The organization of the lamina ganglionaris of the hemipteran insects,Notonecta glauca,Corixa punctata and Gerris lacustris

Summary Neuronal elements, i.e. first and second order neurons, of the first optic ganglion of three waterbugs, N. glauca, C. punctata and G. lacustris, are analyzed on the basis of light and electron microscopy.Eight retinula cell axons, leaving each ommatidium, disperse to different cartridges as they enter the laminar outer plexiform layer. Such a pattern of divergence is one of the conditions for neuronal superposition; it is observed for all three species of waterbugs. The manner in which the receptors of a single bundle of ommatidia split of within the lamina, whereby information from receptors up to three or five horizontal rows away can converge upon the same cartridge, differs among the species. Six of the eight axons of retinula cells R1-6, the short visual fibers end at different levels within the bilayered lamina, whereas the central pair of retinula cells R7/8, the long visual fibers, run directly through the lamina to a corresponding unit of the medulla. Four types of monopolar cells L1–L4 are classified; their branching patterns seem to be correlated to the splitting and termination of retinula cell axons. The topographical relationship and synaptic organization between retinula cell terminals and monopolar cells in the two laminar layers are identified by examination of serial ultrathin sections of single Golgi-stained neurons.An attempt is made to correlate some anatomical findings, especially the neuronal superposition, to results from physiological investigations on the hemipteran retina.     

The first optic ganglion of the bee

Each visual unit (ommatidium) of the compound eye of the honey bee contains nine retinula cells, six of which end as axons in the first synaptic ganglion, the lamina, and three in the second optic ganglion, the medulla. A technique allowing light- and electron microscopy to be performed on the same silver-impregnated sections has made it possible to follow all types of retinula axons of one ommatidium to their terminals in order to study the shape of the terminal branches with their position in the cartridge. 1. The axons of retinula cells 1-6 (numbered according to Menzel and Snyder, 1974) end as three different types of short visual fibres (svf) in the lamina; the axons of retinula cells 7-9 run through the lamina to terminate in the medulla and are known as long visual fibres (lvf). Retinula cells of each type are identified by the location of their cell bodies and by the direction of their microvilli. The retinula cells 1 and 4 (group I according to Gribakin, 1967) end as svf type 1 with three tassel-like branches in stratum B of the first synaptic region. The pair of cells 3, 6 and the pair 2, 5 (group II) end in the first synaptic region in stratum A. Cells 3 and 6 have forked endings, svf type 2, whereas cells 2 and 5 have tapered endings, svf type 3. The remaining retinula cells 7, 8 and 9 have long fibres. Nos. 7 and 8 (group III) have tapered endings and are termed lvf types 1 and 2, respectively. The 9th cell is the lvf type 3 with a highly branched ending. 2. The nine axons in the bundle from one ommatidium have relative positions which do not change from the proximal retina to the monopolar cell body layer. 3. By following silver-stained retinula cells and their corresponding axons, it is possible to describe mirror-image arrangements of fibres in the axon bundles in different parts of the eye. This correlation of numbered retinula cells with specific axon types, together with the highly organized pattern in an axon bundle, allows the correlation between histological and physiological findings on polarization and colour perception.     

Photoreceptor projection and termination pattern in the lamina of gonodactyloid stomatopods (mantis shrimp)

