Anatomy and ultrastructural details of the eye of the passalid beetle Ceracupes yui Okano 1988 (Scarabaeoidea; Passalidae).

One of the least studied eyes of any beetle taxon are those of the scarabaeoid family Passalidae. Some members of this family of around 600 species worldwide are known to have superposition eyes (Aceraius grandis; A. hikidai) while others have apposition eyes (Cylindrocaulus patalis; Ceracupes yui). In C. yui of nearly 3 cm body length (this paper) the retinal layer is very thin and occupies approximately half of an ommatidium's total length, the latter amounting to 284 and 266 µm in the respective dorsal and ventral eye regions. The two eye regions are almost completely separated by a prominent cuticular canthus, a feature usually associated with the presence of a tracheal tapetum, a clear-zone between dioptric and light-perceiving structures and a regular array of smooth facets. In C. yui the facets are smooth (but not very regular) and a tracheal tapetum and a clear-zone are absent. The rhabdoms, formed by 8-9 retinula cells, are complicated, multilobed structures with widths and lengths of around 15 and 80 µm, respectively. The combination of some superposition and mostly apposition eye features, e.g., extensive corneal exocones, relatively small number of ommatidia, absence of a clear-zone and tracheal bush, suggest an adaptation of this species' eye to the fossorial lifestyle of C. yui, and, thus, a manifestation of the passalid eye's plasticity.

The role of Ire1 in Drosophila eye pigmentation revealed by an RNase dead allele.

Ire1 is an endoplasmic reticulum (ER) transmembrane RNase that cleaves substrate mRNAs to help cells adapt to ER stress. Because there are cell types with physiological ER stress, loss of Ire1 results in metabolic and developmental defects in diverse organisms. In Drosophila, Ire1 mutants show developmental defects at early larval stages and in pupal eye photoreceptor differentiation. These Drosophila studies relied on a single Ire1 loss of function allele with a Piggybac insertion in the coding sequence. Here, we report that an Ire1 allele with a specific impairment in the RNase domain, H890A, unmasks previously unrecognized Ire1 phenotypes in Drosophila eye pigmentation. Specifically, we found that the adult eye pigmentation is altered, and the pigment granules are compromised in Ire1H890A homozygous mosaic eyes. Furthermore, the Ire1H890A mutant eyes had dramatically reduced Rhodopsin-1 protein levels. Drosophila eye pigment granules are most notably associated with late endosome/lysosomal defects. Our results indicate that the loss of Ire1, which would impair ER homeostasis, also results in altered adult eye pigmentation.

Ultrastructure and morphology of the compound eyes of the predatory bug Montandoniola moraguesi (Insecta: Hemiptera: Anthocoridae).

The morphology and ultrastructure of the compound eye of the predatory bug, Montandoniola moraguesi (Puton, 1986) was investigated using scanning and transmission electron microscopy. Its compound eyes, which contain ∼195 ommatidia per eye, have the following characteristics: each ommatidium possesses a laminated corneal lens measuring ∼9 µm in diameter and ∼7 µm in thickness, a tetrapartite eucone crystalline cone, which is approximately 5.5 µm long, like a dumbbell with the distal end larger than the proximal end, eight clustered retinula cells ∼25.6 µm in length, two primary pigment cells and eight secondary primary pigment cells. The rhabdomeres of the eight retinula cells form a circular, tiered rhabdom of two elongated and six peripheral retinula cells. The rhabdomeres of cells R7 and R8 are distributed along the basolateral surface of the cone and form a centrally-fused rhabdom that spans nearly the full length of the ommatidium. The microvilli of the peripheral rhabdom (R1-R6) are radially arranged and form a bilobed, V-like shape in the central rhabdom. Based on the similarity of the compound eye of M. moraguesi to the eyes of other predatory insect species, the evolution and function of eyes in predators are briefly discussed.

Visual system characterization of the obligate bat ectoparasite Trichobius frequens (Diptera: Streblidae).

