Geometrical optics of a galatheid compound eye

The eyes of galatheid squat lobsters (Munida rugosa) are shown to be of the reflecting superposition type. In the dark-adapted state corneal lenses focus light at the level of the rhabdoms and light from more than 1000 facets is redirected to the superposition focus by the reflecting surfaces of the crystalline cones. When the eye is light adapted, apposition optics are used. In this state paraxial light is focused by the corneal lens and the parabolic proximal end of the cone onto the distal end of a rhabdomeric lightguide. The latter transmits light across the clear zone to the rhabdom layer. In the dorsal part of the eye the individual ommatidia become progressively shorter until the cones and rhabdoms are no longer separated by a clear zone. Although formerly considered to be developing ommatidia, they are shown to be retained specifically for scanning the downwelling irradiance.Abbreviations RI refractive index - SEM scanning electron microscope     

Hypersensitivity in the anterior median eye of a jumping spider.

Changes in sensitivity of the photoreceptor cells of the anterior median eye of the jumping spider Menemerus confusus Boes. et Str. have been studied by recording electroretinograms (ERGs) and receptor potentials. The amplitudes of the responses (ERGs and receptor potentials) increase during repetitive stimulation, with a maximum increase at 3-5 s intervals. The sensitivity of the photoreceptor cell is greater for about 60 s following illumination (maximum magnitude at 3-5 s) than it is during complete dark adaptation. This phenomenon, which we call 'hypersensitivity', is lost within one day following surgery in physiological saline. Upon loss of hypersensitivity, the sensitivity decrease during light adaptation is greater than for the normal eye and the small increase of sensitivity following the onset of illumination observed for the normal eye is lost.     

Chick eye optics: zero to fourteen days

Ocular dimensions and refractive state data for chicks 0 to 14 days of age were obtained from 234 untreated control eyes of birds treated unilaterally in previous work involving various defocussing lenses and/or translucent goggles. Refractive state and corneal curvatures were measured in vivo by retinoscopy and ophthalmometry respectively. Intraocular dimensions were measured by A-scan ultrasonography, after which the eyes were removed, weighed and measured. In some cases (n=52) intraocular dimensions and lens curvatures were obtained from frozen sections of enucleated eyes. The hyperopia of hatchling chicks (+6.5+4.0 D) initially decreases rapidly and then more gradually to + 2.0 ± 0.5 D by 16 days. The distribution of refractive errors is very broad at Day 0, but becomes leptokurtotic, with a slight myopic skew, by Day 14. Corneal radius is constant for the first four days, possible as a result of pre-hatching lid pressure, and then increases linearly, as do all lens dimensions, axial diameter and equatorial diameter. Schematic eyes were developed for Days 0, 7, and 14.     

Ocular changes accompanying eye loss in the spider family uloboridae

In uloborid spiders, eye loss is accompanied by increased visual angles, optical material investment, and potential visual acuity of the retained eyes. Relative to carapace volume, the six-eyed Hyptiotes cavatus and two four-eyed Miagrammopes species have greater retinal hemisphere areas and lens volumes than do the eight-eyed uloborids Waitkera waitkerensis, Uloborus glomosus, and Octonoba sinensis. In Waitkera, in which the eyes have little visual overlap, and in Miagrammopes, in which eye loss simplifies the spiders' patterns of visual overlap, increased retinal cell density enhances potential visual acuity. However, this occurs at the expense of potential retinal cell sensitivity.     

The ontogeny of facultative superposition optics in a shrimp eye: hatching through metamorphosis

Summary Compound eyes of larval and first postlarval grass shrimp (Palaemonetes pugio Holthuis) were studied with light and electron microscopy following adaptation to darkness or bright light. Larvae have well-developed apposition eyes, including 3 main types of accessory screening and reflecting pigments and a fourth class of putatively reflective granules recently described in adult shrimps. Rhabdoms contain orthogonally layered microvilli, and by the last larval stage, 8 retinular cells. Ocular accessory pigments in both light- and dark-adapted larvae are distributed much like those of light-adapted adults, but the distal mass of reflecting pigment is concentrated dorsally in larvae and ventrally in adults. Since larvae swim upside-down, reflecting pigment is oriented downward in all developmental stages and may function for countershading. Light and dark adaptational migrations of all 3 major accessory pigments commence abruptly at metamorphosis to the first postlarva. Upon dark adaptation in postlarvae, superposition optics remain impossible because (1) distal screening pigment migrates only slightly, (2) no clear zone has developed, and (3) the crystalline cones remain circular in cross section. Nevertheless, a slight improvement in photon catch is expected due to extensive redistributions of reflecting pigment and retinular cell screening pigment granules.

