Structural reactions to polarized light of microvilli in photoreceptor cells of the moth Spodoptera

Summary Intact armyworm moths (Spodoptera exempta, Farn. Noctuidae) were illuminated by polarized monochromatic light to induce structural changes in the rhabdomeres of the compound eyes. The degree of distortion of their microvilli depends on the light energy absorbed per time unit. Under polarized light, the number of quanta absorbed varies with the position of the plane of polarization relative to the axis of the microvilli (intrinsic dichroism). Therefore, in Spodoptera, different degrees of deformations could be demonstrated in differently oriented rhabdomeres of both types of ommatidia. Moreover, in rhabdoms of the lobed type with fan-like arranged microvilli, different reactions were regularly seen in differently oriented microvilli of one rhabdomere. This indicates that microvilli may react to light individually.Supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

PHOTORECEPTOR STRUCTURES : III. DROSOPHILA MELANOGASTER

The eyes of three eye mutants of Drosophila melanogaster were fixed and thin sections studied for its structural detail in the electron microscope. Each ommatidium was found to have seven retinula cells with an equal number of rhabdomeres (visual units). The rhabdomeres average 1.2 µ in diameter and 60 µ in length. Each rhabdomere consists of osmium-fixed dense bands averaging 120 A in thickness, and with less dense interspaces 200 to 400 A. There is an average of 23 dense bands or 46 interfaces per micron within the rhabdomere. The rhabdomere as we have presented it is a single structure of packed rods or tubes. The "fine structure" within the rhabdomere is similar to that observed by electron microscopy for the retinula of the house fly, and to the retinal rods of the vertebrate eye, and to the chloroplasts of plant cells in a variety of animal and plant photoreceptor structures. In addition, the radial arrangements within the ommatidium of radially unsymmetrical units, the rhabdomeres, is probably related to the analysis of polarized light in the insect eye.     

Exocytotic shedding and glial uptake of photoreceptor membrane by a salticid spider.

Receptors in the anterior lateral eyes of salticid spiders possess paired rhabdomeres. The tips of the rhabdomeral microvilli lie adjacent to non-pigmented glial processes. Photoreceptor membrane is lost during turnover by a hitherto undescribed process: individual microvilli lengthen at their tips, taper, and are received by corresponding, coated endocytotic pits in the glial membrane. Pits detach as coated vesicles with coherent fragments of microvilli within them, lose their coats, and accumulate in the glial processes as disorderly membranous detritus. Some microvillus membrane disintegrates before local endocytosis, and appears to get into the glial arms distant from the parent rhabdomere by invaginations which are either endocytotic clefts or a tubulo-cisternal system, but whose precise nature is not yet clear. No photoreceptor membrane is lost by pinocytosis into the receptor cytoplasm. Analogies between the behaviour of this system and the phagocytosis of shed vertebrate photoreceptor membrane are briefly discussed.     

粘虫蛾复眼背、腹区视杆结构的差异

根据光学和电子显微镜的观察,粘虫蛾复眼背、腹区域的视杆结构具有以下主要差异:1)背方小眼视杆的长度短于腹方小眼视杆的长度。2)在横切面上,背方小眼视杆的中段近似方形。该段间细胞的视小杆为三角形,每个具有平行排列的微绒毛。整个视杆包含两个互相垂直的微绒毛轴。腹方小眼视杆的中段为风扇形。间细胞的视小杆为“V”字形,微绒毛排列不平行。3)背方小眼基细胞的视小杆几乎位于气管反光层远侧,而腹方小眼甚至延伸到气管反光层内。 在背方和腹方小眼视杆的内段,每个间细胞的微绒毛均平行,且排列在基细胞的大形视小杆周围。更深层,在其它细胞的轴突均已相继出现的水平上,基细胞的大形视小杆仍然可见。 最后,对形态上的特点,在功能上可能具有的一些意义也进行了初步讨论。     