The apposition compound eyes of gonodactyloid stomatopods are divided into a ventral and a dorsal hemisphere by six equatorial rows of enlarged ommatidia, the mid-band (MB). Whereas the hemispheres are specialized for spatial vision, the MB consists of four dorsal rows of ommatidia specialized for colour vision and two ventral rows specialized for polarization vision. The eight retinula cell axons (RCAs) from each ommatidium project retinotopically onto one corresponding lamina cartridge, so that the three retinal data streams (spatial, colour and polarization) remain anatomically separated. This study investigates whether the retinal specializations are reflected in differences in the RCA arrangement within the corresponding lamina cartridges. We have found that, in all three eye regions, the seven short visual fibres (svfs) formed by retinula cells 1–7 (R1–R7) terminate at two distinct lamina levels, geometrically separating the terminals of photoreceptors sensitive to either orthogonal e-vector directions or different wavelengths of light. This arrangement is required for the establishment of spectral and polarization opponency mechanisms. The long visual fibres (lvfs) of the eighth retinula cells (R8) pass through the lamina and project retinotopically to the distal medulla externa. Differences between the three eye regions exist in the packing of svf terminals and in the branching patterns of the lvfs within the lamina. We hypothesize that the R8 cells of MB rows 1–4 are incorporated into the colour vision system formed by R1–R7, whereas the R8 cells of MB rows 5 and 6 form a separate neural channel from R1 to R7 for polarization processing.This research was supported by the Swiss National Science Foundation (PBSKB-104268/1), the Australian Research Council (LP0214956) and the American Air Force (AOARD/AFOSR) (F62562-03-P-0227).     

The superposition eye of Cloeon dipterum: The organization of the lamina ganglionaris

Summary The lamina ganglionaris of the superposition eye of Cloeon dipterum is composed of separate optic cartridges arranged in a hexagonal pattern. Each optic cartridge consists of one central, radially branched monopolar cell (Li) surrounded by a crown of seven retinula cell terminals and two more unilaterally branched monopolar cells (La1/La2) situated close together outside the cartridge. Projections to neighbouring cartridges have not been observed.In most cases, synaptic contacts could be seen between a presynaptic retinula cell and more than two other postsynaptic profiles, which belong to monopolar cells or sometimes to glial cells.Seven retinula cell fibers of one ommatidium pass in a bundle through the basement membrane, run into their respective cartridges without changing orientation and terminate at approximately equal levels in the lamina. Long visual fibers with endings in the medulla are not visible in the superposition eye lamina, but are present in the lateral apposition eye. The relationship between the behaviour of the animal, optic mechanisms of the superposition eye and the structure of the lamina is discussed.     

The retina-lamina projection in the visual system of the bee,Apis mellifera

Single Golgi impregnated visual cells and their axons were treated from the retina to the first synaptic layer (lamina) in serial electron microscopic sections. This analysis of the retina-lamina projection was undertaken in the upper dorso-median eye region which is known to be involved in the perception of polarized light. For identification of individual visual cells and their fibres a numbering system was used which relates the number of each of the nine visual cells within one retinula to the transverse axis of the rhabdom (TRA) (Fig. 1). Because of the twist of the retinula along its course to the basement membrane (Fig. 6), individual visual cells change their position relative to any eye-constant co-ordinate system. Each axon bundle originating from one 9-celled retinula performs a 180 degrees-rotation before entering the lamina (Fig. 2). The direction of rotation (clockwise or counter-clockwise), which may differ even between adjacent bundles, is related to the two mirror-image types of rhabdoms in the corresponding retinulae and is opposite to the direction of rhabdom twist. Thus, even in small groups of the in total 5500 ommatidia in the eye of the bee, two types of retinulae exist which can be characterized by the geometry of the rhabdoms as well as by the direction of rotation of the retinulae and the axon bundles (Fig. 1). Visual cell numbers 1, 2, and 9, the microvilli of which are oriented in the direction of TRA, form three long visual fibres terminating in the second synaptic layer (medulla). In cross sections of laminar pseudocartridges they appear as the smallest fibre profiles arranged in a symmetrical line of the pseudocartridge bundle (=the transverse axis of the pseudocartridge; TPA) (Fig. 4). The remaining six fibres (cell numbers 3-8) only project to the lamina (short visual fibres; svf's). Two of them (cell numbers 5 and 6), which are the largest cells in the proximal retinula and have their microvilli perpendicularly arranged to TRA (Fig. 1), give rise to the two thickest axons of the underlaying pseudocartridge. In cross sections, t he connecting line of these two axons is orthogonally oriented to TPA (Fig. 5). A model was developed, in which all long visual fibres originate from ultraviolet receptors and in which the polarization sensitivity of the basal ninth cell is enhanced by the twist of the rhabdom. Finally, this model is discussed in light of behavioral experiments revealing the ultraviolet receptors as the only cells involved in the detection of polarized light.     

