The fine structure of the dorsal ocelli in the male bibionid fly

The dorsal ocelli of bibionid flies, details of which have not previously been described, were examined in males of Dilophus febrilis. The three ocelli are combined within an elevated chitin capsule, in a medial position between the enlarged dorsal compound eyes. The biconvex lenses show a multiple layering of up to 150 regularly spaced, clear and dense cuticle zones (100 nm spacing) which probably provide some spectral filtering, suggested by in vivo observations with an epifluorescence microscope. The corneagenous cells and the retina with 100-200 photoreceptor cells are adjoined proximally. A distal retina zone comprises the rhabdoms, which are laterally connected in an hexagonal network. The rhabdoms are between 4 and 15 mum in length; they decrease gradually from the dorsal to the ventral retina region. A middle retina zone comprises the receptor somata, a proximal zone, their axons. Synaptic contacts between axons and interneuron dendrites, feedback synapses to axons, and axo-axonic synapses are found, showing varying pre-synaptic structures. A possible functional role of the ocelli is discussed.     

Anisotropic imaging in the dragonfly median ocellus: a matched filter for horizon detection

It is suggested that the dragonfly median ocellus is specifically adapted to detect horizontally extended features rather than merely changes in overall intensity. Evidence is presented from the optics, tapetal reflections and retinal ultrastructure. The underfocused ocelli of adult insects are generally incapable of resolving images. However, in the dragonfly median ocellus the geometry of the lens indicates that some image detail is present at the retina in the vertical dimension. Details in the horizontal dimension are blurred by the strongly astigmatic lens. In the excised eye the image of a point source forms a horizontal streak at the level of the retina. Tapetal reflections from the intact eye show that the field of view is not circular as in most other insects but elliptical with the major axis horizontal, and that resolution in the vertical direction is better than in the horizontal. Measurements of tapetal reflections in locust ocelli confirm their visual fields are wide and circular and their optics strongly underfocused. The ultrastructure suggests adaptation for resolution, sensitivity and a high metabolic rate, with long, widely separated rhabdoms, retinulae cupped by reflecting pigment, abundant tracheoles and mitochondria, and convoluted, amplified retinula cell plasma membranes.     

The fine structure of a nauplius eye

Summary The nauplius eye of the cyclopoid copepod Macrocyclops albidus has been studied by means of the electron microscope. It is composed of 1 ventral and 2 dorsal ocelli. Each dorsal ocellus consists of a large, pigmented cell, 2 tapetal cells which form a hemispherical cup and are tightly packed with crystals, 9 retinula cells and 5 conjunctival cells. The retinula cells have large masses of endoplasmic reticulum, which can be found in two distinct distributional states, also numerous bodies composed of variously coiled membranes, large amounts of glycogen, mitochondria and scattered neurotubules. The light-sensitive brush borders of these cells are closely coapted and form the irregularly shaped rhabdome. Each of the 9 retinula cells sends an axon by one of three routes to the protocerebrum. In addition, a dendrite emerges from the protocerebrum, enters the ocellus and ends blindly in immediate vicinity to the rhabdome. The observations concerning the structure of the eye made in the present study have been compared to those of light microscopical investigations. Comparison of structure and probable function of the nauplius eye and other arthropod eyes has led to consideration of the probable mode of synaptic transmission between primary and secondary sensory neurons in the ocellus, i.e. between retinula cells and eccentric cell dendrite, and various morphological features that might be of importance in this connection.Supported in part by Postdoctoral Fellowship 41044 and Research Grant G-23972 from the National Science Foundation, and Research Grant HE-005129-04 from the National Institutes of Health to the Oregon Regional Primate Research Center.     

