Retinular fine structure in compound eyes of diurnal and nocturnal sphingid moths

Summary Retinular fine structure has been compared in the superposition compound eyes of three sphingid moths, one nocturnal, Cechenena, and two diurnal, Cephonodes and Macroglossum. Cechenena and Cephonodes have tiered retinas with three kinds of retinular cells: two distal, six regular and one basal. The distal retinular cells in Cechenena are special in having a complex partially intracellular rhabdomere not present in Cephonodes. Macroglossum lacks the distal retinular cell. In Cephonodes a unique rhabdom type, formed by the six regular retinular cells in the middle region of the retinula, is divided into three separate longitudinal plates arranged closely parallel to one another. Their constituent microvilli are consequently all nearly unidirectional. The ratio of rhabdom volume to retinular cell volume in the two diurnal sphingids is 10–27%; this is about the same as that (25%) of skipper butterflies, but significantly smaller than in the nocturnal Cechenena (60%). In the diurnal sphingids retinular cell membranes show elongate meandering profiles with septate junctions between adjacent retinular cells. From the comparative fine structure of their eyes the diurnal sphingids and the skippers would appear to be phylogenetically closely related.Supported in part by grants from Ministry of Education Japan (Special Project Research in Animal Behaviors)     

The Fine Structure of the Compound Eye of Tanais cavolinii Milne-Edwards (Crustacea: Tanaidacea)

Abstract The compound (apposition) eyes of Tanais cavolinii are not well developed: the number of ommatidia is small and there are certain irregularities in structure. The refractive components are formed by the cornea and the cone. The latter is built up by two cone cells. In addition, there are two accessory cone cells confined to the distal part of the cone. The eight pigmented retinular cells extend from the cornea to the basement membrane. Proximal to the cone, they form a fused continuous rhabdom, which in cross section has a rectangular outline. In the middle part of the rhabdom, the microvilli are arranged perpendicular to the long axis of the rhabdom when seen in cross section. The microvilli outside of this area can be arranged either parallel or perpendicular to the microvilli of the middle part. Other irregularities occur in the ommatidium, e.g. the position of the retinular cell nuclei, which are found at different levels. Extensions from the cone cells fuse and form a mesh proximal to the rhabdom. Between the mesh and basal lamina is a basal cell type enveloping the proximal parts of the retinular cells and their axons. These cells also form the basal lamina, which delimits the compound eye from the haemocoel. No special pigment cells are present in the compound eye of Tanais cavolinii.     

The fine structure of the lateral ocellus of the dobsonfly larva

The lateral ocelli of the dobsonfly (Protohermes grandis, Neuroptera) larva have been examined with light and electron microscopy. The larva has six ocelli on both sides of the head, each containing a single corneal lens. A conical crystalline body, of some 10–20 cells is situated immediately posterior to the lens. From 100 to 300 elongated retinular cells are arranged perpendicular to the crystalline body except at the innermost surface of the lens, where they are absent. The distal process of each retinular cell is enclosed by a tube-like rhabdom formed by the close association of microvilli from the same and adjacent distal processes. The distal process contains many mitochondria, multivesicular bodies, microtubles and pigment granules. In the dark-adapted ocellus the pigment granules are concentrated near the nucleus which lies under the rhabdomic layer. The granules diffuse toward the rhabdomic microvilli during light adaptation. Each retinular cell has a single axon, which extends from the ocellus as an ocellar nerve fiber into the optic lobe, where it frequently synapses upon second order neurons. In addition to these afferent synapses, there are two other synaptic combinations: (1) a feedback synapse from a second order neuron to a retinular axon, and (2) a synapse between second order neurons. These results suggest that photic signals reach the more proximal part of the brain via second order neurons after some degree of integration in the optic lobe.     

