Corneal Nipples Research Articles

Osiris gene family defines the cuticle nanopatterns of Drosophila.

Nanostructures of pores and protrusions in the insect cuticle modify molecular permeability and surface wetting and help insects sense various environmental cues. However, the cellular mechanisms that modify cuticle nanostructures are poorly understood. Here, we elucidate how insect-specific Osiris family genes are expressed in various cuticle-secreting cells in the Drosophila head during the early stages of cuticle secretion and cover nearly the entire surface of the head epidermis. Furthermore, we demonstrate how each sense organ cell with various cuticular nanostructures expressed a unique combination of Osiris genes. Osiris gene mutations cause various cuticle defects in the corneal nipples and pores of the chemosensory sensilla. Thus, our study emphasizes on the importance of Osiris genes for elucidating cuticle nanopatterning in insects.

Fine structure of the compound eyes of the crepuscular moth Grapholita molesta (Busck 1916) (Lepidoptera: Tortricidae).

Morphological organization, ultrastructure and adaptational changes under different light intensities (10000, 100, 1, and 0.01mW/m2) of the compound eye of the oriental fruit moth Grapholita molesta (Busck 1916) were investigated. Its superposition type of eyes consist of approximately 1072 ommatidia in males and 1029 ommatidia in females with ommatidial diameters of around 15μm. Each ommatidium features a laminated corneal lens densely covered by corneal nipples of 256nm in height. Crystalline cones are formed by four cone cells, proximally tapering to form a narrow crystalline tract with a diameter of 1.5μm. Eight retinula cells, two primary and six secondary pigment cells per ommatidium are present. The 62.3μm long rhabdom is divided into a thin 1.8μm wide distal and a 5.2μm wide proximal region. Distally the fused rhabdom consists of the rhabdomeres of seven retinula cells (R1-R7) and connects with the crystalline cone. In the proximal rhabdom region, the pigment-containing retinula cell R8 occupies a position in centre of the rhabdom while R1-R7 cells have taken peripheral positions. At this level each ommatidial group of retinula cells is surrounded by a tracheal tapetum. In response to changes from bright-light to dim-light adaptations, the pigment granules in the secondary pigment cells and retinula cells migrate distally, with a decrease in the length of crystalline tract.

Fine Structure of the Compound Eyes of Male and FemaleHeortia vitessoidesMoore (Lepidoptera: Crambidae)

AbstractHeortia vitessoides is a destructive pest of the economically important plant Aquilaria sinensis in China. We investigated the external morphology and ultrastructure of H. vitessoides compound eyes using scanning and transmission electron microscopy. We found that H. vitessoides has superposition eyes, made up of a mean of 3,671 and 3,428 ommatidia in males and females, respectively. The mean ommatidial diameter for males and females was 18.87 and 16.81 µm, respectively. Each ommatidium was capped by a convexly curved corneal facet lens of about 13.85 µm thickness. Corneal nipples measuring 226.36 nm in height in males and 295.48 nm in females, covered the surface of the eyes. Heortia vitessoides eyes are eucone, with each ommatidium having a crystalline cone comprising four cone cells, surrounded by two primary pigment cells. The centrally fused rhabdom of each ommatidium was formed by 8–11 of retinal cells, somewhat more than are typically found in Lepidoptera. The relatively large size of the compound eyes, the large number of ommatidia, large number of retinula cells, and high corneal nipples all suggest that H. vitessoides eyes are adapted to optimize visual sensitivity under dim light conditions.

Some previously unpublished scanning electron micrographs of lepidoptera showing differences in eye structure

Many Lepidoptera have corneal nipples of varying length on the surface of the eye, although corneal nipples are absent in some Lepidoptera species. There are several theories as to the function of these nipples. Figures showing corneal nipples for a number of species are shown here for the first time.

A Comparative Study of Crystallography and Defect Structure of Corneal Nipple Array in Daphnis nerii Moth and Papilio polytes Butterfly Eye.