The Fine Structure of Photoreceptor Terminals in the Compound Eye of Pandalus borealis (Crustacea)

Seven of the photoreceptor axons of each ommatidium in the compound eye of the prawn Pandalus borealis end in two layers in the optic lamina. They have expanded terminals in the optic cartridges; four distally and three proximally in each cartridge. All seven receptor terminals are presynaptic to one lamina monopolar neuron (M₂) of the cartridge. This monopolar neuron is situated centrally in the cartridge and has a thick axis fibre with radially arranged branches, and its axon has a terminal in medulla externa. At the synapses, an arrowlike presynaptic bar is found facing three postsynaptic profiles. The receptor terminals have several characteristics. Their cytoplasm is filled with empty and coated vesicles, and contains numeorus large mitochondria and clusters of tubular elements. There is a longitudinally arranged fascicle of filaments partly surrounded by electron-dense amorphous material in the terminals. Centrally towards M₂, numerous neural spines invaginate into the terminal. Along the entire terminal periphery, there are invaginations from the glial cells. The terminals also form small knoblike protrusions extending into the surrounding glial cells.     

The optic neuropiles and chiasmata of Crustacea

Summary On the basis of ontogeny and adult morphology, an interpretation of the arrangement of optic neuropiles and fibre connexions of the Crustacean compound eye is presented. In the embryo of phyllopods and decapods, the ommatidia, the lamina ganglionaris, and the medulla externa are developed synchronously from a common medial proliferation zone. As this zone persists in all investigated adult Crustacea that possess compound eyes, such a derivation of the mentioned structures is taken to be universal within the group. The direction of growth of the lamina ganglionaris is parallel with the row of ommatidia, the growth direction of the medulla externa is perpendicular to it and parallel with the long axis of the eyestalk. This arrangement is more or less retained in most adult non-Malacostracan Crustacea, and the axons of fully developed neurons pierce the optic neuropiles and leave and enter on the neuropile side. As a result, there is no chiasma in the non-Malacostracan groups.The Malacostraca have an extra neuropile, the medulla interna, derived from the medulla terminalis. Chiasmata occur between the lamina ganglionaris and the medulla externa, and between the medulla externa and the medulla interna. This difference from the non-Malacostracans depends on the course of the fibres. Those coming from the lamina ganglionaris leave the lamina on the neuropile side and enter medulla externa between the cell bodies in the perikaryon layer of the medulla externa neurons and the neuropile of the medulla. The fibres from the medulla externa to the lamina come from T-shaped neurons and emanate from the perikaryon layer side, entering the lamina on its neuropile side. The fibre relations between the medulla externa and the medulla interna are similar. Thus in both cases, chiasmata are present from the beginning, but they become obvious when the medulla externa rotates through part of a circle.The directed growth of the optic neuropiles and the course of the fibre connexions are consequently crucial to the understanding of the topographic relations between the neuropiles. A pattern with short neurons connecting neighbouring optic neuropiles and long neurons connecting the medulla externa with the central nervous system is common to all crustaceans.In memoriam Bertil Hanström.This work has been supported by a grant from the Swedish Natural Science Research Council 2760-3, 99-35.     

Anatomical identification of spectral receptor types in the retina and lamina of the Australian orchard butterfly,Papilio aegeus aegeus D.