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Visual optics: The sandlance eye breaks all the rules

The eyes of the sandlance differ from those of other fish, both optically and in the kinds of movements they make. The predatory behaviour of these tiny fish not only makes their lifestyle similar to that of a chameleon, but has led to several extraordinary examples of convergence in the visual system.     

Polarizing without a c(l)ue

Most cells polarize in response to an asymmetric signal but some can spontaneously establish a discrete axis of polarity in the absence of a directional cue. Many mathematical models predict that locally acting positive feedback loops and globally acting negative regulators are required for pattern formation. Here, we summarize recent progress towards identifying some of the molecular components involved in these positive and negative feedback mechanisms. In addition, we describe the increasing evidence suggesting that the actin cytoskeleton is fundamentally important for the spontaneous symmetry breaking observed in a range of different organisms.     

An owl's eye: Schematic optics and visual performance inStrix aluco L.

Summary The schematic eye ofStrix aluco, a nocturnal owl species, is described. A comparative and ecological context is used to examine the relationships between optical parameters of the eye and its light gathering and resolving powers. It is concluded that the essentially nocturnal feature of the owl eye does not lie in either its light gathering power or the sensitivity of individual rod receptors. Differences in visual performance at low light levels between the owl and the diurnal pigeon appear to be attributable to differences in the retinal neural integration mechanisms of the two species. However, it is hypothesised that the neural mechanisms which mediate the extraction of spatial information from the retinal image throughout the nocturnal luminance range, can function in the owl eye only because of its absolutely large sized retinal image. Thus the primarily nocturnal feature of the owl eye is its absolutely large posterior nodal distance, retinal image brightness is maximised only as a secondary feature.Abbreviation PND posterior nodal distance     

Almost a spider: a 305-million-year-old fossil arachnid and spider origins

Spiders are an important animal group, with a long history. Details of their origins remain limited, with little knowledge of their stem group, and no insights into the sequence of character acquisition during spider evolution. We describe a new fossil arachnid, Idmonarachne brasieri gen. et sp. nov. from the Late Carboniferous (Stephanian, ca 305–299 Ma) of Montceau-les-Mines, France. It is three-dimensionally preserved within a siderite concretion, allowing both laboratory- and synchrotron-based phase-contrast computed tomography reconstruction. The latter is a first for siderite-hosted fossils and has allowed us to investigate fine anatomical details. Although distinctly spider-like in habitus, this remarkable fossil lacks a key diagnostic character of Araneae: spinnerets on the underside of the opisthosoma. It also lacks a flagelliform telson found in the recently recognized, spider-related, Devonian–Permian Uraraneida. Cladistic analysis resolves our new fossil as sister group to the spiders: the spider stem-group comprises the uraraneids and I. brasieri. While we are unable to demonstrate the presence of spigots in this fossil, the recovered phylogeny suggests the earliest character to evolve on the spider stem-group is the secretion of silk. This would have been followed by the loss of a flagelliform telson, and then the ability to spin silk using spinnerets. This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group''s success. The Montceau-les-Mines locality has previously yielded a mesothele spider (with spinnerets). Evidently, Late Palaeozoic spiders lived alongside Palaeozoic arachnid grades which approached the spider condition, but did not express the full suite of crown-group autapomorphies.     

