Simple Eyes Research Articles

Visual Targeting of Motor Actions in Climbing Drosophila

Drosophila melanogaster flies cross surmountable gaps in their walkway of widths exceeding their body length with an astounding maneuver but avoid attempts at insurmountable gaps by visual width estimation. Different mutant lines affect specific aspects of this maneuver, indicating a high complexity and modularity of the underlying motor control. Here we report on two mutants, ocelliless(1) and tay bridge(1), that, although making a correct decision to climb, fail dramatically in aiming at the right direction. Both mutants show structural defects in the protocerebral bridge, a central complex neuropil formed like a handlebar spanning the brain hemispheres. The bridge has been implicated in step-length control in walking flies and celestial E-vector orientation in locusts. In rescue experiments using tay bridge(1) flies, the integrity of the bridge was reestablished, concomitantly leading to a significant improvement of their orientation at the gap. Although producing directional scatter, their attempts were clearly aimed at the landing site. However, this partial rescue was lost in these flies at a reduced-visibility landing site. We therefore conclude that the protocerebral bridge is an essential part of a visual targeting network that transmits directional clues to the motor output via a known projection system.

Applicability of White-Noise Techniques to Analyzing Motion Responses

Motion processing in visual neurons is often understood in terms of how they integrate light stimuli in space and time. These integrative properties, known as the spatiotemporal receptive fields (STRFs), are sometimes obtained using white-noise techniques where a continuous random contrast sequence is delivered to each spatial location within the cell's field of view. In contrast, motion stimuli such as moving bars are usually presented intermittently. Here we compare the STRF prediction of a neuron's response to a moving bar with the measured response in second-order interneurons (L-neurons) of dragonfly ocelli (simple eyes). These low-latency neurons transmit sudden changes in intensity and motion information to mediate flight and gaze stabilization reflexes. A white-noise analysis is made of the responses of L-neurons to random bar stimuli delivered either every frame (densely) or intermittently (sparsely) with a temporal sequence matched to the bar motion stimulus. Linear STRFs estimated using the sparse stimulus were significantly better at predicting the responses to moving bars than the STRFs estimated using a traditional dense white-noise stimulus, even when second-order nonlinear terms were added. Our results strongly suggest that visual adaptation significantly modifies the linear STRF properties of L-neurons in dragonfly ocelli during dense white-noise stimulation. We discuss the ability to predict the responses of visual neurons to arbitrary stimuli based on white-noise analysis. We also discuss the likely functional advantages that adaptive receptive field structures provide for stabilizing attitude during hover and forward flight in dragonflies.

Introduction to the Symposium “Molluscan Models: Advancing Our Understanding of the Eye”*

Since the time of Darwin, the eye has been a subject of evolutionary and comparative biologists alike who were intrigued by the structural complexity and morphological diversity of eyes in nature. Much of what we know about the eye—development, structure, physiology, and function—has been determined from only a handful of model organisms, specifically the mouse and the fly. One major phylum in particular, the Mollusca, has been underutilized in investigating the evolution and development of the eye. This is surprising as molluscs display a myriad of eye types, such as simple pit eyes without any apparatus to focus images, compound eyes that superficially resemble the eyes of flies, camera-type eyes that are similar to vertebrate eyes, and eyes with mirrors, just to name a few. As a result, molluscan eyes comprise more morphological diversity than seen even in the largest animal phylum, the Arthropoda. With all of this incredible diversity, how do we as researchers determine which mollusc species should be developed as models to study the eye? Serb provides background for eye research using traditional model organisms and how using molluscan species would be advantageous to understanding the eye. She describes the research potential of molluscan species as model organisms and identifies criteria that might be used to develop a molluscan model and the questions molluscan models might address. One application of molluscan models is to study the cellular biology of human eye disease. As many degenerative eye diseases, such as macular degeneration, have been linked to the mis-organization of the cytoskeleton within retinal cells, understanding the control of cytoskeleton organization and its influence on photoreceptor cell changes may lead to prevention and possible cures for some eye diseases. Gray, Kelly, and Robles utilize Octopus bimaculoides Pickford and McConnaughey, 1949 as a model organism to study the molecular controls of cytoskeleton organization in the retina. Their work identifies a cell signaling pathway (Rho GTPase) that mediates cytoskeleton rearrangements. Errors in this pathway may prove to be one of the factors that disrupts cytoskeleton formation, leading to retinal degeneration. After developing one or several molluscan models of the eye, how does one set about understanding this great diversity of eyes and place it in an evolutionary context? One way is to use a comparative approach to identify conserved and variable components of eye morphology, such as lens composition, photoreceptor number and organization, and overall eye shape. These morphological features can provide evidence for functional differences and visual capabilities among species. Several authors in the symposium take this approach. Speiser and Johnsen examine eye morphology in four species of scallop and a closely related spondylid (Spondylus americanus [Hermann, 1781]). They show that scallop eye structure varies among species, and these structural differences affect optical resolution and sensitivity. Further, they provide evidence that actively swimming species (e.g., Amusium balloti [Bernardi 1861]) have better optical resolution than non-swimming species. Speiser and Johnsen provide several new and exciting hypotheses on how the scallop eye performs and how visual requirements may differ between mobile and immobile species. Morton takes a broader perspective and reviews the diversity of noncephalic eye types in the Class Bivalvia. He hypothesizes a possible evolutionary path to create the double retina system in Pectinidae and Laternulidae through the duplication of sensory structures on the pallial folds. Zieger and MeyerRochow review the variation of cephalic gastropod eyes, concentrating on pulmonate species, which are the beststudied eyes in gastropods. They discuss eye anatomy, differences in retinal design, and the visual capabilities of different optical components. Finally, they describe the ultrastructure of “additional” or “accessory” eyes associated with cephalic eyes in several lineages. These data provide