Moth and butterfly ommatidial nanostructures have been extensively studied for their anti-reflective properties. Especially, from the point of view of sub-wavelength anti-reflection phenomena, the moth eye structures are the archetype example. Here, a comparative analysis of corneal nipples in moth eye (both Male and Female) and butterfly eye (both Male and Female) is given. The surface of moth(Male and Female) and butterfly(Male and Female) eye is defined with regularly arranged hexagonal facets filled with corneal nipples. A detailed analysis using high-resolution scanning electron microscopy images show the intricate hexagonal arrangement of corneal nipples within the individual hexagonal facet. Individual nipples in moth are circular with an average diameter of about 140/165 nm (Male/Female) and average internipple separation of 165 nm. The moth eye show the ordered arrangement of the corneal nipples and the butterfly eye (Male/Female) show an even more complex arrangement of the nipples. Structurally, the corneal nipples in both male and female butterflies are not circular but are polygons with 5, 6, and 7 sides. The average center-to-center separation in the butterfly(Male/Female) is about 260 nm/204 nm, respectively. We find that these corneal nipples are organized into much more dense hexagonal packing with the internipple (edge-to-edge) separation ranging from 20 to 25 nm. Each hexagonal facet is divided into multiple grains separated by boundaries spanning one or two crystallographic defects. These defects are seen in both moth and butterfly. These are typical 5-coordinated and 7-coordinated defect sites typical for a solid-state material with the hexagonal atomic arrangement. Even though the isolated defects are a rarity, interwoven (7-5) defects form a grain boundary between perfectly ordered grains. These defects introduce a low-angle dislocation, and a detailed analysis of the defects is done. The butterfly eye (Male/Female) is defined with extremely high-density corneal nipple with no apparent grains. Each corneal nipple is a polygon with “n” sides (n = 5, 6, and 7). While the 5- and 7-coordinated defects exist, they do not initiate a grain rotation as seen in the moth eyes. To find out the similarity and the difference in the reflectivity of these nanostructured surfaces, we used the effective medium theory and calculated the reflectivity in moth and butterfly eyes. From this simple analysis, we find that females have better anti-reflective properties compared to the males in both moth and butterfly.

Optical Modelling and Phylogenetic Analysis Provide Clues to the Likely Function of Corneal Nipple Arrays in Butterflies and Moths.

The lenses in compound eyes of butterflies and moths contain an array of nipple-shaped protuberances, or corneal nipples. Previous work has suggested that these nipples increase light transmittance and reduce the eye glare of moths that are inactive during the day. This work builds on but goes further than earlier analyses suggesting a functional role for these structures including, for the first time, an explanation of why moths are attracted to UV light. Using a phylogenetic approach and 3D optical modelling, we show empirically that these arrays have been independently lost from different groups of moths and butterflies and vary within families. We find differences in the shape of nipples between nocturnal and diurnal species, and that anti-glow reflectance levels are different at different wave-lengths, a result thereby contradicting the currently accepted theory of eye glow for predator avoidance. We find that there is reduced reflectance, and hence greater photon absorption, at UV light, which is probably a reason why moths are attracted to UV. We note that the effective refractive index at the end of the nipples is very close to the refractive index of water, allowing almost all the species with nipples to see without distortion when the eye is partially or completely wet and providing the potential to keep eyes dry. These observations provide a functional explanation for these arrays. Of special interest is the finding that their repeated and independent loss across lepidopteran phylogeny is inconsistent with the explanation that they are being lost in the ‘higher’, more active butterflies.

An apposition compound eye adapted for nocturnal vision in the moth midge Clogmia albipunctata (Williston) (Diptera: Psychodidae)