Summary The nine receptor cells examined in each ommatidium of the butterfly Papilio aegeus aegeus can be named according to their positional orientation across the fused rhabdom. Six of them end as short visual fibres (svf) in the second stratum of the lamina, whereas the remaining three retinula cells (lvf) pass together with the lamina fibres (L-fibres) the first optic ganglion and the outer chiasma to end in the three most distal layers of the second optic ganglion, the medulla. The organization of the retinula-cell axons within the pseudocartridge and the cartridge remains almost uniform throughout the first optic ganglion. Five L-fibres, which have their origin in the fenestrated layer (FL), join each laminar cartridge before entering the neuropil of the first optic region. Four of these L-fibres (L-1, L-2, L-3 and L-4) could be definitely located and characterized using Golgi-stained light- and electron-microscopic techniques. Whereas L-1 and L-3 show a lateral branching pattern reaching only fibres of the same cartridge, L-2 and L-4 have long collaterals interconnecting several neighbouring cartridges in a characteristic pattern. Serial sections of silver-impregnated retinula-cell axons as well as L-fibres were investigated for their synaptic connectivity patterns between and within these fibres. These cellular interactions and possible information processing are discussed.     

The first optic ganglion of the bee

Summary The arrangement of first and second order neurons in an optic cartridge and the topographical relationships of the second order neurons within a cartridge and to groups of surrounding cartridges have been analyzed in the visual system of the bee, Apis mellifera, from light and electron microscope studies on Golgi preparations. At the level of the monopolar cell body layer, the nine retinula cell fibres of each ommatidium, the six short visual fibres arranged in a circle surrounding the three long visual fibres, become cartridges as a consequence of the appearance of the second order neurons (L-fibres) which join the R-fibre bundles. Two of the four different L-fibre types, L-1 and L-2, remain together in the centre of the cartridge throughout the lamina. The axons of the L-3 and L-4 fibres, however, have their position integrated into the circle formed by the endings of the short visual fibres. On the basis of further examination of light and especially electron microscopical Golgi material, the different L-fibres can be classified into four types which appear in each cartridge. The clear stratification in the first synaptic region (A, B and C) seems to be the best criterion for a morphological classification since such a classification necessarily also includes a functional basis. According to a naming system based on the position of the lateral processes, L-fibres with side branches in strata A, B and C are called L-1 fibres. Fibres with lateral processes in strata A and B are L-2 fibres; monopolar cell fibres with branches only in the second stratum B are L-fibres of type 3; and all monopolar cells with branches only in stratum C are called L-4 fibres. In addition to the branching pattern covering only the parent cartridge, two of the four fibre types (L-2 and L-4) have long collaterals reaching neighbouring cartridges: L-2 in stratum A and L-4 in stratum C. These collaterals presumably form a substrate for lateral interactions.     

Neuronal connectivity patterns in the compound eyes of Artemia salina and Daphnia magna (Crustacea: Branchiopoda)

The neuronal types and patterns in the visual system of the species Artemia salina and Daphina magna have been studied with the Golgi method and electron microscopy. The lamina contains five classes of neurons: photoreceptor axons, monopolar, centrifugal, tangential and amacrine neurons. The terminals of the receptor axons are distributed in two (A. salina) or three (D. magna) layers. The dilated terminals have an extensive and wide array of fine branches. One axon from each ommatidium bypasses the lamina and terminates in the medula in A. salina. A. salina has four types of monopolar neurons, two of which are stratified, whereas in D. magna only two types are found, one of which is bistratified. Tangential T-neurons connect the lamina with the protocerebrum. D. magna has in addition one tangential T-neuron connecting both the lamina and the medulla with the protocerebrum. In both species monopolar-type centrifugal neurons connect the medulla and the lamina, whereas that of A. salina has a wide laminar distribution. Both species also have amacrine cells in the lamina. The medulla contains, besides those shared with the lamina, transmedullary neurons (two types in A. salina), amacrine cells and neurons originating in the protocerebrum. "Cartridge"-type synaptic compartments are lacking in the investigated species, although a periodic arrangement is discernible in the distal portion of the lamina of A. salina. The receptors from three types of specialized contacts in Artemia, one of which involves a dyad. D. magna has only one-to-one synapses. Neurosecretory fibres are absent in A. salina.     

