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.     

The eyes of mesopelagic crustaceans: I. Gennadas sp. (Penaeidae)

Summary The eye of the deep-sea penaeid shrimp Gennadas consists of approximately 700 square ommatidia with a side length of 15 n. 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 m in diameter) and is 75 m long. In cross sections adjacent rhabdoms, all approximately 8 m 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 authors wish to thank the director of the Meat Industry Research Institute in Hamilton and his staff for the use of their electron microscope facilities     

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.     

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.     

MORPHOLOGY OF THE OMMATIDIA OF THE COMPOUND EYE OF LIMULUS

The sensory portion of the ommatidium of the compound eye of Limulus has been studied with the electron microscope. In axial longitudinal section the rhabdom appears to be made up of small polygons, and in transverse section the rhabdom appears as a banded structure of dark lines. Thus in three dimensions the rhabdom resembles a honeycomb composed of tubular units, the long axes of which lie in transverse planes and are oriented perpendicular to the retinula cell's contours. The tubular units, which are about 140 mµ in diameter in Limulus (70 mµ in diameter in the spider and Scutigera), are microvilli of the borders of the retinula cells. The walls of these microvilli are continuous with fine linear structures (membranes) in the cytoplasm of the retinula cells. In transverse sections of the ommatidium oval bodies interpreted as mitochondria are observed in an annular zone at the tips of the rhabdom's rays. These mitochondria, which are 2 to 10 µ in diameter, are crowded with irregular closed outlines about 100 mµ in diameter. Possible functions of components of the ommatidium are discussed.     

Fine structural description of the compound eye of the Madagascar 'hissing cockroach'Gromphadorhina portentosa (Dictyoptera: Blaberidae)

The compound eyes of the wingless adults of the Madagascar ‘hissing cockroach’Gromphadorhina portentosa Sachum, 1853 were examined by light and electron microscopy. Each eye contains 2 400‐2 500 mostly hexagonal facets. However, irregularities affecting both shape and size of the ommatidia are relatively common, especially towards the margins of the eye. An individual ommatidium of this eucone type of apposition eye contains eight retinula cells, which give rise to a centrally‐fused, tiered rhabdom. The distal end of the latter is funnel‐shaped and accommodates the proximal end of the cone in its midst. Further below, the rhabdom (then formed by the rhabdomeres of four retinula cells) assumes a squarish profile with microvilli aligned in two directions at right‐angle to each other. Cross sections through the proximal regions of the rhabdom display triangular rhabdom outlines and microvilli (belonging to 3‐4 retinula cells different from those involved in the squarish more distal rhabdom) that run in three directions inclined to one another by 120°. Overall the organization of the eye conforms to the orthopteroid pattern and particularly closely resembles that of the American cockroach Periplaneta americana. However, since G. portentosa possesses fewer ommatidia, this could be a consequence of its inability to fly. On the other hand, the large size of the facets and the voluminous rhabdoms suggest considerable absolute sensitivity and an ability to detect the plane of linearly polarized light. Based on the pattern of microvillus orientations in combination with the crepuscular lifestyle G. portentosa leads and the habitat it occurs in, the prediction is made that this insect uses its green receptors for e‐vector discrimination in the environment of down‐welling light that reaches the forest floor.     

The Anostracan Rhabdom and the Basement Membrane. An Ultrastructural Study of the Artemia Compound Eye (Crustacea)

Abstract The ommatidia of the compound eyes of Artemia salina L. are normally composed of four crystalline cone cells containing glycogen. The cells are enveloped by two so-called “cellules épidermiques juxta-cristallines”. There are also six pigmented retinula cells, all contributing to the rhabdom. A peculiar feature of the Artemia crystalline cone cells is that their elongated parts, the so-called cone cell roots, widen and flatten proximally, forming interdigitating “endfeet”. The basement membrane thus consists of a cellular portion combined with the basal lamina. The main mass of the rhabdom of the Artemia eye is built up by five retinula cells, two contributing a smaller part. The microvilli are oriented in four directions, two being orthogonal. The sixth cell contributes on two small portions to the rhabdom in the distalmost and a more proximal position. The rest of it runs axon-like outside the omnatidium. Where the sixth cell wedges in, the direction of the microvilli is changed and has no orthogonal pattern. Two rhabdom types of compound eyes are distinguished: the decapod or banded or layered rhabdom: and the anostracan rhabdom with continuous rhabdomeres.     