As an obligate ectoparasite of bats, the bat fly Trichobius frequens (Diptera: Streblidae) inhabits the same subterranean environment as their nocturnal bat hosts. In this study, we characterize the macromorphology, optical architecture, rhabdom anatomy, photoreceptor absorbance, and opsin expression of the significantly reduced visual system in T. frequens resulting from evolution in the dark. The eyes develop over a 21-22 day pupal developmental period, with pigmentation appearing on pupal day 11. After eclosion as an adult, T. frequens eyes consist of on average 8 facets, each overlying a fused rhabdom consisting of anywhere from 11 to 18 estimated retinula cells. The dimensions of the facets and fused rhabdoms are similar to those measured in other nocturnal insects. T. frequens eyes are functional as shown by expression of a Rh1 opsin forming a visual pigment with a peak sensitivity to 487 nm, similar to other dipteran Rh1 opsins. Future studies will evaluate how individuals with such reduced capabilities for spatial vision as well as sensitivity still capture enough visual information to use flight to maneuver through dark habitats.

Ultrastructural evidence for the origin of the subretinal pigment shield in the compound eye of Drosophila melanogaster.

Little morphological information is available about subretinal pigment shields in insect compound eyes, especially ultrastructural information. The latter is however needed in order to detect possible smallest projections that emanate from pigment-granule-bearing cells and pass through the basal matrix (BM), but that are not visible in light micrographs. Previous work on the subretinal pigment shield in Drosophila melanogaster suggests that the pigment cell population located below the BM is closely associated with secondary and tertiary pigment cells. Whether these cells stay in connection throughout life with the subretinal regions via thin projections that pass through the fenestrae of the BM, or whether the subretinal parts later become separated during eye development remained so far unknown. Our investigation of the periphery of the BM by three-dimensional reconstruction based on serial-sectioning transmission electron microscopy has revealed that the secondary and tertiary pigment cells possess thin projections that pass through the fenestrae of the BM and thus connect the cellular regions above and below the BM in the adult compound eye. The subretinal pigment shield of D. melanogaster is therefore of retinal origin and is not composed of additional subretinal pigment cells. The maintained bond allows the active displacement of pigment granules below the BM during the process of dark and light adaptation of the compound eye.

The brain and the corresponding sense organs in calanoid copepods - Evidence of vestiges of compound eyes.

Copepoda is one of the crustacean taxa with still unresolved phylogenetic relationships within Tetraconata. Recent phylogenomic studies place them close to Malacostraca and Cirripedia. Little is known about the morphological details of the copepod nervous system, and the available data are sometimes contradictory. We investigated several representatives of the subgroup Calanoida using immunohistochemical labeling against alpha-tubulin and various neuroactive substances, combining this with confocal laser scanning analysis and 3D reconstruction. Our results show that the studied copepods exhibit only a single anterior protocerebral neuropil which is connected to the nerves of two protocerebral sense organs: the frontal filament organ and a photoreceptor known as the Gicklhorn's organ. We suggest, on the basis of its position and the innervation it provides, that Gicklhorn's organ is homologous to the compound eye in arthropods. With regard to the frontal filament organ, we reveal detailed innervation to the lateral protocerebrum and the appearance of spherical bodies that stain intensely against alpha tubulin. A potential homology of these bodies to the onion bodies in malacostacan crustaceans and in Mystacocarida is suggested. The nauplius eye in all the examined calanoids shows the same basic pattern of innervation with the middle cup sending its neurites into the median nerve, while the axons of the lateral cups proceed into both the median and the lateral nerves. The early development of the axonal scaffold of the nauplius eye neuropil from the proximal parts of the nauplius eye nerves follows the same pattern as in other crustaceans. In our view, this specific innervation pattern is a further feature supporting the homology of the nauplius eye in crustaceans.

Novel type of sub-retinal pigment shield in the miniaturized compound eye of Trichogramma evanescens.