The retina-lamina projection in the crab Leptograpsus variegatus

Summary In the crab, Leptograpsus variegatus, the projection of retinula cell axons to the lamina was investigated by tracing them through a series of semi-thin sections. Forty-four such axons were traced from a single group of ommatidia as far as the distal layers of the lamina. The eight receptor axons of one ommatidium project to a single lamina cartridge. Therefore, because the crab has a fused rhabdom, angular information is conserved in vision, and the outside world is projected literally onto the lamina, just as it is in the standard non-dipteran pattern of insects. The belief of previous workers that other decapod eyes show neural superposition was an inference based primarily on the patterns of penetration of the basement membrane by receptor axons, and on degeneration experiments. This evidence is reviewed, shown to be inadequate and discussed in the light of the projection now demonstrated for Leptograpsus.     

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.     

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 Organization of the lamina ganglionaris of the prawn,Pandalus borealis (Kröyer)

The Lamina ganglionaris (first optic neuropile) of the decapod crustacean Pandalus borealis has its optic cartridges (synaptic compartments) arranged in horizontal rows. Each optic cartridge contains seven receptor axon terminals and the branching axis fibres of five monopolar second order neurons. Four types of monopolar neurons are classified. Their cell bodies are arranged in two layers. The inner layer contains the cell bodies of exclusively one of these types, and each cartridge is invaded by two neurons of this neuron type (type M 1:a and M 1:b). The outer layer contains the cell bodies of the remaining three types (M 2, M3 and M4). One gives rise to a large radially branched axis fibre in the centre of the cartridge. The other two have wide branches which may make inter-cartridge contacts, one proximally and the other distally in the plexiform layer, which is clearly bistratified. The receptor axons terminate in two levels corresponding to these strata. Two sets of tangenital fibres form networks in the proximal and the mid-portion of the lamina. Both networks have fibres with primary branches in the vertical plane and secondary branches in the horizontal plane. The fibres of the networks are derived from axons that pass from the second optic neuropile, the medulla externa.     

Identificaton of UV,green and red receptors,and their projection to lamina in the cabbage butterfly,Pieris rapae

Summary The photoreceptors in the compound eye of a cabbage butterfly, Pieris rapae, were examined by conventional and intracellular-labeling electron microscopy by the use of the cobalt(III)-lysine complex as an ionized marker. Five types of spectral sensitivity were recorded intracellularly in electrophysiological experiments. They peaked at about 340, 380, 480, 560 and 620 nm, respectively. One of the distal retinula cells (R2) was a UV receptor, whereas the R4 distal retinula cell was a green receptor. The basal retinula cell, R9, was found to be a red receptor; it was localized near the basement membrane, having a bilobed cell body with an individual nucleus in each lobe. A small number of rhabdomere microvilli were present in a narrow cytoplasmic bridge connecting the two lobes. The axons of six retinula cells (R3–R8) in each ommatidium terminated at the cartridge in the lamina (short visual fiber), whereas those of the other three retinula cells, R1, R2 and R9, extended to the medulla (long visual fiber). The information from the UV and red receptors is therefore probably delivered directly to the medulla neurons, independent of that from the other spectral receptor types.     

The organisation of the lamina ganglionaris of the crabs Scylla serrata and Leptograpsus variegatus

Summary The gross structure and neuronal elements of the first optic ganglion of two crabs, Scylla serrata and Leptograpsus variegatus, are described on the basis of Golgi (selective silver) and reduced silver preparations. Of the eight retinula cells of each ommatidium, seven end within the lamina, while the eighth cell sends a long fibre to the external medulla. Five types of monopolar neurons are described, three types of large tangential fibres, and one fibre which may be centrifugal. The marked stratification of the lamina is produced by several features. The main synaptic region, the plexiform layer, is divided by a band of tangential fibres; the short retinula fibres end at two levels in the plexiform layer; and two types of monopolar cells have arborisations confined to the distal or proximal parts of the plexiform layer. The information presently available concerning the retina-lamina projection in Crustacea is examined. Some of the implications of retina and lamina structure are discussed in conjunction with what is known about their electrophysiology.     

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.     