A re-investigation and re-interpretation of the cumacean photoreceptor

Evidence for and against the view that the singular eye in Diastylis rathkei (Cumacea) represents a regressed compound eye is summarised and supplemented with new ultrastructural observations. Neither in the adult nor in the larval manca stage are even traces of a facetation found. The eye consists of two closely apposed eye halves and the four lenticular complexes in each appear to increase in size with age along an isometric growth curve. Each lenticular complex consists of a lens rich in glycogen-like particles and a rhabdom made up of regularly aligned microvilli 0.075 μ m in diameter. No more than three retinula cells contribute to each lenticular compkx. Retinula cells contain presumed carotenoid bodies 0.4–0.5 μm in diameter and clusters of electron-opaque glycogen matcrial near the proximally-located nuclei. The bulk of the eye is occupied by cells crowded with reflecting vesicles of approximately 0.8 μ m diameter. Though it appears too early to offer a final and decisive conclusion as to the nature and origin of the eye of D. rathkei , comparisons with compound eyes of other peracaridan crustaceans and anatomical parallels to isopod and other malacostracan ocelli make clear that the long-held view that the cumacean photoreceuptor represents a regressed compound eye is not necessarily the correct one.     

Fine structural organization of the lateral ocelli in two species of Scolopendra (Chilopoda: Pleurostigmophora): an evolutionary evaluation

The lateral ocelli of Scolopendra cingulata and Scolopendra oraniensis were examined by electron microscopy. A pigmented ocellar field with four eyes arranged in a rhomboid configuration is present frontolaterally on both sides of the head. Each lateral ocellus is cup-shaped and consists of a deeply set biconvex corneal lens, which is formed by 230–2,240 cornea-secreting epithelial cells. A crystalline cone is not developed. Two kinds of photoreceptive cells are present in the retinula. 561–1,026 cylindrical retinula cells with circumapically developed microvilli form a large distal rhabdom. Arranged in 13–18 horizontal rings, the distal retinula cells display a multilayered appearance. Each cell layer forms an axial ring of maximally 75 rhabdomeres. In addition, 71–127 club-shaped proximal retinula cells make up uni- or bidirectional rhabdomeres, whose microvilli interdigitate. 150–250 sheath cells are located at the periphery of the eye. Radial sheath cell processes encompass the soma of all retinula cells. Outside the eye cup there are several thin layers of external pigment cells, which not only ensheath the ocelli but also underlie the entire ocellar field, causing its darkly pigmented. The cornea-secreting epithelial cells, sheath cells and external pigment cells form a part of the basal matrix extending around the entire eye cup. Scolopendromorph lateral ocelli differ remarkably with respect to the eyes of other chilopods. The dual type retinula in scolopendromorph eyes supports the hypothesis of its homology with scutigeromorph ommatidia. Other features (e.g. cup-shaped profile of the eye, horizontally multilayered distal retinula cells, interdigitating proximal rhabdomeres, lack of a crystalline cone, presence of external pigment and sheath cells enveloping the entire retinula) do not have any equivalents in scutigeromorph ommatidia and would, therefore, not directly support homology. In fact, most of them (except the external pigment cells) might be interpreted as autapomorphies defining the Pleurostigmophora. Certain structures (e.g. sheath cells, interdigitating proximal rhabdomeres, discontinuous layer of cornea-secreting epithelial cells) are similar to those found in some lithobiid ocelli (e.g. Lithobius). The external pigment cells in Scolopendra species, however, must presently be regarded as an autapomorphy of the Scolopendromorpha.     

Regional differences in a nematoceran retina (Insecta,Diptera)

Summary The compound eye of Psychoda cinerea comprises two types of ommatidia, arranged so as to divide the retina into distinct dorsal and ventral regions. The P-type ommatidium, in the ventral part of the eye, differs fundamentally from the other dipteran ommatidia so far described, and is regarded as a primitive ommatidium. The acone dioptric apparatus is the same in both types, with a spherical lens and four Semper cells, the processes of which expand below the rhabdom to form a ring of pigment sacs. Only the distal region of the rhabdom is surrounded by a continuous ring of screening pigment, formed by 2 primary and 12–16 secondary pigment cells. The highly pigmented retinula cells penetrate the basement membrane proximally at about the level of their nuclei; in this region they are separated from the hemolymph by glial elements. The rhabdomeres R1–6 are fused to form a tube. The two types of ommatidia are defined by the arrangement of the retinula cells R7/8: in the T type the central rhabdomeres are one below the other, in the usual tandem position, whereas in the P type only R8 is central, with R7 in the peripheral ring. In the proximal region of the retina, retinula cells with parallel microvilli in neighboring ommatidia are joined in rows by lateral processes from the R8 cells. All the rhabdomeres are short and not twisted, which suggests that the retinula cells are highly sensitive to direction of polarization. The eye can adapt by a number of retinomotor processes. These findings, together with observations of behavior, imply that the psychodids have well-developed visual abilities.     