FINE STRUCTURE OF THE RETINULAE IN THE COMPOUND EYE OF THE HONEY-BEE

The retinula of the compound eye of the worker honey-bee has been examined with the electron microscope. The rhabdom lies on the ommatidial axis; it is usually cylindrical in shape, about 3 to 4 µ in diameter, and lacks an axial cavity. Cross-sections show it to be four parted, although it is formed from eight retinular cells (Figs. 2, 3). Each quadrant of the rhabdom consists of a closely packed parallel array of tubules with long axes perpendicular to the axis of the rhabdom. The tubules in adjacent quadrants of the rhabdom are mutually perpendicular. At the distal end of the ommatidium these tubules are seen to be microvilli of the retinular cells. Immediately surrounding the rhabdom, the cytoplasm of the retinular cells contains a membranous endoplasmic reticulum which is oriented approximately radially with respect to the axis of the ommatidium. Farther away from the rhabdom the cytoplasm contains numerous mitochondria.     

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.     

Ultrastructure of the larval stemmata,the stemmata nerves and the optic neuropils of the larval cotton bollworm,Heliothis armigera (Hübner) (Lepidoptera : Noctuidae)

Ultrastructure of stemmata (larval eyes), stemmatal nerves, and the optic neuropils of 5th-instar larvae of cotton bollworm, Heliothis armigera (Hübner) (Lepidoptera : Noctuidae), were examined with scanning and transmission electron microscopes. Six stemmata are on each side of the head. Each stemma consists of 7 retinula cells arranged into 2 tiers. Stemmata I and II have 4 distal retinula cells and 3 proximal cells, the other 4 stemmata (III–IV) have 3 distal cells and 4 proximal cells. Stemmata I and IV have a short proximal rhabdom and the rhabdomere of each proximal cell has its microvilli projecting in only one direction. On the other hand, each stemma (in stemmata II–V) has a long proximal rhabdom and the rhabdomere of each proximal cell has microvilli pitched in several different directions relative to the horizontal plane. An axon projects proximally from each retinula cell body. The stemmatal nerve is composed of the 42 retinular axons from all of the 6 stemmata on the same side of the head. Each stemmatal nerve projects to the ipsilateral optic neuropil. Axons from each stemma are in a fasicle (within the stemmatal nerve), which consists of 7 axons, 3–4 of them are thick and terminate synaptically in the proximal neuropil; the others are thinner and terminate in the distal neuropil. Organelles, particularly lysosomes, undergo ultrastructural transformations relative to ambient light levels. The functional significance of abovementioned structures are discussed in light of current knowledge.     

Morphology of the compound eye of the giant deep-sea isopod Bathynomus giganteus

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.     

Fine structure of the compound eye of Porcellio scaber in light and dark adaption.

The compound eyes of the terrestrial isopod Porcellio scaber comprises about 20 ommatidia. The dioptric apparatus of each ommatidia includes a biconvex corneal lens and a spherical crystalline cone that is secreted by two cone cells. The closed rhabdom is formed by the microvillar extensions of seven pigmented retinula cells and one apical eccentric cell. All retinular axons exit the eye in one bundle. During dark-adaption pigment granules in the retinula cells rapidly withdrew from around the rhabdom and the cell periphery, and migrated basally. Rhabdoms thickened because of movement of the microvilli, and mitochondria moved medially and basally. During light adaption these processes were reversed. Multivesicular bodies became less numerous and rough endoplasmic reticulum and ribosomes proliferated during the initial stages of light adaption.     

The retina of the phalangid,Opilio ravennae,with particular reference to arhabdomeric cells