The mapping of visual space by dragonfly lateral ocelli

We study the extent to which the lateral ocelli of dragonflies are able to resolve and map spatial information, following the recent finding that the median ocellus is adapted for spatial resolution around the horizon. Physiological optics are investigated by the hanging-drop technique and related to morphology as determined by sectioning and three-dimensional reconstruction. L-neuron morphology and physiology are investigated by intracellular electrophysiology, white noise analysis and iontophoretic dye injection. The lateral ocellar lens consists of a strongly curved outer surface, and two distinct inner surfaces that separate the retina into dorsal and ventral components. The focal plane lies within the dorsal retina but proximal to the ventral retina. Three identified L-neurons innervate the dorsal retina and extend the one-dimensional mapping arrangement of median ocellar L-neurons, with fields of view that are directed at the horizon. One further L-neuron innervates the ventral retina and is adapted for wide-field intensity summation. In both median and lateral ocelli, a distinct subclass of descending L-neuron carries multi-sensory information via graded and regenerative potentials. Dragonfly ocelli are adapted for high sensitivity as well as a modicum of resolution, especially in elevation, suggesting a role for attitude stabilisation by localization of the horizon.     

The fine structure of the ocelli of Triatoma infestans (Hemiptera: Reduviidae)

The morphology and fine structure of the ocelli of Triatoma infestans have been analyzed by means of light and electron microscopy. The two dorsal ocelli of this species are located behind the compound eyes, looking dorsally and frontally. Externally, the ocelli are marked by the corneal lenses virtually spherical in form and limited internally by a cuticular apodeme. The lens focuses the incoming rays beyond the retina. A single layer of corneagen cells lies below the cuticular lens. The corneagen cells and photoreceptors are arranged in a cup-like fashion beneath the cuticular lens. A distal retinal zone comprises the rhabdoms, which are laterally connected in an hexagonal meshwork. A middle retinal zone comprises the receptor cell segment free of rhabdom, and a proximal zone their axons. In the middle zone, the oviform nuclei and spheroids are located. Screening pigment granules are present within the retinal cell. Spherical mitochondria are homogeneously distributed in the cytoplasm of the cell body. In the axonal zone, mitochondria are found in the peripheral region. Axons from receptor cells extend into the ocellar neuropile at the base of the ocelli, to synapse with second order neurons. The large axons of second order neurons are bundled by glial cells. The ocellar plexus exhibits a high diversity of synaptic unions (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic).     

Sexual Dimorphism in the Compound Eye of Rhagophthalmus ohbai (Coleoptera: Rhagophthalmidae): I. Morphology and Ultrastructure

The eyes of the winged males and larvi-form, wingless females of the firefly Rhagophthalmus ohbai differ from each other in several respects. Compared with the eyes of the males, those of the females contain fewer (35 versus ca. 3500) and smaller (20 μm versus 24-31 μm) facets and anatomically they are of the apposition type. Their main function appears to be to detect light intensity changes from day to nighttime; resolving power of the female eye must be poor and e-vector discrimination would be absent. The eyes of the males consist of a smaller, dorsal region of ca. 500 om-matidia of about 250 μm length and a larger, ventral region of ca. 2000 ommatidia of about 640 urn length. The microvilli of the dorsal eye region are somewhat wider than those of the ventral region (55 nm versus 45 nm) and are less regularly arranged. A tapetal reflecting layer is only present in the dorsal eye region. The small clear-zone between dioptric apparatus and retina in the dorsal eye region would not allow as good a superposition image to be produced as in the ventral eye region with its 5 times wider clear-zone. The regular orientations of the microvilli in the rhabdoms and the lack of a proper tapetum in the ventral eye region suggest that e-vector discrimination should be possible.     

Ocellar fine structure inCaenis robusta (Ephemeroptera), Trichostegia minor,Agrypnia varia,andLimnephilus flavicornis (Trichoptera)

Summary Three dorsal ocelli are present inCaenis robusta (Ephemeroptera), Trichostegia minor, Agrypnia varia, andLimnephilus flavicornis (Trichoptera). The dioptric apparatus of the ocelli differs between the four species. InTrichostegia andAgrypnia a biconvex corneal lens is present, inLimnephilus the corneal lens is convexo-concave complemented by an underlying haemocoelic space, whereas a cellular vitreous body is found between the cuticle and the retinal layer in the ephemerid. In the three trichopteroid species the ocelli are surrounded by an array of longitudinally arranged tracheoles; inCaenis a layer of screening pigments is found in this position. In this species the rhabdoms formed by microvilli of neighbouring retinula cells have a randomly arranged meshwork pattern; in the three trichopteroid species the rhabdoms are isolated, built up of four retinula cells. Cells with basally situated nuclei and lamellar extensions between the retinular cells are found in the ocelli ofTrichostegia, Agrypnia, andLimnephilus.     