The visual system of male scale insects

Animal eyes generally fall into two categories: (1) their photoreceptive array is convex, as is typical for camera eyes, including the human eye, or (2) their photoreceptive array is concave, as is typical for the compound eye of insects. There are a few rare examples of the latter eye type having secondarily evolved into the former one. When viewed in a phylogenetic framework, the head morphology of a variety of male scale insects suggests that this group could be one such example. In the Margarodidae (Hemiptera, Coccoidea), males have been described as having compound eyes, while males of some more derived groups only have two single-chamber eyes on each side of the head. Those eyes are situated in the place occupied by the compound eye of other insects. Since male scale insects tend to be rare, little is known about how their visual systems are organized, and what anatomical traits are associated with this evolutionary transition. In adult male Margarodidae, one single-chamber eye (stemmateran ocellus) is present in addition to a compound eye-like region. Our histological investigation reveals that the stemmateran ocellus has an extended retina which is formed by concrete clusters of receptor cells that connect to its own first-order neuropil. In addition, we find that the ommatidia of the compound eyes also share several anatomical characteristics with simple camera eyes. These include shallow units with extended retinas, each of which is connected by its own small nerve to the lamina. These anatomical changes suggest that the margarodid compound eye represents a transitional form to the giant unicornal eyes that have been described in more derived species.

Exceptional Variation on a Common Theme: The Evolution of Crustacean Compound Eyes

The Crustacea contain an amazing, and often (to humans) bizarre, array of visual designs. This diversity includes many different examples of both simple and compound eyes, each with standard or uniquely crustacean features. In this review, we focus on the anatomical variation, optical principles, and molecular diversity of crustacean compound eyes to illustrate how the complicated structures involved in vision are adapted for particular environments. Using this knowledge as a starting point, and considering what is known of crustacean evolution overall, we present the most recent ideas of how crustacean compound eyes have evolved and show how eyes that are based on fundamentally different optical principles can in fact be derived from each other and thus be closely related through common descent.

External microstructure of the ambrosia beetle Platypus koryoensis (Coleoptera: Curculionidae: Platypodinae)

Abstract Platypus koryoensis is a minute ambrosia beetle found in forests. It can cause significant economic damage to oak trees. Recently in Korea, it has been reported as a major pest of oak trees, because it causes sooty mold of oak by introducing the pathogenic fungus Raffaelea sp. In this paper, we demonstrate the fine structural aspects of the external body of the ambrosia beetle using field emission scanning electron microscopy, as a part of basic research into this pest so that strategies for its control might be developed. This beetle has a sensory system well developed to respond to both visual and chemical stimuli. Both sexes have a pair of faceted compound eyes and a pair of knobbed antennae, but simple eyes are absent. The mouthparts on its distinct snouts are effective devices for penetration and for boring holes. The mouthparts consist of the labrum, a pair of mandibles, a pair of maxillae and the labium. Both the maxillary and the labial palpi have the function of directing the food to the mouth and holding it while the mandibles chew the food. The distal ends of these palpi are flattened and have shovel‐like setae. The thorax has a particularly hard exoskeleton and hard elytra, including powerful muscles that operate both the wings and the legs. The legs are multi‐segmented and have a strong femur and tibia, including one pair of claws on the end of each tarsal segment. Characteristically, both male and female beetles have mycangial cavities for storing spores and other microorganisms, but only females have three pairs of large depressions on their dorsal thorax.