Summary The retina of the phalangid, Opilio ravennae, consists of retinula cells with distal rhabdomeres, arhabdomeric cells, and sheath cells. The receptive segment of retinula cells shows a clear separation into a Proximal rhabdom, organized into distinct rhabdom units formed by three or four retinula cells, and a Distal rhabdom, consisting of an uniterrupted layer of contiguous rhabdomeres. One of the cells comprising a retinula unit, the so-called distal retinula cell (DRC), has two or three branches that pass laterally alongside the rhabdom, thereby separating the two or three principal retinula cells of a unit. The two morphologically distinct layers of the receptive segment differ with respect to the cellular origin of rhabdomeral microvilli: DRC-branches contribute very few microvilli to the proximal rhabdom and develop extremely large rhabdomeres in the distal rhabdom only, causing the rhabdom units to fuse. Principal retinula cells, on the other hand, comprise the majority of microvilli of the proximal rhabdom, but their rhabdomeres diminish in the distal rhabdom. It is argued that proximal and distal rhabdoms serve different functions in relation to the intensity of incident light.In animals fixed 4 h after sunset, pigment granules retreat from the distal two thirds of the receptive segment. A comparison of retinae of day- and night-adapted animals shows that there is a slight (approximately 15%) increase in the cross-sectional area of rhabdomeral microvilli in dark-adapted animals, which in volume corresponds to the loss of pigment granules from the receptive segment. The length of the receptive segment as well as the pattern and shape of rhabdom units, however, remain unchanged.Each retinula unit is associated with one arhabdomeric cell. Their cell bodies are located close to those of retinula cells, but are much smaller and do not contain pigment granules. The most remarkable feature is a long, slender distal dendrite that extends up to the base of the fused rhabdom where it increases in diameter and develops a number of lateral processes interdigitating with microvilli of the rhabdom. The most distal dendrite portion extends through the center of the fused rhabdom and has again a smooth outline. All dendrites end in the distal third of the proximal rhabdom and are never present in the layer of the contiguous distal rhabdom. Arhabdomeric cells are of essentially the same morphology in day- and night-adapted animals. They are interpreted as photoinsensitive secondary neurons involved in visual information-processing that channel current collected from retinula cells of the proximal rhabdom along the optic nerve. A comparison is made with morphological equivalents of these cells in other chelicerate species.     

The compound eye of Parnara guttata (Insecta,Lepidoptera, Hesperiidae): Fine structure of the ommatidium

Summary The fine structure of an ommatidium of a skipper butterfly, Parnara guttata, has been studied using the electron microscope. Each ommatidium has nine retinula cells, which were classified into three groups: two distal, six medial and one basal retinula cells. The rhabdomeres of the distal retinula cells are localized in the distal part of the rhabdom, while those of the six medial retinula cells appear throughout most of the rhabdom. The rhabdomere of the basal retinula cell occupies only the basal part of the rhabdom. The rhabdomeres of four medial cells are constructed of parallel microvilli, while fan-like microvilli form the rhabdomeres of other two medial retinula cells. The distal and basal retinula cells have rhabdomeres consisting of both parallel and fan-like microvilli. This is the first time the construction of the rhabdomeres of the distal and basal retinula cells has been described in such fine detail for a skipper butterfly. Nine retinula cell axons of each ommatidium extend to the first neuropile of the optic lobe, the lamina ganglionaris. No difference was found in the number of retinula cells of an ommatidium or the shape of the rhabdom between the dorsal and ventral regions of the compound eye.     

Der Feinbau des Auges der Mehlmotte,Ephestia kuehniella Zeller (Lepidoptera,Pyralididae)

Zusammenfassung Die Retinula im Ommatidium der Mehlmotte besteht aus einer wechselnden Anzahl (9–12, meist 11) langgestreckter, prismatischer Sinneszellen. Außerdem enthält jede Retinula nahe der Basalmembran im Zentrum zwischen diesen distalen Retinulazellen noch eine basale Retinulazelle. Die Längsachse der Retinula wird von der Achsenstruktur eingenommen, die aus Mikrovilli besteht. Ihr distaler Teil ist der Achsenfaden, der breitere, proximale Teil bildet das Rhabdom. Dieses erscheint im Querschnitt meist vierstrahlig gelappt, da seine Außenseite in Längsrichtung tief gekehlt ist. Der Rhabdomquerschnitt gliedert sich in mehrere Schöpfe parallel angeordneter Mikrovilli (Rhabdomsektoren); jeder Rhabdomsektor besteht aus 1 oder 2 Rhabdomeren. Die basale Retinulazelle entsendet einen kleinen Schopf von Mikrovilli in die proximale Spitze des Rhabdoms. Die distalen Retinulazellen setzen sich proximal in Neuriten fort, welche sich in Einkehlungen der basalen Retinulazelle bzw. der Tracheenendzelle einschmiegen. Jeweils eine Tracheole durchbricht zusammen mit dem Neuritenstrang einer Retinula die Basalmembran; sie verzweigt sich distal zu ca. 30 Tracheolen, die die Retinula umhüllen.Die Kristallkegelzellen grenzen distal an die Cornea; proximal laufen die Kristallkegelzellen eines Ommatidiums in einen gemeinsamen Fortsatz aus, der zwischen den Retinulazellen unmittelbar am Achsenfaden endet. — Nur das helladaptierte Auge wurde untersucht. Hierbei erscheint im distalen Teil der Retinula nur der Achsenfaden lichtdurchlässig, das Cytoplasma der Retinulazellen hingegen von Pigmentgrana durchsetzt und für Licht undurchlässig.