Fine structure of the dorsal ocellus of the worker honeybee

The three dorsal ocelli of worker honeybees have been studied by light and electron microscopy. Each ocellus has a single flattened spheroidal lens and about 800 elongated retinular cells. Retinular cells are paired and form a two-part plate-like rhabdom between their distal processes. Each rhabdomere comprises parallel microvilli projecting laterally from the apposed retinular cells. Primary receptor cell axons synapse within the ocellus with ocellar nerve fibers of two different calibers. Each ocellus has eight thick fibers ca 10 m?m in diameter and several thinner ones less than 3 m?m in diameter. Fine structural evidence suggests that retinular axons end presynaptically on both types of ocellar nerve fibers. Since all retinular cells apparently synapse repeatedly with the thick fibers this involves a convergence of about 100:1. Thick fibers always terminate postsynaptically within the ocellus while thin fibers terminate presynaptically on other thin fibers, thick fibers or retinular axons. Structural evidence for synaptic polarization indicates that retinular cells and thick fibers are afferent, thin fibers efferent. Thus complex processing of the ocellar visual input can occur before the secondary neurons of the three ocelli converge to form the single short ocellar nerve which runs to the posterior forebrain.     

Visual behaviour and the structure of dark and light-adapted larval and adult eyes of the New Zealand glowworm Arachnocampa luminosa (Mycetophilidae: Diptera)

Both larval and adult New Zealand cave glowworms exhibit reactions to light; their photoreceptors must, therefore, be regarded as functional. The two principal stemmata of the larva possess large biconvex lenses and voluminous rhabdoms. Approximately 12 retinula cells are present. In light-adapted larvae the diameter of the rhabdom is 8 μm and that of an individual microvillus is 49.5 nm. Dark-adapted eyes have rhabdoms that measure 14 μm in cross section and microvilli with an average diameter of 54 nm. The compound eye of the adult comprises approximately 750 ommatidia, each with a facet diameter of 27–28 μm. A facet is surrounded by 1–6 interommatidial hairs which are up to 30 μm long. The interommatidial angle is 5.5°. Cones, consisting of 4 crystalline cone cells, are of the ‘acone’ type. Pigment granules in the primary pigment cells are twice as large as those of the retinula cells which measure 0.6–0.75 μm in diameter. The rhabdom is basically of the dipteran type, i.e. six open peripheral rhabdomeres surround 2 central rhabdomers arranged in a tandem position. The microvilli of cells 1–6 and cell 8 have diameters ranging from 68 to 73 nm, but those of the distally-located central rhabdomere 7 are 20% larger. This is irrespective of whether the eye is dark or light-adapted. In the latter the cones are long and narrow, the screening pigment granules closely surround the rhabdomeres, and the rhabdom is less voluminous than that of the dark-adapted eye.     

东方蜜蜂背单眼的结构和胚后发育

本研究通过形态解剖和BrdU免疫组织化学方法对东方蜜蜂Apis cerana Fabricius背单眼的胚后发育过程进行了比较研究,结果表明:东方蜜蜂的每一个背单眼都包括角膜晶体、角膜生成细胞、小网膜细胞以及后部的单眼神经。蜜蜂的背单眼起源自头壳上皮;其胚后发育的高峰期集中在蛹发育的前3d;其新细胞主要来源于上皮细胞和圆锥形单眼囊周围细胞的有丝分裂;单眼同脑的联系在P1期前后就已经建立;角膜晶体的形成在P5以后。说明单眼的结构和发育同其功能密切相关。     

Retinal ultrastructure of the dorsal eye region of Pararge aegeria (Linné) (Lepidoptera: Satyridae)