Female and Male Polymorphism in Two Species of Melittobia Parasitoid Wasps (Hymenoptera: Eulophidae)

Polymorphism appears to be fundamental in Melittobia wasps, but uncertainty exists as to its extent and form. Most researchers recognize 2 basic female forms--a long-winged dispersive “type form” and an early-maturing, short-winged gravid “second form.” However, some investigators have recognized two macropterous forms, “jumpers” and “flyers.” Many others have suggested that males, which normally comprise only about 5% of the population, also may exist in 2 forms in various Melittobia species. This study examined the role of maturational factors in 2 widespread representatives of different species groups, M. australica (hawaiiensis group) and M. digitata (acasta group). Individuals of both sexes from different points in the emergence curve were examined and measured immediately after eclosion and 5 d later. Both sets of measurements supported the existence of 2 clearly defined female morphs. However, when newly eclosed macropterous females were randomly assigned to 5-d placements in empty vials or with prepupal hosts (Trypoxylon politum Say), subsequent tests in a flight arena demonstrated that “jumpers” and “flyers” were simple reflections of macropterous female physiological state, with heavier, gravid females reluctant to do more than a slow crawl, and lighter, more nutritionally stressed females being more inclined to fly. Thus we conclude that there is no justification for recognizing a “jumper” morph. Male morphometrics indicated continuous variability in size and pigmentation of simple eyes and ocelli, but no morphologically distinctive male morphs at any point in the emergence curve. However, reports of distinct male morphs in other Melittobia species cannot be dismissed.

Your Core Energy is Within Your Grasp

This paper, an abridgement of a 55-page paper, Curling: Exercises and Notes, highlights a new gentle approach to stimulating instinctual life-energy, helping restore natural energetic pulsation, and providing containment for building boundaries. A soft curling and uncurling of the fingers and/or toes, in rhythm with the regular breathing pattern, brings up a mild, expansive, pleasurable energetic charge, which can then be augmented with simple body movements, eyes and words to help repossess repressed natural instincts. If stronger, more assertive energy is desired, the curl can be held tighter and longer. This paper outlines six Curling exercises, useful both in the therapeutic setting and as self-help. The exercises can be done standing, sitting or lying down, and can be both relaxing and/or revitalizing. Some exercises can be done in bed, for helping to fall asleep, for example, or for an invigorating way to wake up.

E-PTFE (Gore-Tex®) implant with or without low-dosage mitomycin-C as an adjuvant in penetrating glaucoma surgery: 2 year randomized clinical trial

E-PTFE (Gore-Tex^®) implant with or without low-dosage mitomycin-C as an adjuvant in penetrating glaucoma surgery: 2 year randomized clinical trial

Directional Selectivity in the Simple Eye of an Insect

Among other sensory modalities, flight stabilization in insects is performed with the aid of visual feedback from three simple eyes (ocelli). It is thought that each ocellus acts as a single wide-field sensor that detects changes in light intensity. We challenge this notion by providing evidence that, when light-adapted, the large retinal L-neurons in the median ocellus of the dragonfly respond in a directional way to upward moving bars and gratings. This ability is pronounced under UV illumination but weak or nonexistent in green light and is optimal at angular velocities of approximately 750 degrees s(-1). Using a reverse-correlation technique, we analyze the functional organization of the receptive fields of the L-neurons. Our results reveal that L-neurons alter the structure of their linear spatiotemporal receptive fields with changes in the illuminating wavelength, becoming more inseparable and directional in UV light than in green. For moving bars and gratings, the strength of directionality predicted from the receptive fields is consistent with the measured values. Our results strongly suggest that, during the day, the retinal circuitry of the dragonfly median ocellus performs an early linear stage of motion processing. The likely advantage of this computation is to enhance pitch control.