Pigment granules, found in different cell types of the retina in insect compound eyes, fulfill important functions. They isolate the individual ommatidia from stray light, regulate the angular sensitivity, and restrict the light that reaches the photoreceptor according to ambient light intensities. Descriptions of pigment cells within the retina are included in ultrastructural eye descriptions, but knowledge of pigment cell types beneath the retina and basal matrix (BM) are relatively limited in insects. In the miniaturized parasitoid wasp Trichogramma evanescens Westwood 1833, a sub-retinal pigment shield is formed by pigment-bearing cells, which appear in two-dimensional TEM sections to form a separate population beneath the BM. By using three-dimensional reconstructions of serial-section transmission electron microscopy, it was possible to reveal that the sub-retinal pigment shield of T. evanescens is not formed by a separate cell type, but by extensions of the lateral rim pigment cells that penetrate gaps in the BM. The reconstruction is supported by evidence from a statistical analysis of pigment granule volumes of all pigment bearing cell types in the retina and rim region. The study reveals the first known case of the participation of lateral rim cells in a sub-retinal pigment shield in an insect eye. As neither pigmented extensions of secondary pigment cells, nor pigment granules in the extensions of the cone cell projections are present above the BM in T. evanescens, the sub-retinal extensions of the lateral rim cells can be seen as a functional adaptation to miniaturization in order to maintain a proximal shielding function.

Analysis of a cellular structure observed in the compound eyes of Drosophila white; yata mutants and white mutants.

We previously identified the Drosophila yata mutant, which showed phenotypes including progressive vacuolization of the white-coloured compound eye, progressive shrinkage of the brain and a shortened lifespan. The yata gene was shown to be involved in controlling intracellular trafficking of the Amyloid precursor protein-like protein, which is an orthologue of Amyloid precursor protein, which is a causative molecule of Alzheimer's disease. In this study, we examined the phenotype of the compound eye of the yata mutant using electron microscopy and confocal microscopy. We found that abnormal cellular structures that seemed to originate from bleb-like structures and contained vesicles and organelles, such as multivesicular bodies and autophagosomes, were observed in aged white; yata mutants and aged white mutants. These structures were not observed in newly eclosed flies and the presence of the structures was suppressed in flies grown under constant dark conditions after eclosion. The structures were not observed in newly eclosed red-eyed yata mutants or wild-type flies, but were observed in very aged red-eyed wild-type flies. Thus, our data suggest that the observed structures are formed as a result of changes associated with exposure to light after eclosion in white mutants, white; yata mutants and aged flies.

Ultrastructural comparison of the compound eyes of the Asian corn borer Ostrinia furnacalis (Lepidoptera: Crambidae) under light/dark adaptation.

The Asian corn borer Ostrinia furnacalis is one of the most destructive pests of maize throughout eastern Asia and the South Pacific. In the present study the fine structure of the compound eyes of adult O. furnacalis was investigated under light/dark adaptation using light and electron microscopy. The compound eyes of male and female O. furnacalis are superposition eyes with electron-lucent clear zones. The sexual differences of the compound eyes of O. furnacalis are mainly reflected in eye size rather than ommatidial ultrastructure. Each ommatidium of both sexes contains 12 retinula cells, one of which is the basal retinula cell. All the retinula cells form a centrally-fused, two-tiered rhabdom, whose distal layer passes through the clear zone and distally connects with the crystalline cone. The ultrastructural changes under light/dark conditions mainly involve the rhabdom occupation ratio to retinula cell volume in the proximal layer of the rhabdom as well as the dimensions of the subcorneal zone and the crystalline tract. Pigment movements occur within the retinula cells and primary pigment cells, but are undetectable within the secondary pigment cells. Regardless of light or dark adaptation, in other words, the pigments never migrate into the clear zone.

A study of the role of vision in the foraging behaviour of the pyrrhocorid bug Antilochus conquebertii (Insecta; Hemiptera; Pyrrhocoridae).