Retina and dioptric apparatus of the dung beetle Euoniticellus africanus

Retinal fine structure and optics of the eye of the dung beetle Euoniticellus africanus have been studied and compared with those of three other scarabaeid beetles: Repsimus manicatus, Anoplognathus pallidicollis and Sericesthis geminata. The eye of Euoniticellus, in common with that of the other three species, possesses a dioptric system in which light first passes through a thick optically homogeneous cornea, and then enters a non-homogeneous crystalline cone. The lens cylinder properties of the latter cause the light rays to become partially focused across the clear-zone upon the rhabdom layer. Rays traced through a large scale drawing of the eye, with refractive indices measured for each component, predict an acceptance angle of approximately 26°. Since no significant aperture changes, lengthening of crystalline thread, cell or pigment migrations appear to be associated with dark/light adaptation, the eye may be assumed to be permanently poorly focused. In optomotor experiments the beetles did not show their characteristic antennal following response to black and white stripes when the latter had repeat periods of <30°. Structurally the eye of Euoniticellus differs markedly from that of other scarabaeids. It is totally divided into dorsal and ventral eye which are of a different size (the dorsal eye is smaller), but whose structural organization is basically the same. Principal pigment cells (they do not fully surround the cone) as well as accessory pigment cells (they accompany the retinula cells in an extraordinarily regular fashion as far as to the basement membrane) exhibit some unusual features. On the proximal side of the clear-zone, at a level where all retinula cell membranes form complex meanders and convolutions, cell 1 is the first to possess a rhabdomere. In it, all microvilli run parallel. This rhabdomere becomes part of the rectangular proximal rhabdom over the upper 20% of its length. Below this level the rhabdom consists of 6 rhabdomeres, but throughout its length microvilli are oriented in 2 orthogonal directions. It is thought that polarization sensitivity in dung beetles generally is related to the rhabdom organization described for Euoniticellus. An eighth (basal) cell is present in each ommatidium, but it lacks a rhabdomere. A tracheal tapetum is not developed. Finally, the point is made not to regard all different eye structures in insects as perfect adaptations to a particular environment or way of living, for specializations of photoreceptors may either follow, parallel or precede any ecological adaptation.     

The fine structure of the eyes of some bristly millipedes (Penicillata, Diplopoda): additional support for the homology of mandibulate ommatidia

The eyes of adult Phryssonotus platycephalus (Synxenidae) and Polyxenus lagurus (Polyxenidae) were investigated by light and electron microscopy. At each side of the head, various numbers of eye cups are situated on projections, the eye hills. The eye cups of P. platycephalus and P. lagurus are similarly structured and considered homologous sense organs. Each corneal lens is biconvex and formed by four to six pigmented corneagenous cells with their nuclei displaced towards the mid-periphery of the eye cup. The corneal surface displays a conspicuous nanostructure of fingerprint-like ridges in P. platycephalus. However, the corneal surface appears smooth in P. lagurus. In P. platycephalus. A rudimentary crystalline cone is observed in each eye cup, always produced by a constant number of three eucone cells. The crystalline cone is wedged between the corneal lens and the distal rhabdom and consists of three distinct compartments. Each cone compartment is connected to the voluminous proximal nuclear region by one elongated cytoplasmic process, which runs through the infraretinular space. A dual type retinula is always arranged in two distinct horizontal cell layers. The distal retinula contains an unfixed number of four to five cells in P. lagurus, whereas it contains five to eight cells in P. platycephalus. The distal retinula cells form a large and fused axial rhabdom. A constant number of three proximal retinula cells give rise to a small axial rhabdom, which looks more or less triangular in cross sections. The basal matrix is rather thin, inconspicuous and lines the bases of the eye cups. The ultrastructure of the eye cups of P. platycephalus resembles that observed in the ommatidia of the centipede Scutigera coleoptrata. The present study lends additional support to the homology of mandibulate ommatidia, because of the common possession of crystalline cone cells and a bilayered dual type retinula in the eye cups of P. platycephalus. Ommatidia or unicorneal eyes that include eucone cells with nuclei displaced outside the cone compartments, as found in Scutigeromorpha and Penicillata, might also be interpreted as an additional autapomorphy of the Myriapoda. The suggested homology of scutigeromorph and penicillate eyes implies that penicillate eye cups have to be considered modified, probably miniaturized ommatidia.     

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.     