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Fine structure of the eye of the meal moth, Ephestia kuehniella Zeller (Lepidoptera, Pyralididae)

Summary In each ommatidium of the meal moth a retinula is formed from a varying number (9–12, mostly 11) of elongated, prismatic sense cells. In addition, a basal retinular cell is situated near the basement membrane in the center of the other (distal) retinular cells. The axis of the retinula is occupied by many microvilli forming the axial structure, the distal section of which is the slender axial thread. Proximally, the axial structure widens (to 8.5 m instead of 1 m in diameter) and is now called rhabdom. Cross sections of the rhabdom mostly look like a petaloid with four petals; this figure is due to longitudinal infoldings along the length of the rhabdom surface. The rhabdom cross section is subdivided into several brushes of microvilli (rhabdom sectors), each one being characterized by an approximately parallel arrangement of its microvilli. One rhabdom sector may be composed of one or two rhabdomeres respectively.The basal retinular cell participates in rhabdom formation through a small brush of microvilli at the proximal end of the rhabdom. Proximally, the distal retinular cells taper into slender neurites which are embedded in grooves at the surface of the basal retinular cell and the tracheal end cell respectively. One tracheole piercing the basement membrane together with the neurites of one retinula branches into about 30 tracheoles surrounding the retinula.The crystalline cone cells touch the cornea; proximally, their cytoplasm forms a point which eventually terminates amongst the distal tips of the retinular cells, immediately at the axial thread.—Our work was restricted to light adapted eyes; in this condition, light transmission in the distal part of the retinula seems to be blocked by retinular cell pigment except inside the axial thread.

Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Ultrastructure of the compound eye of the diploid female beetle,Xyleborus ferrugineus

Summary The compound eye of female (diploid) Xyleborus ferrugineus beetles was examined with scanning and transmission electron microscopy. The eye is emarginate, and externally consists of roughly 70–100 facets. Each ommatidium is composed of a thickly biconvex lenslet with about 50 electron dense and rare layers. The lens facet overlies a crystalline cone of the acone type which is roughly hourglass-shaped. Pigment cells envelop the entire ommatidium, and pigment granules also are abundant throughout the cytoplasm of the 8 retinular cells. The rhabdomeres of 2 centrally situated photoreceptor cells effectively fuse into a rhabdom that extends from the base of the crystalline cone deeply into the ommatidium. Six distal peripheral retinular cells encircle the 2 central cells, and their rhabdomeres join laterally to form a rhabdomeric ring around the central rhabdom. The rhabdom and rhabdomeric ring are effectively separated by the cytoplasm of the two central retinular cells which contains the usual organelles and an abundance of shielding pigment granules. Eight axons per ommatidium gather in a tracheae-less fascicle before exiting the eye through the fenestrate basement membrane. No tracheation was observed among the retinular cells. Each Semper cell of each observed crystalline cone contained an abundance of virus-like particles near the cell nucleus. The insect is laboratory reared, and the visual system seems very amenable to photoreceptor investigations.This research was supported by the Director of the Research Division, C.A.L.S., University of Wisconsin, Madison; and in part by research grant No. RR-00779 from the Division of Research Resources, National Institutes of Health and by funds from the Schoenleber Foundation, Milwaukee, WI to D.M.N.     

Ultrastructure within the Lateral Plexus of the Limulus Eye

The ultrastructure of the lateral plexus in the compound eye of Limulus is investigated by serial section technique. "Cores" of tissue containing the axons, lateral plexus, and neuropile associated with one sensory ommatidium show the following features: (a) collateral branches from retinular cells do not contribute to the lateral plexus proper, but do form retinular neuropile by contacting collaterals of a self-contained cluster of retinular axons; (b) collateral branches from eccentric cell axons always branch repeatedly upon leaving the parent axon, and compose the bulk of the lateral plexus; (c) the most distal collateral branches from an eccentric cell axon appear to form neuropile and synaptic contacts with each other, whereas more proximal branches form synaptic contacts with collaterals from eccentric cell axons of neighboring ommatidia. We conclude that the ribbon synapses and associated transmitter substance in eccentric cell collaterals must be inhibitory, and that two pathways for self-inhibition may exist. We suggest, as a working hypothesis for the structure of the lateral plexus, a branching pattern with depth that mirrors the horizontal spread of lateral inhibition measured physiologically.     