Our study aims to describe (1) external morphology of the compound eye of Antilochus conquebertii, (2) postembryonic changes involving the eye's shape and size and (3) behaviour of the animal with respect to the organization of the compound eye. With each moult of the insect, the structural units of the compound eye increase in size as well as the number, resulting in an overall increase in eye size. The resolution of the adult eye is better than the young one. The adult possesses UV and polarization sensitivity in its eye. Parallel to the changes of the eye the behaviour of the adult animal changes, rendering it increasingly nocturnal and less active in under illuminated conditions. The current study describes the eye and its functional relationship with the behaviour of the animal at the nymphal and adult developmental stage.

Anatomy of the stemmata in the Photuris firefly larva.

Fireflies (Coleoptera: Lampyridae) have distinct visual systems at different stages of development. Larvae have stemmata and adults have compound eyes. Adults use compound eyes to mediate photic communication during courtship. Larvae do not manifest this behavior, yet they are bioluminescent. We investigated the structure of stemmata in Photuris firefly larvae to identify anatomical substrates (i.e., rhabdomeres) conferring visual function. Stemmata were located bilaterally on the antero-lateral surfaces of the head. Beneath the ~ 130 µm diameter lens, we identified a pigmented eye-cup. At its widest point, the eye-cup was ~ 150 µm in diameter. The optic nerve exited the eye-cup opposite the lens. Two distinct regions, asymmetric in size and devoid of pigmentation, were characterized in stemmata cross-sections. We refer to these regions as lobes. Each lobe contained a rhabdom of a radial network of rhabdomeres. Pairs of rhabdomeres formed interdigitating microvilli contributed from neighboring photoreceptor cell bodies. The optic nerve contained 88 axons separable into two populations based on size. The number of axons in the optic nerve together with distinct rhabdoms suggests these structures were formed from 'fusion stemmata.' This structural specialization provides an anatomical substrate for future studies of visually mediated behaviors in Photuris larvae.

Morphology and scaling of compound eyes in the smallest beetles (Coleoptera: Ptiliidae).

The coleopteran family Ptiliidae (featherwing beetles) includes some of the smallest insects known with most of the representatives of this family measuring less than 1 mm in body length. A small body size largely determines the morphology, physiology, and biology of an organism and affects the organization of complex sense organs. Information on the organization of the compound eyes of Ptiliidae is scarce. Using scanning electron microscopy we analyzed the eyes of representatives of all subfamilies and tribes and provide a detailed description of the eye ultrastructure of four species (Nephanes titan, Porophila mystacea, Nanosella sp. and Acrotrichis grandicollis) using transmission electron microscopy. The results are compared with available data on larger species of related groups of Staphyliniformia and scale quantitative analyses are performed. The eyes of Ptiliidae consist of 15-50 ommatidia 6-13 µm in diameter and all conform to the apposition acone type of eye with fused rhabdoms of banded organization. Each ommatidium has the typical cellular arrangement present also in the eyes of larger staphyliniform beetles, but strongly curved lenses, short cones, reduced pigment cells, a high density of pigment granules and certain modifications of the rhabdom seem typical of ptiliid eyes. Allometric analyses show that as body size decreases, the number of facets drops more steeply than their average size does.

Three-dimensional ultrastructural organization of the ommatidium of the minute parasitoid wasp Trichogramma evanescens.

Existing information on insect compound eyes is mainly limited to two-dimensional information derived from histological or ultrathin sections. These allow a basic description of eye morphology, but are limited in z-axis resolution because of the section thickness or intervals between sections, so that accurate volumetric information cannot be generated. Here we use serial-sectioning transmission electron microscopy to present a 3-D reconstruction at ultrastructural level of a complete ommatidium of a miniaturized insect compound eye. Besides the general presentation of the three dimensional arrangement of the different cell types within the ommatidium, the reconstruction allowed volumetric measurements and numerical analyses to be undertaken, revealing new insights into the number, size and distribution of cell organelles in insect ommatidia. Morphological features that can be related to miniaturization, namely the dimensions and displacement of nuclei, reduction of average pigment granule volume and loss of pigment granules in the terminals of the cone cells, the impact of metabolic activity of cell types on miniaturization, as well as maintenance of rhabdomere volume and limits to its miniaturization, are all discussed.