Sperm dumping in a haplogyne spider

The present study shows that females of Silhouettella loricatula (Arachnida: Araneae: Oonopidae) manage to process sperm in an unusual and previously unknown way. The male ejaculate consisting of spermatozoa and globular secretion is enclosed in a secretory sac. This may avoid the mixing of sperm from different males and at least severely limit sperm competition. The process of sperm enclosure occurs within the female's sperm storage site (receptaculum) as the ejaculate is not surrounded by a sac inside the male's sperm-transferring organs (palpal bulbs). The secretion forming the sac is produced by glands adjoining the receptaculum. The possibility that globular secretions in the male palpal bulbs partly contribute to the sac cannot be ruled out completely. It is suggested that in S. loricatula , the main function of sperm enclosure in a sac is enabling females to dump the ejaculate of a male. The present study represents the first report on sperm dumping in the family Oonopidae. During five first and three second copulations in the laboratory, the dumping of a sac was observed. One dumped sac was sectioned and contained spermatozoa. Two couples were flash-fixed with liquid nitrogen early during copulation, which revealed the mechanism of the sac dumping. By muscle contractions, the receptaculum is bent backwards and the sac moved into the genital opening. The actual sac dumping occurs most probably in cooperation with the male, which moves his pedipalps rhythmically during the entire copulation. Extensions and furrows on the emboli suggest that they may additionally be used as copulatory courtship devices. The enclosure of sperm from the current male in secretion takes place during or immediately following copulation as all mated females sacrificed after copulation had a new sac containing spermatozoa in the receptaculum. Dumping sperm of a previous male during the next copulation may allow females to bias sperm precedence.     

Kin recognition in a social spider

Social spiders accept immigrant spiders into their kin-based groups, suggesting that spiders cannot recognise kin and may lose inclusive fitness benefits. A field and two laboratory experiments on Diaea ergandros, a social crab spider, demonstrated that younger and older instar D. ergandros do discriminate siblings, but potential benefits were variable and not equally distributed. First, proportional survival was greater in large groups regardless of the within-group relatedness, so accepting immigrants increases probability of group survival (although relatedness was more important among smaller groups). Second, juvenile D. ergandros ate unrelated spiders instead of siblings when starved, so immigrants might represent a food reserve in times of food shortage. Third, subadult resident, sibling females cannibalised unrelated, immigrant females and their brothers instead of immigrant males when starved, suggesting that subadult female spiders may maximise outbreeding opportunities. These benefits provide selective pressure for groups to accept immigrants, but as benefits are realised differentially, conflict and cooperation will exist within spider groups similar to that shown in other group-living taxa.     

Efferent fibers and daily rhabdomal changes in the anteromedial eye of the liphistiid spider,Heptathela kimurai

The presence of efferent fibers in the retina of liphistiid spiders, kept in natural daily cycles of illuminance, was examined by electron microscopy. The efferent fibers were observed to extend their processes through the ocellar nerve to the retina. They contained characteristic large electron-dense granules and branched repeatedly within the retina with varicosities, to provide synaptoid contacts with the receptor cells. They ran mostly among receptor cells and glial cells but sometimes protruded into receptor cells to establish invaginated synaptoid contacts. The synaptoid structures were characterized by spherical clear vesicles located at the presynaptic region, with electron-dense material adhering to the plasma membranes of the receptor cell and the efferent fiber, and a cleft about 10 nm wide formed by the two opposed parallel membranes. The clear vesicles and the electron-dense granules were secreted by exocytosis. The efferent fiber was characteristically presynaptic in relation to the receptor cell. In addition, the rhabdoms differed in size from day to night.     

Terrestrial optics in an aquatic eye: The sandlance,Limnichthytes fasciatus (Creediidae,Teleostei)

The sandlance, Limnichthyes fasciatus (Creediidae, Teleostei), behaves like a marine chameleon, with independent movements of its turret-like eyes, highly-effective camouflage and rapid strikes for isolated, mobile prey at close quarters. The optical system has a fixed circular pupil, a deep pit fovea and a flattened lens unlike any other teleost lens so far described. The convex, laminated structure of the cornea is also unparalleled in a teleost which suggests that the cornea may play a refractive role that might compensate for the reduced power of the flattened lens. This suggestion has been supported in the present investigation by four independent sets of observations:- i. Purkinje images formed underwater by the cornea; ii. Measurements of the magnification of intra-corneal iridophores viewed through the corneal lenticle; iii. Measurements of the magnification produced by the dissected corneal lenticle and lens when viewed over a grating; iv. Ray tracing experiments comparing the degree of refraction produced by the lens and by the corneal lenticle. All experimental observations confirm that the cornea of the sandlance has a significant refractive role, with a power of approximately 200 D compared with a power of 550 D for the lens. This is the first report of a significant refractive role played by the cornea in a teleost. The optical system of lens plus cornea, in combination with a deep pit fovea, may be more suitable for the detection and visual localisation of small, moving, underwater prey than the conventional wide-field spherical lens system of other teleosts. The evolutionary convergence of this marine optical system and lifestyle with those of the chameleon is remarkable, given the constraints imposed by underwater optics.