We examined the fine structure of dorsal rim ommatidia of the compound eye of Pararge aegeria (Lepidoptera: Satyridae) and compared them with ommatidia of the large dorsal region described by Riesenberg (1983 Diploma, University of Munich). 1. The ommatidia of the dorsal rim show morphological specializations known to be typical of the perception of polarized light: (a) the dumb-bell-shaped rhabdoms contain linearly aligned rhabdomeres with only 2 orthogonally arranged microvilli orientations. The rhabdoms are composed of the rhabdomeres of 9 receptor cells, 8 of which are radially arranged. The rhabdomeres of receptor cells VI and V5, as well as D2, D4, D6 and D8 are dorsoventrally aligned, whereas the rhabdomeres of the cells H3 and H7 are perpendicular to them. The rhabdomere of the bilobed 9th retinula cell lies basally and is dorsoventrally aligned, where retinula cell VI and V5 are already axonal. (b) There is no rhabdomeric twist, and (c) the rhabdoms are rather short. 2. However, in the ommatidia of the large dorsal region, only 2 retinula cells (H3 and H7) are suitable for perception of polarized light. 3. Lucifer yellow and horse radish peroxidase were used as tracers to visualize the projections of retinula cell axons of the dorsal rim area and the large dorsal region into the optic neuropils (lamina and medulla). Two receptors (VI and V5) from both the dorsal rim area and the large dorsal region, have long visual fibres projecting into the medulla. The 7 remaining retinula cells of both eye regions, including those that meet the structural requirements for detection of polarized light in the large dorsal region, terminate in the lamina (short visual fibres). These results provide a starting point for further studies to reveal the possible neuronal pathways by which polarized light may be processed.     

蟋蟀视觉系统的解剖结构与生理机能

蟋蟀视觉系统由单眼、复眼、视叶三部分组成。蟋蟀的单眼为背单眼,由角膜、角膜生成细胞、视网膜等组成,是提高昆虫复眼所感知的视觉刺激的兴奋水平部位;复眼是最主要的视觉器官,由角膜、晶锥、感杆束和网膜细胞、基膜组成,是光电转导和视觉级联反应的中心;视叶由神经节层、外髓和内髓组成,是视觉神经系统的中心。     

The direction of incident light and its perception in the control of pupal melanization in Pieris brassicae

The rôle of the direction of incident light in pupal melanization of the cabbage white butterfly, Pieris brassicae, was examined by exposing larvae during their sensitive period to various combinations of a light and dark background and of melanization inhibiting and promoting light. The results show that melanization is strongly controlled by the direction of the incident light.By blinding different eyes with black varnish, their rôle in the perception of the brightness contrast was studied. Nearly the same degrees of melanization are achieved as by corresponding arrangements of brightness contrast in the environment. Blinding the most ventrally located ocellus (No. 1) results in maximal melanization; blinding additional ocelli has no additional effect. On the other hand, no influence is observed if only the most dorsal ocellus (No. 6) is blinded.By coating the eyes with colours, their rôle in perception of melanization inhibiting and promoting light was investigated. Melanization is markedly influenced by light perceived via the ventral situated ocelli. In this case, the effect also depends on the number of ocelli involved. All effects may be explained by a mechanism which differentiates and transforms signals received through two or more ocelli, light perceived via the most ventral ocellus (No. 1) being the most important signal.An additional extraocular light receptor appears to be located dorsally in the head. This extraocular receptor can discriminate between different spectral ranges and perceive a brightness contrast (if the ocelli are blinded and the trunk is exposed to white illumination).     

Ocellar optics in nocturnal and diurnal bees and wasps

Nocturnal bees, wasps and ants have considerably larger ocelli than their diurnal relatives, suggesting an active role in vision at night. In a first step to understanding what this role might be, the morphology and physiological optics of ocelli were investigated in three tropical rainforest species – the nocturnal sweat bee Megalopta genalis, the nocturnal paper wasp Apoica pallens and the diurnal paper wasp Polistes occidentalis – using hanging-drop techniques and standard histological methods. Ocellar image quality, in addition to lens focal length and back focal distance, was determined in all three species. During flight, the ocellar receptive fields of both nocturnal species are centred very dorsally, possibly in order to maximise sensitivity to the narrow dorsal field of light that enters through gaps in the rainforest canopy. Since all ocelli investigated had a slightly oval shape, images were found to be astigmatic: images formed by the major axis of the ocellus were located further from the proximal surface of the lens than images formed by the minor axis. Despite being astigmatic, images formed at either focal plane were reasonably sharp in all ocelli investigated. When compared to the position of the retina below the lens, measurements of back focal distance reveal that the ocelli of Megalopta are highly underfocused and unable to resolve spatial detail. This together with their very large and tightly packed rhabdoms suggests a role in making sensitive measurements of ambient light intensity. In contrast, the ocelli of the two wasps form images near the proximal boundary of the retina, suggesting the potential for modest resolving power. In light of these results, possible roles for ocelli in nocturnal bees and wasps are discussed, including the hypothesis that they might be involved in nocturnal homing and navigation, using two main cues: the spatial pattern of bright patches of daylight visible through the rainforest canopy, and compass information obtained from polarised skylight (from the setting sun or the moon) that penetrates these patches.     