The giant butterfly-moth Paysandisia archon has spectrally rich apposition eyes with unique light-dependent photoreceptor dynamics.

The palm borer moth Paysandisia archon (Burmeister, 1880) (fam. Castniidae) is a large, diurnally active palm pest. Its compound eyes consist of ~ 20,000 ommatidia and have apposition optics with interommatidial angles below 1°. The ommatidia contain nine photoreceptor cells and appear structurally similar to those in nymphalid butterflies. Two morphological ommatidial types were identified. Using the butterfly numbering scheme, in type I ommatidia, the distal rhabdom consists exclusively of the rhabdomeres of photoreceptors R1-2; the medial rhabdom has contributions from R1-8. The rhabdom in type II ommatidia is distally split into two sub-rhabdoms, with contributions from photoreceptors R2, R3, R5, R6 and R1, R4, R7, R8, respectively; medially, only R3-8 and not R1-2 contribute to the fused rhabdom. In both types, the pigmented bilobed photoreceptors R9 contribute to the rhabdom basally. Their nuclei reside in one of the lobes. Upon light adaptation, in both ommatidial types, the rhabdoms secede from the crystalline cones and pigment granules invade the gap. Intracellular recordings identified four photoreceptor classes with peak sensitivities in the ultraviolet, blue, green and orange wavelength regions (at 360, 465, 550, 580 nm, respectively). We discuss the eye morphology and optics, the photoreceptor spectral sensitivities, and the adaptation to daytime activity from a phylogenetic perspective.

Asymmetric ommatidia count and behavioural lateralization in the ant Temnothorax albipennis.

Workers of the house-hunting ant Temnothorax albipennis rely on visual edge following and landmark recognition to navigate their rocky environment, and they also exhibit a leftward turning bias when exploring unknown nest sites. We used electron microscopy to count the number of ommatidia composing the compound eyes of workers, males and queens, to make an approximate assessment of their relative sampling resolution; and to establish whether there is an asymmetry in the number of ommatidia composing the workers' eyes, which might provide an observable, mechanistic explanation for the turning bias. We hypothesise that even small asymmetries in relative visual acuity between left and right eyes could be magnified by developmental experience into a symmetry-breaking turning preference that results in the inferior eye pointing toward the wall. Fifty-six workers were examined: 45% had more ommatidia in the right eye, 36% more in the left, and 20% an equal number. A tentative connection between relative ommatidia count for each eye and turning behaviour was identified, with a stronger assessment of behavioural lateralization before imaging and a larger sample suggested for further work. There was a clear sexual dimorphism in ommatidia counts between queens and males.

Unique Temporal Expression of Triplicated Long-Wavelength Opsins in Developing Butterfly Eyes.

Following gene duplication events, the expression patterns of the resulting gene copies can often diverge both spatially and temporally. Here we report on gene duplicates that are expressed in distinct but overlapping patterns, and which exhibit temporally divergent expression. Butterflies have sophisticated color vision and spectrally complex eyes, typically with three types of heterogeneous ommatidia. The eyes of the butterfly Papilio xuthus express two green- and one red-absorbing visual pigment, which came about via gene duplication events, in addition to one ultraviolet (UV)- and one blue-absorbing visual pigment. We localized mRNAs encoding opsins of these visual pigments in developing eye disks throughout the pupal stage. The mRNAs of the UV and blue opsin are expressed early in pupal development (pd), specifying the type of the ommatidium in which they appear. Red sensitive photoreceptors first express a green opsin mRNA, which is replaced later by the red opsin mRNA. Broadband photoreceptors (that coexpress the green and red opsins) first express the green opsin mRNA, later change to red opsin mRNA and finally re-express the green opsin mRNA in addition to the red mRNA. Such a unique temporal and spatial expression pattern of opsin mRNAs may reflect the evolution of visual pigments and provide clues toward understanding how the spectrally complex eyes of butterflies evolved.