Impact of Diabetic Retinopathy on Quantitative Retinal Nerve Fiber Layer Measurement and Glaucoma Screening

To investigate the impact of diabetic retinopathy on quantitative retinal nerve fiber layer (RNFL) assessment and diagnostic power for glaucoma by scanning laser polarimetry (GDx-VCC) and optical coherence tomography (StratusOCT). The individual RNFL parameters of GDx and OCT were obtained for 170 eyes (one eye from each of 170 subjects [45 healthy, 47 glaucoma, 40 diabetes, and 38 glaucoma with diabetes]) and were compared among the four groups. Diabetic eyes had mild to moderate nonproliferative diabetic retinopathy (NPDR) without maculopathy. In glaucomatous eyes with or without diabetes, the ability to discriminate glaucoma was assessed by the areas under the receiver operating characteristic curves (AUROCs) and the sensitivities at more than 80% and 90% of specificities for each technique. Using GDx-VCC, significant differences (P < 0.05) in RNFL measurement parameters were found for all comparisons except those between glaucomatous eyes with diabetes and without diabetes. StratusOCT parameters did not detect significant differences between age-matched healthy and diabetic eyes. Among the parameters included, the nerve fiber indicator (NFI) of GDx-VCC and the inferior quadrant thickness (IQT) of StratusOCT had the largest AUROCs and sensitivities at specificities greater than 80%: NFI (0.912, 86%) and IQT (0.902, 85%) in glaucomatous eyes with diabetes; NFI (0.935, 92%) and IQT (0.921, 91%) in simple glaucomatous eyes. Mild to moderate NPDR causes a quantitative discrepancy in RNFL measurements between GDx-VCC and StratusOCT in simple diabetic eyes. However, mild to moderate glaucomatous optic neuropathy can be highly discriminated by the two imaging devices in eyes with diabetic retinopathy.

Expression of a retinal homeobox (Rx) gene during planarian regeneration

Retinal homeobox (Rx) genes, with representatives in vertebrates and invertebrates, encode fundamental regulators of early eye and brain formation. Here we describe the spatio-temporal expression profile of a candidate planarian orthologue of Rx during regeneration in Dugesia japonica and Schmidtea mediterranea. Although low levels of Rx transcripts were found throughout the body of intact planarians, high levels of Rx expression were specific to regenerating tissue in both head and tail fragments. We also observed that Rx was never expressed in the simple rhabdomeric planarian eyes, supporting the notion that only formation of eyes that use the ciliary type of photoreceptors requires Rx function.

A motion-sensitive neurone responds to signals from the two visual systems of the blowfly, the compound eyes and ocelli

In the blowfly Calliphora vicina, lobula plate tangential cells (LPTCs) estimate self-motion by integrating local motion information from the compound eyes. Each LPTC is sensitive to a particular (preferred) rotation of the fly's head. The fly can also sense rotation using its three ocelli (simple eyes), by comparing the light intensities measured at each ocellus. We report that an individually identified tangential cell, V1, responds in an apparently rotation-specific manner to stimulation of the ocelli. This effect was seen with or without additional stimulation of the compound eye. We delivered stimuli to the ocelli which mimicked rotation of the fly's head close to that of the preferred axis of rotation of V1. Alternating between preferred and anti-preferred rotation elicited a strongly phasic response, the amplitude of which increased with the rate of change of light intensity at the ocelli. With combined stimulation of one compound eye and the ocelli, V1 displayed a robust response to ocellar stimuli over its entire response range. These findings provide the opportunity to study quantitatively the interactions of two different visual mechanisms which both encode the same variable--the animal's rotation in space.

Cambrian view

The analysis of visual systems is a valuable method of assessing phylogenetic processes. As in the present animal world, we find simple and complex systems in the Lower Cambrian. One may detect “simple eyes” for example with an advanced design in lobopodians, while the existence of even more simple “simple eyes” is very probable but still to be proved. More complex systems are to be found. In Leanchoilia illecebrosa Hou, 1987 and Leanchoilia superlata Walcott, 1912 there are probable dorsal median eyes and a pair of fine, stalked ventral eyes. Both systems may contribute to phylogenetic and systematic discussions. These presumably movable stalked eyes may be regarded as an adaptation to a mobile lifestyle. They suggest that the physiologic principle of nystagmus to stabilise the visual world of an animal in motion was already realised in Leanchoilia, perhaps for the first time. To analyse the surface of the early eyes from the Lower Cambrian – not only of Leanchoilia, but of any other forms as well – the number, shape and other parameters of the lenses could lead to further knowledge regarding vision in early invertebrates.