The ocelli of arctiid moths: ultrastructure of the retina during light and dark adaptation

The ultrastructure of the dorsal ocelli of two arctiid moths (Arctia caja (A. caja) and Creatonotos transiens (C. transiens) was investigated. The two ocelli are positioned laterally on the vertex of the head posterior to the antennae, close to the dorsal margin of the compound eyes. The biconvex corneal lens is located at the apex of a cone-shaped cuticular elevation, which encapsulates the retina. The corneagenous cell layer and the cup-like retina with about 100-130 receptor cells in A. caja (70-90 receptor cells in C. transiens) are adjoined proximally. The retina is completely enclosed by the perineurium and thus separated from the corneagenous cells and the surrounding hemolymph. Irregularly shaped rhabdomeres, consisting of densely packed microvilli, are present in the distal region of the receptor cells. Up to three cells may form a rhabdom. Thus a loose network of photoreceptive structures over the whole retina results. A unique feature of these arctiid ocelli are photoreceptor vacuoles containing microvilli. The function of these organelles is unknown. The rhabdomeric arrangement within the light and dark adapted retina differs considerably. The ultrastructure of the rhabdomeres indicates an intense membrane turnover. However, changes in adaptation state are not accompanied by dramatic changes in the photoreceptive area of an ocellus.     

Remodeling of the Nauplius Eye into the Adult Ocelli during Metamorphosis of the Barnacle, Balanus amphitrite hawaiiensis

The planktonic barnacle larva has a single median ocellus (nauplius eye), while the adult possesses two distinct sets of photoreceptors; a pair of lateral ocelli and a single median ocellus. The nauplius eye of the cypris larva of Balanus amphitrite hawaiiensis is composed of 14 visual cells grouped into three components (a pair of lateral components and a single ventral component) surrounding two centrally located pigment cells; each lateral component consists of 5 visual cells and the ventral component, 4 visual cells. In each component, the rhabdom is made up of apposing microvilli arising directly from the neighboring visual cell bodies.
During metamorphosis into the adult form, the three components of the median ocellus become separated. Each lateral component migrates laterally on the mantle and is remodeled into the adult lateral ocellus, losing two visual cells but gaining new pigment and tapetum cells in the process. The ventral component remains in the mid portion and becomes the adult median ocellus without fundamental modification in composition. The visual cells in both ocelli undergo a marked increase in volume and form many finger-like dendrites. Rhabdomes are made up of interdigitating microvilli arising from the the dendrite tips.     

The eyes of a “nobody”,Anoplodactylus petiolatus (Pantopoda; Anoplodactylidae)

The fine structure of the four ocelli ofAnoplodactylus petiolatus was examined using serial longitudinal and transversal sections of the eye hill. Each pigment cup ocellus is composed of a (planconvex) cuticular lens, lens forming hypodermal cells, inverse retinula cells with latticed rhabdom and surrounding tapetum and pigment layers. Within the retinula cells a distal “vitreous” zone, a nucleus zone and a proximal rhabdomeric zone can be distinguished. Retinula cell axons originate proximally. The tapetum cells contain several layers of reflecting crystals. Distally, they have a common microvillous region. The intraretinal “vitreous” zone contains glycogen-like particles in the centre and rough ER in the periphery. Contrary to other Pantopoda vitreous cells, a praeretinal membrane and a vertical lens groove have not been observed inAnoplodactylus. While the presence of four (median) ocelli appears to be a primitive characteristic, the inverse orientation of the retinula cells in combination with a tapetum lucidum represents a highly derived characteristic among arthropod median eyes.     