Alternative moth-eye nanostructures: antireflective properties and composition of dimpled corneal nanocoatings in silk-moth ancestors.

Moth-eye nanostructures are a well-known example of biological antireflective surfaces formed by pseudoregular arrays of nipples and are often used as a template for biomimetic materials. Here, we provide morphological characterization of corneal nanostructures of moths from the Bombycidae family, including strains of domesticated Bombyx mori silk-moth, its wild ancestor Bombyx mandarina, and a more distantly related Apatelodes torrefacta. We find high diversification of the nanostructures and strong antireflective properties they provide. Curiously, the nano-dimple pattern of B. mandarina is found to reduce reflectance as efficiently as the nanopillars of A. torrefacta. Access to genome sequence of Bombyx further permitted us to pinpoint corneal proteins, likely contributing to formation of the antireflective nanocoatings. These findings open the door to bioengineering of nanostructures with novel properties, as well as invite industry to expand traditional moth-eye nanocoatings with the alternative ones described here.

Antireflective nanocoatings for UV-sensation: the case of predatory owlfly insects.

Moth-eye nanostructures, discovered to coat corneae of certain nocturnal insects, have inspired numerous technological applications to reduce light reflectance from solar cells, light-emitting diodes, and optical detectors. Technological developments require such nanocoatings to possess broadband antireflective properties, transcending the visual light spectrum, in which animals typically operate. Here we describe the corneal nanostructures of the visual organ exclusive in UV sensation of the hunting insect Libelloides macaronius and report their supreme anti-light-reflectance capacity.

A population of G2-arrested cells are selected as sensory organ precursors for the interommatidial bristles of the Drosophila eye.

Cell cycle progression and differentiation are highly coordinated during the development of multicellular organisms. The mechanisms by which these processes are coordinated and how their coordination contributes to normal development are not fully understood. Here, we determine the developmental fate of a population of precursor cells in the developing Drosophila melanogaster retina that arrest in G2 phase of the cell cycle and investigate whether cell cycle phase-specific arrest influences the fate of these cells. We demonstrate that retinal precursor cells that arrest in G2 during larval development are selected as sensory organ precursors (SOPs) during pupal development and undergo two cell divisions to generate the four-cell interommatidial mechanosensory bristles. While G2 arrest is not required for bristle development, preventing G2 arrest results in incorrect bristle positioning in the adult eye. We conclude that G2-arrested cells provide a positional cue during development to ensure proper spacing of bristles in the eye. Our results suggest that the control of cell cycle progression refines cell fate decisions and that the relationship between these two processes is not necessarily deterministic.

Not flying blind: a comparative study of photoreceptor function in flying and non-flying cockroaches.

Flying is often associated with superior visual performance, as good vision is crucial for detection and implementation of rapid visually guided aerial movements. To understand the evolution of insect visual systems it is therefore important to compare phylogenetically related species with different investments in flight capability. Here, we describe and compare morphological and electrophysiological properties of photoreceptors from the habitually flying green cockroach Panchlora nivea and the American cockroach Periplaneta americana, which flies only at high ambient temperatures. In contrast to Periplaneta, ommatidia in Panchlora were characterized by two-tiered rhabdom, which might facilitate detection of polarized light while flying in the dark. In patch-clamp experiments, we assessed the absolute sensitivity to light, elementary and macroscopic light-activated current and voltage responses, voltage-activated potassium (Kv) conductances, and information transfer. Both species are nocturnal, and their photoreceptors were similarly sensitive to light. However, a number of important differences were found, including the presence in Panchlora of a prominent transient Kv current and a generally low variability in photoreceptor properties. The maximal information rate in Panchlora was one-third higher than in Periplaneta, owing to a substantially higher gain and membrane corner frequency. The differences in performance could not be completely explained by dissimilarities in the light-activated or Kv conductances; instead, we suggest that the superior performance of Panchlora photoreceptors mainly originates from better synchronization of elementary responses. These findings raise the issue of whether the evolutionary tuning of photoreceptor properties to visual demands proceeded differently in Blattodea than in Diptera.