The first optic ganglion of the bee

Summary The synaptic relationships between and within receptor-cell axons (RCAs), first-order interneurones (L-fibres) and accessory fibres (acc) in the first optic ganglion (the lamina) of the worker bee were studied in serial sections with Golgi-EM and routine transmission electron microscopy. The ommatidium contains nine retinular (photoreceptor) cells all of which project as RCAs to a single optical cartridge in the lamina. Six of the RCAs end as short visual fibres (svf) in the lamina, while the remaining three, the so-called long visual fibres (lvf), pass the lamina and end in the second optic ganglion, the medulla. In addition to the RCAs and an unknown number of accessory fibres, the cartridge also contains four L-fibres (L 1–4). The spatial arrangement of the RCAs and L-fibres within a cartridge is constant throughout the depth of the lamina. Serial sections reveal a great number of chemical synapses interconnecting RCAs, L-and acc fibres. Double T-shaped presynaptic dense projections are surrounded and in close association with either spherical or flattened synaptic vesicles. The finding of gap junctions between and within identified RCAs and L-fibres suggest that these axons may be electronically coupled. A model for information processing in the lamina of the bee is suggested from observations of synaptic connectivity between and within fibres of one cartridge.     

The fine structure of the retina of the honey bee drone

Summary The eye of the honey bee drone is composed of approximately 8,000 photoreceptive units or ommatidia, each topped by a crystalline cone and a corneal facet. An ommatidium contains 9 visual or retinula cells whose processes or axons pierce a basement membrane and enter the optic lobe underlying the sensory retina. The visual cells of the ommatidium are of unequal size: six are large and three, small. In the center of the ommatidium, the visual cells bear a brush of microvilli called rhabdomere. The rhabdome is a closed-type one and formed mainly by the rhabdomeres of the six large retinula cells. The rhabdomeric microvilli probably contain the photopigment (rhodopsin), whose modification by light lead to the receptor potential in the retinula cells. The cytoplasm of the retinula cells contains various organelles including pigment granules (ommochromes), and peculiar structures called the subrhabdomeric cisternae. The cisternae, probably composed of agranular endoplasmic reticulum undergo swelling during dark adaptation and appear in frequent connection with Golgi cisternae. Three types of pigment cells are associated with each ommatidium. The crystalline cone is entirely surrounded by two corneal pigment cells. The ommatidium, including its dioptric apparatus and corneal pigment cells, is surrounded by a sleeve of about 30 elongated cells called the outer pigment cells. These extend from the base of the corneal facet to the basement membrane. Near the basement membrane the center of the ommatidium is occupied by a basal pigment cell. Open extracellular channels are present between pigment cells as well as between retinula cells. Tight junctions within the ommatidium are restricted to the contact points between the rhabdomeric microvilli. These results are discussed in view of their functional implications in the drone vision, as well as in view of the data of comparative morphology.This work was supported by a grant from the Fonds National Suisse de la Recherche Scientifique.     

The neurons of the first synaptic region of the optic neuropil of the firefly,Phausis splendidula L. (Coleoptera)

On the basis of Golgi preparations the neuronal elements of the lamina ganglionaris (first synaptic region of the visual system) of the firefly. Phausis splendidula L., are described. Of the set of 8 retinula fibres that originate from each ommatidium of the compound eye, at least 6 terminate in the esternal plexiform layer. At least one, probably two, retinula fibres per ommatidium penetrate this layer to end in the medulla, via the first optic chiasma. Five types (m1-m5) of monopolar cells can be distinguished. Only two of these, m1 and m3 have dendritic fields limited to one column of the lamina mosaic; all other monopolar cells have larger fields of up to 45 mum diameter. m2 and m4 have various field spreads in different strata of the external plexiform layer. m5 has process in only one stratum of the external plexiform layer. Medulla-to-lamina cells with arborisations associated with only a single column of the lamina mosaic were not observed; medulla-to-lamina cells whose fields coincide with the various strata of the external plexiform layer were found, however. The present observations are briefly compared with those made on another beetle, Hoplia farinosa L. Comparisons with other species of insects, and the relationship between structure of the eye and structure of the lamina are also discussed.