An ultrastructural study of the eye of Gomphiocephalus hodgsoni, a collembolan from Antarctica

The collembolan Gomphiocephalus hodgsoni is one of a few hexapods occurring in Antarctica. Male and female individuals do not differ with regard to their eyes. Both possess eight single-lens eyes. In the adults, each lens has a diameter of 10–12 m and covers an almost spherical crystalline cone made up of four unequal moieties. When we accepted homogenous refractive indices, known from pterygote insect eyes, and then calculated the focal length of the dioptric apparatus, we found that light could be focused on the retina. The retinal cells of each eye are grouped in two tiers and surround the centrally fused rhabdom. Rhabdomeric microvilli, approximately 1.3 m long, possess a diameter of 80 nm and are typically aligned in one direction. More proximally, however, two rhabdomeres with microvilli perpendicular to the larger, more distal, rhabdom appear. Signs of light-induced damage, despite the bright summer radiation in Antarctica, were not seen. Ricegrain-like screening pigment granules, measuring 0.8×0.45 m in dimensions, densely surround each rhabdom and shield the cytoplasm of the surrounding cells. The orthogonal arrangement of the microvilli suggests that G. hodgsoni could be sensitive to light-polarization. This ability might be of importance in detecting tiny amounts of meltwater in close proximity to the insect.The order of the authors names reflects the sequence in which the individuals joined in this project.     

Fine structure of light- and dark-adapted eyes of desert ants,Cataglyphis bicolor (Formicidae,Hymenoptera)

Among ants, Cataglyphis bicolor shows the best performance in optical orientation. Its eye is of the apposition type with a fused rhabdom. Morphological studies on the general struture of the eye as well as the effect of light have been carried out with transmission and scanning electron microscopy. An ommatidium is composed of a dioptric apparatus, consisting of a cornea, corneal process and a crystalline cone, the sensory retinula, which is made up of eight retinula cells in the distal half and of an additional ninth one in the proximal half. The ommatidia are separated from each other by two primary pigment cells, which surround the crystalline cone and an average of 12 secondary pigment cells, which reach from cornea to the basement membrane. The eye of Cataglyphis bicolor possesses a light intensity dependent adaptation mechanism, which causes a radial and distal movement of the pigment granules within the retinula cells and a dilatation of cisternae of the ER along the rhabdom. Until now, no overall order in arrangement of retinula cells or direction of microvilli has been found from ommatidium to ommatidium. Such an order, however, must exist, either on the retina or the lamina level, since we have proven the ant's capacity for polarized light analysis.     

Structure of the dorsal eye region of the moth,Adoxophyes reticulana Hb. (Lepidoptera : Tortricidae)

Ommatidia of the eucon compound eye of Adoxophyes reticulana (Lepidoptera : Tortricidae) were investigated elect ronmicroscopically. The dorsofrontal part and the dorsal rim region were examined in serial sections. Seven radially arranged retinula cells RC₁₋₇ form the rhabdom from distal to proximal region (Fig. 1). The 8th retinula cell RC₈ joins the first 7 at their bases; this cell enlarges proximally (Fig. 1C, D). In the dorsofrontal region, 2 types of rhabdoms are distinguished; Type II (Figs. 1B₂;3b) outnumbers Type I (Figs. 1B₁;3a by a ratio of 4 : l. In the dorsal rim area, the first 2 rows are occupied exclusively by Type 11-rhabdoms; beyond this, the rhabdom of the dorsal rim area is characterized by the fact that its middle and proximal parts are considerably larger in diameter than in the dorsofrontal part; in this region, the microvilli of the horizontally oriented rhabdomeres are also parallel to the ;,-axis of the eye (Figs. 1B₃;3d). Thus, this small eye region meets the structural requirements for the detection of polarized light. The eye is interpreted as an intermediate between apposition and superposition eyes, because the rhabdom begins at the tip of the crystalline tract and the retinula cells are pigmented like those of an apposition eye. On the other hand, the structure of the dioptric apparatus and the tracheal system corresponds to those of superposition eyes. Parallels with the Ephestia eye in basic structural features are discussed in regard to the possible function of this eye and to the systematic position of A. reticulana.     

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.     

棉铃虫蛾复眼的微细结构及其区域性差异

用电子显微镜观察棉铃虫蛾复眼的微细结构及其区域性差异。此复眼具有小网膜细胞柱的透明带。每个小眼包括一个外凸内平的角膜,一个晶锥,四个形成晶锥、晶束的晶锥细胞和两个围绕着晶锥的主虹膜细胞,六至八个小网膜细胞和一个基细胞。晶锥末端有一短小固定的晶束。小网膜细胞柱远侧中央有似微绒毛结构的视杆束。每个小眼被六个附色素细胞围绕。 微细结构的区域性差异:1.背方小眼视杆中段横切面近似矩形,主要由六个微绒毛平行排列的三角形视小杯组成,整个视杆包含两个互相垂直的微绒毛轴;腹方、前方、后方和侧方区域的小眼视杆中段横切面为风扇形,“V”字形视小杆内微绒毛排列不平行;2.前方区域小眼视杆中段的横切面要比后方大;3.前方、腹方区域内,有的相邻小眼的小网膜细胞柱互相连结,背方、后方区域未观察到这一现象。     

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.