The lateral photoreceptor of the barnacle,Balanus eburneus

Summary The lateral eye of the barnacle, Balanus eburneus, fixed in highly concentrated osmium is a lens-shaped body of approximately 250 m in diameter and about 75 m thick. It contains three photoreceptor cells which occupy about 42% of its volume. The photoreceptor cells are irregularly shaped and extend countless dendritic processes which bear rhabdomeres at their ends. Individual rhabdomeres come into contact with rhabdomeres originating from dendrites of the same or of one of the other visual cells. Thirteen per cent of the volume of the photoreceptor cells is taken up by the rhabdomeres. The membranes of the rhabdomeric microvilli contain globular subunits which suggest a 70 Å spacing of rhodopsin molecules. There are two kinds of glial cells. One kind, type A glial cells, makes contact with the fibrous capsule of the photoreceptor. The other kind, type B glial cells, is associated with the photoreceptor cells and extends countless tiny cytoplasmic extensions which interdigitate with similar extensions of the receptor cells. There are approximately 95 type B glial cells and 130 type A glial cells in the receptor. The cytoplasm of the photoreceptor cells contains countless small Golgi fields, mitochondria, microtubules, multivesicular and multilamellar bodies. The extracellular space of the photoreceptor is less than 0.1% of its total volume.The authors wish to thank Mrs. G. Theisen and Miss D. Hupp for expert technical assistance and Drs. M. Behrens, C. Helrich, and C.C. Krischer for many inspiring discussions. This study was partly supported by the SFB 160 of the Deutsche Forschungsgemeinschaft     

Specialized ommatidia for polarization vision in the compound eye of cockchafers,Melolontha melolontha (Coleoptera,Scarabaeidae)

Summary The superposition eye of the cockchafer, Melolontha melolontha, exhibits the typical features of many nocturnal and crepuscular scarabaeid beetles: the dioptric apparatus of each ommatidium consists of a thick corneal lens with a strong inner convexity attached to a crystalline cone, that is surrounded by two primary and 9–11 secondary pigment cells. The clear zone contains the unpigmented extensions of the secondary pigment cells, which surround the cell bodies of seven retinula (receptor) cells per ommatidium and a retinular tract formed by them. The seven-lobed fused rhabdoms are composed by the rhabdomeres of the receptor cells 1–7. The rhabdoms are optically separated from each other by a tracheal sheath around the retinulae. The orientation of the microvilli diverges in a fan-like fashion within each rhabdomere. The proximally situated retinula cell 8 does not form a rhabdomere. This standard form of ommatidium stands in contrast to another type of ommatidium found in the dorsal rim area of the eye. The dorsal rim ommatidia are characterized by the following anatomical specializations: (1) The corneal lenses are not clear but contain light-scattering, bubble-like inclusions. (2) The rhabdom length is increased approximately by a factor of two. (3) The rhabdoms have unlobed shapes. (4) Within each rhabdomere the microvilli are parallel to each other. The microvilli of receptor 1 are oriented 90° to those of receptors 2–7. (5) The tracheal sheaths around the retinulae are missing. These findings indicate that the photoreceptors of the dorsal rim area are strongly polarization sensitive and have large visual fields. In the dorsal rim ommatidia of other insects, functionally similar anatomical specializations have been found. In these species, the dorsal rim area of the eye was demonstrated to be the eye region that is responsible for the detection of polarized light. We suggest that the dorsal rim area of the cockchafer eye subserves the same function and that the beetles use the polarization pattern of the sky for orientation during their migrations.     

Extracellular shedding of photoreceptor membrane in the open rhabdom of a tipulid fly

Summary The compound eyes of the Australian tipulid fly, Ptilogyna, shed the bulk of their rhabdomeral membrane to extracellular space during turnover. The rhabdomeres of the retinulae lie in a common extracellular space (ECS), which is subdivided in the proximal retina. Before dawn, a distal region of the microvilli in each rhabdomere differentiates and becomes less electron-dense after conventional fixation. The differentiated region then dilates and develops an irregular profile. A few hours after dawn, the transformed tips break off and form a detritus in the ECS. The degraded membrane is internalised back into the retinula cells by mass endocytosis. Retinulae develop pseudopodia at sites bordering the ECS and engulf the membrane detritus, which comes to lie first of all in vacuoles within the receptor cells and then forms very large multivesicular bodies. The latter transform to multilamellar and residual bodies and are, presumably, lysed. Surrounding these secondary lysosomes are rough endoplasmic reticulum and smooth tubular systems, tentatively considered on comparative grounds to provide hydrolases. The literature concerning the ultrastructure of compound eyes offers a small number of instances where extracellular shedding can be suspected for morphological reasons. Attention is drawn to analogies with the shedding of photoreceptor membranes in vertebrate retinae.     