Morphology and anatomy, dark/light adaptational changes and optics of the compound eyes of the nocturnal moth midge Clogmia albipunctata (Williston) are studied. Its apposition type of eye consists of approximately 260 well-separated ommatidia. Each ommatidium features a biconvex corneal lens covered by corneal nipples measuring around 17nm in height; a crystalline cone of the acone type; and an open (laterally fused) rhabdom formed by eight retinular cells (R1-R8). The corneal lens, whose biological significance is addressed, is composed of a thick yellow-coloured inner lens unit (ILU) surrounded by a thin, colourless outer lens unit (OLU). We identified two types of ommatidia: dorsally located T-type ommatidia and ventrally located P-type ommatidia. In the T-type ommatidia, the rhabdomeres of the retinular cells R7 and R8 are centrally located and are arranged in tandem with R7 above R8. In comparison, in the P-type ommatidia, only the R8 rhabdomere is central, whereas the R7 rhabdomere locates in the peripheral ring. Above the distal tip of the rhabdom, the crystalline cone and the PPCs form an aperture that dynamically changes its size under dark/light conditions, thus modulating the amount of light that reaches the photoreceptive layer. The Clogmia albipunctata eye has a low F-number of 1.2, a high interommatidial angle of 11° and a large eye parameter of 4.6μm·rad. The eye is characterized by relatively poor spatial resolution, but exhibits high absolute sensitivity.

An RNAi Screen for Genes Involved in Nanoscale Protrusion Formation on Corneal Lens in Drosophila melanogaster

The "moth-eye" structure, which is observed on the surface of corneal lens in several insects, supports anti-reflective and self-cleaning functions due to nanoscale protrusions known as corneal nipples. Although the morphology and function of the "moth-eye" structure, are relatively well studied, the mechanism of protrusion formation from cell-secreted substances is unknown. In Drosophila melanogaster, a compound eye consists of approximately 800 facets, the surface of which is formed by the corneal lens with nanoscale protrusions. In the present study, we sought to identify genes involved in "moth-eye" structure, formation in order to elucidate the developmental mechanism of the protrusions in Drosophila. We re-examined the aberrant patterns in classical glossy-eye mutants by scanning electron microscope and classified the aberrant patterns into groups. Next, we screened genes encoding putative structural cuticular proteins and genes involved in cuticular formation using eye specific RNAi silencing methods combined with the Gal4/UAS expression system. We identified 12 of 100 candidate genes, such as cuticular proteins family genes (Cuticular protein 23B and Cuticular protein 49Ah), cuticle secretion-related genes (Syntaxin 1A and Sec61 ββ subunit), ecdysone signaling and biosynthesis-related genes (Ecdysone receptor, Blimp-1, and shroud), and genes involved in cell polarity/cell architecture (Actin 5C, shotgun, armadillo, discs large1, and coracle). Although some of the genes we identified may affect corneal protrusion formation indirectly through general patterning defects in eye formation, these initial findings have encouraged us to more systematically explore the precise mechanisms underlying the formation of nanoscale protrusions in Drosophila.

Remarkable crystal and defect structures in butterfly eye nano-nipple arrays

The corneal nipple structures on the eyes of two nymphalid butterfly species (Nymphalis antiopa and Polygonia interrogationis) are analyzed in terms of nipple arrangements and associated defects. The nipple arrays in both species have close-packed hexagonal lattices with lattice parameters of about 200 nm. The most abundant defects observed are 5–7 coordination defects that generate dislocations, dislocation-type low angle and structural unit-like high angle grain boundaries, as well as closed-loop defects. These disordered structures are compared with imperfections found in other 2D and 3D crystal structures, and it is concluded that the defects in the nipple arrays are likely not due to random growth accidents. Instead, they could be the result of geometric constraints due to eye curvature or serve a yet undiscovered purpose in the optical properties of these eyes.

Does the aquatic invertebrate nipple array prevent bubble adhesion? An experiment using nanopillar sheets

The nipple array is a submicrometre-scale structure found on the cuticle surfaces of various invertebrate taxa. Corneal nipples are an antiglare surface in nocturnal insects, but the functional significance of the nipple array has not been experimentally investigated for aquatic organisms. Using nanopillar sheets as a mimetic model of the nipple array, we demonstrated that significantly fewer bubbles adhered to the nanopillar surface versus a flat surface when the sheets were hydrophilic. Many more bubbles adhered to the hydrophobic surface than the hydrophilic surfaces. Bubbles on the body surface may cause buoyancy problems, movement interference and water flow occlusion. Here, bubble repellence is proposed as a function of the hydrophilic nipple array in aquatic invertebrates and its properties are considered based on bubble adhesion energy.