Orthogonal microvillus pattern in the eighth rhabdomere of the rock crab Grapsus

Summary The eighth retinular cell (R 8) of Grapsus lacks cytoplasmic pigment granules and basically resembles those previously known in the ghost crab Ocypode and the mysid Praunus. Distally located, R 8 comprises four lobes inserted between the outer ends of the seven regular retinular cells (R 1–R 7). A thin cytoplasmic bridge connects these lobes. One lobe adjacent to R 1 contains the nucleus of R 8 and gives rise proximally to the cell's axon. The short distal eighth rhabdomere consists of microvilli (mvl) protruding axially from all four lobes. Similar R 8's were found also in two other crab families and in two other genera of mysids.In Grapsus the eighth rhabdomere is extraordinary in possessing mvl oriented in two orthogonal directions parallel to the mvl of R 1–R 7. The distal 20% of the rhabdom consists of mvl originating exclusively from R 8. These appear in somewhat irregular bands and are alternately oriented parallel to the animal's vertical or horizontal axis. More proximally the retinula contains eleven sectors but the rhabdom still comprises bands of alternating mvl with those from R 8 joined respectively by the rhabdomeres of R 1, 4, and 5 (horizontal) and R 2, 3, 6 and 7 (vertical). The rest of the rhabdom shows typical decapod organization with seven interdigitating rhabdomeres.This research has been aided by grants from the United States Public Health Service (5 RO1 EY 00405) and the National Geographic Society. The authors are grateful to Mabelita Campbell for her helpful assistance.     

Microvillar orientation in the photoreceptors of the ant Cataglyphis bicolor

Polarization sensitivity in arthropod photoreceptors is crucially dependent on the arrangement of the microvilli within the rhabdom. Here, we present an electron-microscopical study in which the degree of microvillar alignment and changes in the cross-sectional areas of the rhabdoms along their length were studied in the compound eye of the desert ant, Cataglyphis bicolor. Serial cross-sections through the retina were taken and the orientation of the microvilli was determined in the photoreceptors of individually identified ommatidia. The reconstructions of microvillar alignment were made in the three anatomically and functionally distinct regions of the Cataglyphis compound eye: the dorsal rim area (DRA), the dorsal area (DA), and the ventral area (VA). The following morphological findings are consistent with polarization sensitivities measured previously by intracellular recordings. (1) The microvilli of the DRA photoreceptors are aligned in parallel along the entire length of the cell from the distal tip of the rhabdom down to its proximal end, near the basement membrane. The microvilli of the retinular cells R1 and R5 are always parallel to each other and perfectly perpendicular, with only minor deviation, to the microvillar orientation of the remaining receptor cells. (2) In the DA and VA regions of the eye, the microvillar tufts of the small receptors R1, R3, R5, R7, and R9 change their direction repetitively every 1-4 7m for up to 90°. In contrast, the large receptor cells R2, R4, R6, and R8 maintain their microvillar orientation rigidly. (3) In the DRA ommatidia, the cross-sectional areas of the rhabdomeres do not change along the length of the rhabdom, but substantial changes occur in the DA and VA ommatidia.     

The Ventral Photoreceptor Cells of Limulus : I. The microanatomy

The ventral photoreceptor cells of Limulus polyphemus resemble the retinular cells of the lateral eyes both in electrical behavior and in morphology. Because of the great size of the ventral photoreceptor cells they are easy to impale with glass capillary micropipettes. Their location along the length of the ventral eye nerve makes them easy to dissect out and fix for electron microscopy. Each cell has a large, ellipsoidal soma that tapers into an axon whose length depends upon the distance of the cell from the brain. The cell body contains a rich variety of cytoplasmic organelles with an especially abundant endoplasmic reticulum. The most prominent structural feature is the microvillous rhabdomere, a highly modified infolding of the plasmalemma. The microvilli are tightly packed together within the rhabdomere, and quintuple-layered junctions are encountered wherever microvillar membranes touch each other. Glial cells cover the surface of the photoreceptor cell and send long, sheet-like projections of their cytoplasm into the cell body of the photoreceptor cell. Some of these projections penetrate the rhabdomere deep within the cell and form quintuple-layered junctions with the microvilli. Junctions between glial cells and the photoreceptor cell and between adjacent glial cells are rarely encountered elsewhere, indicating that there is an open pathway between the intermicrovillous space and the extracellular medium. The axon has a normal morphology but it is electrically inexcitable.     