The mapping of visual space by identified large second-order neurons in the dragonfly median ocellus

In adult dragonflies, the compound eyes are augmented by three simple eyes known as the dorsal ocelli. The outputs of ocellar photoreceptors converge on relatively few second-order neurons with large axonal diameters (L-neurons). We determine L-neuron morphology by iontophoretic dye injection combined with three-dimensional reconstructions. Using intracellular recording and white noise analysis, we also determine the physiological receptive fields of the L-neurons, in order to identify the extent to which they preserve spatial information. We find a total of 11 median ocellar L-neurons, consisting of five symmetrical pairs and one unpaired neuron. L-neurons are distinguishable by the extent and location of their terminations within the ocellar plexus and brain. In the horizontal dimension, L-neurons project to different regions of the ocellar plexus, in close correlation with their receptive fields. In the vertical dimension, dendritic arborizations overlap widely, paralleled by receptive fields that are narrow and do not differ between different neurons. These results provide the first evidence for the preservation of spatial information by the second-order neurons of any dorsal ocellus. The system essentially forms a one-dimensional image of the equator over a wide azimuthal area, possibly forming an internal representation of the horizon. Potential behavioural roles for the system are discussed.

A Spatiotemporal White Noise Analysis of Photoreceptor Responses to UV and Green Light in the Dragonfly Median Ocellus

Adult dragonflies augment their compound eyes with three simple eyes known as the dorsal ocelli. While the ocellar system is known to mediate stabilizing head reflexes during flight, the ability of the ocellar retina to dynamically resolve the environment is unknown. For the first time, we directly measured the angular sensitivities of the photoreceptors of the dragonfly median (middle) ocellus. We performed a second-order Wiener Kernel analysis of intracellular recordings of light-adapted photoreceptors. These were stimulated with one-dimensional horizontal or vertical patterns of concurrent UV and green light with different contrast levels and at different ambient temperatures. The photoreceptors were found to have anisotropic receptive fields with vertical and horizontal acceptance angles of 15° and 28°, respectively. The first-order (linear) temporal kernels contained significant undershoots whose amplitudes are invariant under changes in the contrast of the stimulus but significantly reduced at higher temperatures. The second-order kernels showed evidence of two distinct nonlinear components: a fast acting self-facilitation, which is dominant in the UV, followed by delayed self- and cross-inhibition of UV and green light responses. No facilitatory interactions between the UV and green light were found, indicating that facilitation of the green and UV responses occurs in isolated compartments. Inhibition between UV and green stimuli was present, indicating that inhibition occurs at a common point in the UV and green response pathways. We present a nonlinear cascade model (NLN) with initial stages consisting of separate UV and green pathways. Each pathway contains a fast facilitating nonlinearity coupled to a linear response. The linear response is described by an extended log-normal model, accounting for the phasic component. The final nonlinearity is composed of self-inhibition in the UV and green pathways and inhibition between these pathways. The model can largely predict the response of the photoreceptors to UV and green light.

Pteropsin: A vertebrate-like non-visual opsin expressed in the honey bee brain

Insects have excellent color vision based on the expression of different opsins in specific sets of photoreceptive cells. Opsins are members of the rhodopsin superfamily of G-protein coupled receptors, and are transmembrane proteins found coupled to light-sensitive chromophores in animal photoreceptors. Diversification of opsins during animal evolution provided the basis for the development of wavelength-specific behavior and color vision, but with the exception of the recently discovered non-visual melanopsins, vertebrate and invertebrate opsins have generally been viewed as representing distinct lineages. We report a novel lineage of insect opsins, designated pteropsins. On the basis of sequence analysis and intron location, pteropsins are more closely related to vertebrate visual opsins than to invertebrate opsins. Of note is that the pteropsins are missing entirely from the genome of drosophilid flies. In situ hybridization studies of the honey bee, Apis mellifera, revealed that pteropsin is expressed in the brain of this species and not in either the simple or compound eyes. It was also possible, on the basis of in situ hybridization studies, to assign different long wavelength opsins to the compound eyes (AmLop1) and ocelli (AmLop2). Insect pteropsin might be orthologous to a ciliary opsin recently described from the annelid Platynereis, and therefore represents the presence of this vertebrate-like light-detecting system in insects.