Studying nanostructured nipple arrays of moth eye facets helps to design better thin film solar cells

Nipples on the surface of moth eye facets exhibit almost perfect broadband anti-reflection properties. We have studied the facet surface micro-protuberances, known as corneal nipples, of the chestnut leafminer moth Cameraria ohridella by atomic force microscopy, and simulated the optics of the nipple arrays by three-dimensional electromagnetic simulation. The influence of the dimensions and shapes of the nipples on the optics was studied. In particular, the shape of the nipples has a major influence on the anti-reflection properties. Furthermore, we transferred the structure of the almost perfect broadband anti-reflection coatings to amorphous silicon thin film solar cells. The coating that imitates the moth-eye array allows for an increase of the short circuit current and conversion efficiency of more than 40%.

Always on the bright side of life: anti-adhesive properties of insect ommatidia grating

The surface of some insect eyes consists of arrays of cuticular protuberances, which are 50-300 nm in diameter, and are termed corneal nipples or ommatidia gratings. They were widely reported to reduce the reflectance for normally incident light, contributing to camouflage by reducing glare to predators, while furthermore enhancing the intake of light, which is especially important for nocturnal insects. Our preliminary observations suggest a third function: in contrast to the rest of the body, ommatidia of various insects remain clean, even in a heavy contaminated environment. In order to prove such an anti-contamination hypothesis of these structures, we measured the adhesive properties of polymer moulds of insect ommatidia, and compared these data with control surfaces having the same curvature radii but lacking such a nanostructure. A scanning electron microscope (SEM) study and force measurements using an atomic force microscope (AFM) on the eye surfaces of three different insect species, dragonfly Aeshna mixta (Odonata), moth Laothoe populi (Lepidoptera) and fly Volucella pellucens (Diptera), were undertaken. We revealed that adhesion is greatly reduced by corneal grating in L. populi and V. pellucens when compared with their smooth controls. The smooth cornea of A. mixta showed no statistically significant difference to its control. We assume that this anti-adhesive phenomenon is due to a decrease in the real contact area between contaminating particles and the eye's surface. Such a combination of three functions in one nanostructure can be interesting for the development of industrial multifunctional surfaces capable of enhancing light harvesting while reducing light reflection and adhesion.

The eye of the parthenogenetic and minute moth Ectoedemia argyropeza (Lepidoptera: Nepticulidae)

Ectoedemia argyropeza (Zeller, 1839) possesses a compound eye that exhibits features of both apposition and superposition type eyes. Like apposition eyes, the eye of E. argyropeza lacks a clear-zone, which in superposition eyes separates the distal dioptric from the proximal light-perceiving structures. On the other hand, a tracheal layer around the proximal ends of the rhabdom as well as a well-developed corneal nipple array on the corneal surfaces are features that E. argyropeza shares with the larger moths. Unique, and so far only seen to this extreme degree in any insect, is the hourglass-shape of E. argyropeza's rhabdom, in which two almost equally voluminous regions (one distal, one proximal and formed in both cases by seven rhabdomeres) are connected by a narrow waist-like region of the retinula. An eighth retinula cell, not participating in rhabdom formation, is developed as a basal cell, just above the basement membrane. The eye responds with photomechanical changes to dark/light adaptation, but while the proximal rhabdom moiety slightly expands (as expected) in the dark, the distal rhabdom increases its diameter only upon light-adaptation. Owing to the tandem position of the two rhabdom moities, it is in the light-adapted state that the distally-placed rhabdom is favoured, while the proximal rhabdom plays a more important role at low ambient light levels. With screening pigments withdrawn, tracheal tapetum exposed, and distal rhabdom diameters reduced, the proximal and in the dark enlarged rhabdom is then in a position to capture photons that have entered the eye through not only the ommatidial window above, but other facets as well even in the absence of a clear-zone and superposition optics.