Cellular identification of the violet receptor in the crayfish eye

Summary Ten violet receptors in the retinas of crayfish (Procambarus) were injected intracellularly with the fluorescent dye Lucifer Yellow-CH and subsequently identified in histological preparations. All had their cell body located distal to the main rhabdom, in the position of the small, 8th retinular cell. In nine cases it was possible to trace the axon of the violet receptor beyond thelamina ganglionaris, and in four cases, to its termination in themedulla.By contrast, 22 green receptors similarly injected were all found to contribute to the main rhabdom, which is formed by retinular cells 1–7. Their axons synapsed in thelamina ganglionaris.Microspectrophotometry of the 8th cell reveals an absorption peak at 440 nm. As previous microspectrophotometric observations indicated that retinular cells 1–7 all contain a visual pigment with _max at 530 nm, the microspectrophotometric data confirm that the violet receptor is cell 8.This work was supported by USPHS grant EY00222 to Yale University. D.C. is a USPHS predoctoral trainee supported by National Research Service Award 5-T32-GM07527. We are grateful to Dr. W.W. Steward for a gift of Lucifer Yellow-CH and J.D. Collins for technical assistance.     

Ultrastructure of the eye of a euphausiid crustacean

The compound eye of the Antarctic euphausiid Euphausia superba is a spherical clear zone eye. The dioptric system consists of a hexagonally-faceted cornea, two corneagenous cells, two crystalline cone cells which form the bipartite crystalline cone, and two accessory cone cells. The dioptric system of each ommatidium is separated from that of adjacent ommatidia by six distal pigment cells and a basement membrane. The proximal tip of the crystalline cone is cupped by the distal ends of the seven retinula cells whose nuclei are arranged in a staggered array slightly distal to the middle of the clear zone. In the distal half of the clear zone, each narrow retinula cell column is surrounded by large proximal extensions of the six distal pigment cells. The pigment cells narrow more proximally and terminate at the proximal basement membrane. A specialized axial channel complex extends from the crystalline cone through the clear zone, and is continuous with a conical refractive element which caps the distal end of the rhabdom. The rhabdom is fused, and made up of alternating highly birefringent layers of orthogonally-oriented microvilli. It is surrounded by a narrow extra-cellular space which is continuous with the distal refractive element and a second conical refractive element at the proximal end of the rhabdom.     

Ultrastructural study of the naupliar eye of the ostracodeVargula graminicola (Crustacea,Ostracoda)

Summary Ostracodes, like other crustaceans, have a simple naupliar eye that is built upon a theme of three eye cups surrounded by a layer of screening pigments. The single naupliar eye of the ostracodeVargula graminicola is situated medially on the dorsal-anterior side of the body and has three fused eye cups, two dorso-lateral and one ventral. Each eye cup has the following components: (1) pigment cells between the eye cups, (2) tapetal cells, (3) retinular cells with (4) microvillar rhabdomeres, and (5) axons extending into the protocerebrum. Typically two retinular cells contribute lateral microvilli to each rhabdom. The two dorso-lateral eye cups have about 40 retinular cells (20 rhabdoms) and the ventral eye cup has about 30 retinular cells (15 rhabdoms). Typical of myodocopid naupliar eyes (as reported from light microscopic studies), no lens cells or cuticular lenses were observed. The presence of tapetal cells identifies theVargula eye as a maxillopod-ostracode type crustacean naupliar eye. It is unlikely that the naupliar eye ofV. graminicola functions in image formation, rather it probably functions in the mediation of simple taxis towards and away from light.     

Larval and adult eye of the Western Rock Lobster (Panulirus longipes)

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.