UV SETS THE BEE CLOCK

If you see a bee flitting between flowers, it's a good bet that it's relying on UV light for its deft navigation. Given that we've known for almost a century that bees can see UV, we know surprisingly little about the nature and location of UV visual pigments in bees' brains. To explore the possible functions of such pigments, Adriana Briscoe and Johannes Spaethe set out to locate UV visual pigments in bumblebees' brains(p. 2347).Briscoe explains that insects' visual pigments consist of opsin proteins. If you want to find out where an opsin protein is expressed, the obvious place to look is in an insect's retina. But Briscoe knew that some butterfly visual pigments are only expressed in the butterfly's brain, not in its retina. Could the same be true for bumblebees?Before they could tackle this question, Briscoe and Spaethe needed to identify the gene for bumblebees' UV-sensitive opsin protein. They extracted RNA from bumblebee eyes, and from it sequenced a putative UV opsin gene. To find out if their sequence really encodes a UV opsin, they searched other insect genomes for similar sequences. They found that their putative bumblebee UV opsin only produced hits to other known insect UV opsins. But was their bumblebee opsin a functional UV-sensitive visual pigment? To show that the bumblebee sequence contains a critical amino acid residue that confers UV sensitivity, Briscoe and Spaethe aligned their bumblebee sequence with the known UV opsin sequence of Drosophila. Briscoe was relieved to find that `the bumblebee sequence contained the same mutation found in Drosophila that we know is functionally important to make a UV-sensitive pigment.'Now, Briscoe and Spaethe could probe where this UV opsin protein is expressed in the bumblebee brain. They had an anti-opsin antibody for a butterfly UV visual pigment, which they discovered also revealed UV opsin expression in bumblebees' retinas and brains. In the bumblebee retina, they found that the ommatidia – the repeating structures that make up the compound eye – come in three types; they can contain two, one or no UV opsin-expressing cells. Briscoe didn't expect to find such heterogeneity because honeybees, bumblebees' closest relatives, are thought to have just one ommatidial type. Briscoe and Spaethe also found that the UV opsin is expressed in bees' ocelli, the three simple eyes that detect polarised light and allow bees to forage until dusk. `This is the first opsin to be localised in bee ocelli,' Briscoe says.But when Briscoe and Spaethe examined bumblebees' brains, they were in for a few more surprises. They found UV opsin expression in various brain parts,but two in particular caught their eye – the optic and antennal lobes,the regions that process vision and olfaction. Briscoe and Spaethe knew that some insects express period, a circadian clock protein, in the outer layer of the optic lobe. Curious to find out if this is also true for bumblebees, they searched for period expression in bumblebee brains. Sure enough, bumblebees express period in the optic and antennal lobes. Since period and the UV opsin are both expressed in the optic and antennal lobes, Briscoe and Spaethe conclude that UV light might play a role in bumblebee circadian rhythm regulation, mediated through these two brain regions.

Duplicated Pax6 genes in Glomeris marginata (Myriapoda: Diplopoda), an arthropod with simple lateral eyes

Composite (facetted) eyes comprised by several units, termed ommatidia, are an ancestral feature in the arthropods. Some arthropods, however, do not possess composite eyes, obviously by secondary reduction. Reductions on the level of conserved eye developmental genes are one possibility to reduce the visual system. The genes of the Pax6 family have been shown to be key regulators of visual system development in a wide variety of animals. Reduction of Pax6 expression may therefore be expected in a species with reduced eyes. Here I have investigated the myriapod Glomeris marginata that displays very simple eyes. Glomeris, however, possesses two Pax6 genes that, based on their sequence, are similar to Drosophila eyeless ( ey) and twin of eyeless ( toy), respectively. Both genes are highly expressed in the optic lobes and the ventral nerve cord of developing embryos. Furthermore, homologs of other high-ranking eye developmental genes like hedgehog, decapentaplegic, dachshund, and homothorax are expressed in the optic lobes. This indicates that eye reduction in Glomeris is not realized at the level of the Pax6 genes or other genes on the upper levels of the eye development network. I suggest instead that the simple eyes of Glomeris are the product of changes at a much lower level in the network, probably at the level of genes directly regulating ommatidia development or ommatidia number and arrangement.

The unusual visual system of the Strepsiptera: external eye and neuropils.

Adult males of the insect order Strepsiptera are characterized by an unusual visual system that may use design principles from compound as well as simple eyes. The lenses of this eye are unusually large and focus images onto extended retinae. The light-gathering ability of the lens is sufficient to resolve multiple points of an image in each optical unit. We regard each unit as an independent image-forming eye that contributes an inverted partial image. Each partial image is re-inverted by optic chiasmata between the retinae and the lamina, where the complete image could be assembled from the neighboring units. The lamina, medulla and lobula are present, but their organization into cartridges is not clearly discernable. Fluorescent fills, whole-tissue stains, and synaptotagmin immunohistochemistry show that the optic neuropils nevertheless are densely packed, and that several parallel channels within the medulla underlie each of the lenses. The size and shape of the rhabdoms, as well as a relatively slow flicker-fusion frequency could suggest that these eyes evolved through a nocturnal life stage.