The natural transparency and piezoelectric response of the Greta oto butterfly wing

The Greta oto, or the "glasswing butterfly", is a member of the Ithomiini tribe. Although rare among lepidopteran, the G. oto's wings are naturally transparent. To understand the material properties of natural transparency, the various structures on the surface of the wings, both the transparent and brown "veins" and wing parameter regions were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), reflectance spectrometry, transmission spectrometry and scanning probe microscopy (SPM) in order to investigate their local structure, periodic character, and electromechanical response. Nanosized protuberances in a highly ordered array were found on the surface of the transparent part similar to that of the "corneal nipple array" found in other insects as an antireflective device.

Sexual dimorphism in the compound eye of the moth Operophtera brumata (Lepidoptera, Geometridae)

Abstract. Brachyptery in female moths is a common phenomenon. This article examines the eyes of the moth Operophtera brumata, in which only the males have wings. Both sexes possess eyes of the superposition type, have facets with corneal nipples, and exhibit eyeglow, but the eye of the male has 2174 facets, measures 725 μm in diameter, and has a clear zone of 112‐μm width. That of the female has 1352 facets, measures 557 μm across, and has a 77‐μm‐wide clear zone. There were no significant differences in rhabdom diameters (10 μm) and tracheal sheath development, but male rhabdoms were significantly longer (85 vs. 65 μm). Dark/light adaptational changes manifested themselves in the positions of the distal screening pigment granules: between the cones during darkness and into the clear zone during light adaptation. The combination of a larger radius of curvature, wider clear zone, and longer rhabdoms gives the males a sensitivity at least twice that of the female. The narrower interommatidial angles of the male (2.8° vs. 3.2°) together with the well‐developed tracheal rhabdom sheaths, furthermore, provide the eyes of the males with better resolution. Finally, eye‐glow intensity in males diminishes considerably faster in light than it does in females. The differences between males and females appear to reflect their different roles: males begin to search after sunset in and around stands of trees for the pheromone‐emitting females, which sit on the trunk of the tree under which they had emerged from their pupal case. Flying males encounter rapid light intensity changes and have to evade obstacles and enemies. The females are considerably more sedentary, but once mated, still seek suitable oviposition sites long after the males have ceased searching for them. During this activity, the females also need visual sensitivity and acuity, but less than the actively flying males.

Sexual dimorphism and light/dark adaptation in the compound eyes of male and female Acentria ephemerella (Lepidoptera: Pyraloidea: Crambidae)

In the highly sexual-dimorphic nocturnal moth, Acentria ephemerella Denis & Schiffermuller 1775, the aquatic and win- gless female possesses a refracting superposition eye, whose gross structural organization agrees with that of the fully-winged male. The possession of an extensive corneal nipple array, a wide clear-zone in combination with a voluminous rhabdom and a reflecting tracheal sheath are proof that the eyes of both sexes are adapted to function in a dimly lit environment. However, the ommatidium of the male eye has statistically significantly longer dioptric structures (i.e., crystalline cones) and light-perceiving elements (i.e., rhab- doms), as well as a much wider clear-zone than the female. Photomechanical changes upon light/dark adaptation in both male and female eyes result in screening pigment translocations that reduce or dilate ommatidial apertures, but because of the larger number of smaller facets of the male eye in combination with the structural differences of dioptric apparatus and retina (see above) the male eye would enjoy superior absolute visual sensitivity under dim conditions and a greater resolving power and ability to detect movement during the day. The arrangement of the microvilli in the rhabdom of both genders suggests that their eyes are polarization-sensitive, an ability they would share with many aquatic insects that have to recognize water surfaces. Although sexual recognition in A. ephe- merella is thought to chiefly rely on pheromones, vision must still be important for both sexes, even if the females are wingless and never leave their watery habitat. Females swim actively under water and like their male counterparts, which fly above the surface of the water, they would have to see and avoid obstacles as well as potential predators. This, together with a small incidence of winged females, we believe, could be the reason why the eyes of female A. ephemerella are less regressed than those of other sexually dimorphic moths, like for instance Orgyia antiqua. Another, but difficult to test, possibility is that male and female A. ephemerella have diverged in their behaviour and habitat preferences less long ago than other sexually dimo rphic moths.

