within the basement membrane; otherwise they, too, are absent from the eye of Gennadas. The rhabdom is massive and occupies 50% of the eye. It consists of orthogonally oriented microvilli (the latter measuring 0.07 micrometer in diameter) and is 75 micrometer long. In cross sections adjacent rhabdoms, all approximately 8 micrometer in diameter, form an almost continuous sheet and leave little space for retinula cell cytoplasm. In spite of a one h exposure to light, rhabdom microvilli show no disintegration or disruption of membranes. Vesicles of various kinds, however, are present in all seven retinula cells near the basement membrane. Bundles of seven axons penetrate the basement membrane. On their way to the lamina they often combine and form larger aggregations.

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.