Intercellular junctions and rhombic particle arrays in the developing and adult dorsal ocelli of the honeybee

Intercellular junctions and particle arrays in the developing and mature dorsal ocelli of the honeybee Apis mellifera have been studied with conventional and freeze-fracture electron microscopy. Four types of junctions are found in the lentigenic and retinogenic part during development. These are desmosomes, septate junctions, tight junctions, and gap junctions. Gap junctions and septate junctions are found between differentiating photoreceptor cells only as long as the rhabdoms are beginning to form. Their disappearance after differentiation indicates that they could play a part in cell determination. Desmosomes connect photoreceptor cells into the early imaginai stage and then disappear. Other junctions, once they have formed, remain for the life of the animal, but can change considerably in structure, distribution and frequency. The cells of the perineurium surrounding the ocellus are connected by septate and gap junctions, which may be the basis of the blood-eye barrier. Rhombic particle arrays on the E-face of the glial membrane attached to the photoreceptor cell membrane first appear in small groups one day before emergence. In the further course of life these arrays become more extensive and apparent. Their significance may be to play some role in receptor function.     

Etude topographique des interneurones ocellaires et de quelques uns de leurs prolongements chez Periplaneta americana

The topography of the largest ocellar interneurons in the brain of the cockroach Periplaneta americana was shown with cobalt chloride. The ocellar interneurons coloured from one nerve are confined to the ipsilateral side of the pars intercerebralis; their number and their position vary along the ocellar tract. If two ocellar nerves colour from the ocelli, the interneurons show a bilateral symmetry. Only one interneuron runs through the brain between each ocellus and the contralateral connective to the mesothoracic ganglion. When the injection of cobalt chloride is done without any current from the ocellus, the second-order ocellar neurons only are coloured, but when it is done using a current the higher order interneurons are also coloured.Axonal iontophoresis from a cervical connective back into the brain, has revealed that the cellular body of the contralateral higher-order interneuron is situated in the postero-ventral part of the protocerebrum. This pericaryon with a long cellular process is the largest of the ocellar ones (Ø = 50–60 μm). These results are discussed in relation to the ocellar and visual pathways of Schistocerca.     

Specialized ommatidia for polarization vision in the compound eye of cockchafers,Melolontha melolontha (Coleoptera,Scarabaeidae)

Summary The superposition eye of the cockchafer, Melolontha melolontha, exhibits the typical features of many nocturnal and crepuscular scarabaeid beetles: the dioptric apparatus of each ommatidium consists of a thick corneal lens with a strong inner convexity attached to a crystalline cone, that is surrounded by two primary and 9–11 secondary pigment cells. The clear zone contains the unpigmented extensions of the secondary pigment cells, which surround the cell bodies of seven retinula (receptor) cells per ommatidium and a retinular tract formed by them. The seven-lobed fused rhabdoms are composed by the rhabdomeres of the receptor cells 1–7. The rhabdoms are optically separated from each other by a tracheal sheath around the retinulae. The orientation of the microvilli diverges in a fan-like fashion within each rhabdomere. The proximally situated retinula cell 8 does not form a rhabdomere. This standard form of ommatidium stands in contrast to another type of ommatidium found in the dorsal rim area of the eye. The dorsal rim ommatidia are characterized by the following anatomical specializations: (1) The corneal lenses are not clear but contain light-scattering, bubble-like inclusions. (2) The rhabdom length is increased approximately by a factor of two. (3) The rhabdoms have unlobed shapes. (4) Within each rhabdomere the microvilli are parallel to each other. The microvilli of receptor 1 are oriented 90° to those of receptors 2–7. (5) The tracheal sheaths around the retinulae are missing. These findings indicate that the photoreceptors of the dorsal rim area are strongly polarization sensitive and have large visual fields. In the dorsal rim ommatidia of other insects, functionally similar anatomical specializations have been found. In these species, the dorsal rim area of the eye was demonstrated to be the eye region that is responsible for the detection of polarized light. We suggest that the dorsal rim area of the cockchafer eye subserves the same function and that the beetles use the polarization pattern of the sky for orientation during their migrations.