Structure and putative function of dark- and light-adapted as well as UV-exposed eyes of the food store pest Psyllipsocus ramburi Sélys-longchamps (Insecta: Psocoptera: Psyllipsocidae)

The psocopteran Psyllipsocus ramburi Sélys-Longchamps can render food stuffs unpalatable and may serve as an intermediate host for cestodes. Its two circular compound eyes consist of about 26 facets, capped by strongly convexly curved corneae of 10–18 μm in diameter. Corneal nipples or interommatidial hairs are not developed. Beneath each corneal lens a cluster of four cone cells, enveloped by two primary pigment cells, separates an ommatidial group of eight retinula cells from the inner corneal surface. Membrane specializations of the retinula cells, known as the microvilli, measure 60 nm in diameter, and collectively make up the rhabdom, which is columnar in shape and has a distal diameter of 4 or 5 μm, depending on whether it is day- or night-adapted. Cone cell lengths measure 4.5 μm during the day and 8.5 μm at night and retinula cell screening pigments closely approach the edge of the rhabdom during the day. A 1-h exposure to UV-A ( λ max=351 nm) of ca. 1200 lx causes an almost total destruction of the photoreceptive membranes of the rhabdom and bleached all retinula cell screening pigments, but not the pigment grains of the primary pigment cells. Calculations, based on the anatomical data, suggest that the eyes are adapted to function under dim light levels, but cannot produce sharp images since their best possible acceptance angles are 22° and 28° in light- and dark-adapted states, respectively. Destruction of vision, likely affecting biorhythm and reproduction, by exposing the insects to UV-A may offer an alternative to the use of chemicals in controlling these insects.

Partial coherence and other optical delicacies of lepidopteran superposition eyes

Superposition eyes are generally thought to function ideally when the eye is spherical and with rhabdom tips in the focal plane of the imaging optics of facet lenses and crystalline cones. Anatomical data as well as direct optical measurements demonstrate that the superposition eyes of moths and skippers often deviate severely from the expected ideal case. Part of the deviation has been attributed to diffraction at the single facet lens, which was taken to be an essential limit to spatial resolution, because light traveling through different facet lenses was assumed to be incoherent. By considering the two-dimensional facet lens lattice, it is here demonstrated that many facets within a superposition aperture transmit coherent light, allowing a much sharper image than possible with single facet lens diffraction. Partial coherence therefore is an important aspect of superposition imaging. It is argued that broadening of the photoreceptor acceptance angles occurs because of optical errors in the facet lens-crystalline cone system other than diffraction. The transmittance of the superposition aperture of moths and skippers is improved by the corneal nipple arrays of the facet lenses, but quantitative assessment shows that the effect is minor.

Light on the moth-eye corneal nipple array of butterflies

The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids.

Fine Structure of the Compound Eye of Mallada basalis (Neuroptera: Chrysopidae)

The morphology of the compound eye of the lacewing Mallada basalis (Walker) was investigated by scanning and transmission electron microscopy. The optical superposition eye is composed of ≈600 ommatidia that are identical in both sexes. The length of an ommatidiumis ≈100 μm. Each ommatidium contains a lens covered with small corneal nipples, 4 cone cells forming a crystalline cone and tract, 2 principal pigment cells that encompass the crystalline cone, and 8 retinula cells (numbers 1–8). Each ommatidium is surrounded by 12 accessory pigment cells, some of which are located in the center of a triangle formed by 3 adjacent ommatidia. Thus, accessory pigment cells belong to part of 3 different ommatidia. The proximal part of the ommatidium is tightly enclosed by the tracheae. In dark adaptation, 7 retinula cells (numbers 1–7) are arranged in a circle at the tip of the crystalline tract. The rhabdomere of retinula cell number 7 lies at the proximal end of the crystalline tract, and number 8 lies at the distal end of the rhabdom column. All 8 retinula cells penetrate the basement membrane and continue as 8 axons. Each ommatidium shows the fused-type rhabdom that is of great advantage to spectral sensitivity. The fine structure of the ommatidia is further discussed in detail.