Specialized ommatidia of the polarization-sensitive dorsal rim area in the eye of monarch butterflies have non-functional reflecting tapeta

Many insects exploit sky light polarization for navigation or cruising-course control. The detection of polarized sky light is mediated by the ommatidia of a small specialized part of the compound eye: the dorsal rim area (DRA). We describe the morphology and fine structure of the DRA in monarch butterflies (Danaus plexippus). The DRA consists of approximately 100 ommatidia forming a narrow ribbon along the dorsal eye margin. Each ommatidium contains two types of photoreceptor with mutually orthogonal microvilli orientations occurring in a 2:6 ratio. Within each rhabdomere, the microvilli are well aligned. Rhabdom structure and orientation remain constant at all retinal levels, but the rhabdom profiles, as seen in tangential sections through the DRA, change their orientations in a fan-like fashion from the frontal to the caudal end of the DRA. Whereas these properties (two microvillar orientations per rhabdom, microvillar alignment along rhabdomeres, ommatidial fan array) are typical for insect DRAs in general, we also report and discuss here a novel feature. The ommatidia of monarch butterflies are equipped with reflecting tapeta, which are directly connected to the proximal ends of the rhabdoms. Although tapeta are also present in the DRA, they are separated from the rhabdoms by a space of approximately 55 μm effectively inactivating them. This reduces self-screening effects, keeping polarization sensitivity of all photoreceptors of the DRA ommatidia both high and approximately equal.     

Rapid synthesis of photoreceptor membrane and assembly of new microvilli in a crab at dusk

Summary Large areas of photoreceptor membrane are synthesized in the retinula cells of the crab Leptograpsus variegatus at dusk. Initially, new membrane differentiates from rough endoplasmic reticulum (ER) as large tubules of smooth ER. These tubules transform to concentric ellipsoids of closely apposed pairs of membranes (doublet ER), sometimes passing through an intervening crenate form. The new membrane is transported through bridges of cytoplasm that cross the palisade to the rhabdom region, from which the remains of the rhabdomeres that were built during the previous dusk have been dissolved. The degradation of the old microvilli of one rhabdomere is accomplished without affecting neighbouring rhabdomeres of other cells. New microvilli are assembled in situ from sheets of doublet ER, which are converted to tubules oriented in the same direction as the future microvilli. The cytoplasmic face of the ER remains the cytoplasmic face of the tubules, which become progressively narrower, partly by further longitudinal division, until the final diameter of the microvillus is reached. A central core is often seen in transverse sections of mature microvilli. It may be involved in the final consolidation, but rhabdomeric microvilli are not formed in the same manner as those of intestinal brush border cells. There is no evidence that new membrane passes through the Golgi compartment before incorporation into the rhabdom, as is the case for rod outer segment membrane in vertebrate photoreceptors.     

Fundamental differences in the optical structure of the eyes of nocturnal and diurnal mosquitoes

We have studied the anatomy and optics of the eyes of a range of mosquito species from the wholly dark-active blood-feeding Anopheles gambiae to the diurnal plant-feeder Toxorhynchites brevipalpis. Consistent with studies by Satô in the 1950s, we find that dark-active and crepuscular species have short fused rhabdoms with a conical construction. This maximises the amount of light the rhabdoms receive from the almost hemispherical wide-aperture lenses. Toxorhynchites, on the other hand, has long narrow rhabdomeres that are separated from each other over their entire length, and so resemble the open rhabdoms of advanced flies (Brachycera and Cyclorrhapha). These findings are confirmed by studies of the pseudopupil, whose form indicates the layout of the rhabdomere tips in the focal plane of each ommatidial lens. In anopheline species the pseudopupil is a single undivided ellipse, indicating a fused rhabdom structure, whereas in Toxorhynchites there is a ring of six outer elements surrounding a central one. This means that each rhabdomere views a separate direction in space, and our measurements indicate that, as in higher Diptera, adjacent rhabdomeres share their fields of view with one of the rhabdomeres in the immediately adjacent ommatidia. This in turn means that in the diurnal type of mosquito eye there is a basis for neural superposition, but the fused construction of anopheline rhabdoms precludes this. The Aedes species studied were similar to Anopheles but with lenses of less extreme aperture, and Sabethes cyaneus, a diurnal blood-feeder, was intermediate in structure, with fused conical rhabdoms in the centre of the eye and unfused rhabdomeres around the periphery.     

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.     

Fine structure of the ommatidia and the occurrence of rhabdomeric twist in the dorsal eye of male Bibio marci (Diptera,Nematocera, Bibionidae)

Summary The ommatidia in the dorsal eye of male Bibio marci (March flies) are comprised of eight retinula cells (R1–8). In the distal region, the open rhabdomeres of retinula cells 1–6 are arranged in a symmetrically circular pattern with their microvilli directed radially. Immediately beneath the crystalline cone, cell 7 forms a rhabdomere that is about 1 m long and lies in the center of the circle formed by the rhabdomeres of cells 1–6. For the remaining length of an ommatidium it is replaced by the rhabdomere of retinula cell 8. The cell body of this retinula cell almost encloses its own rhabdomere by forming a deep invagination. Consequently, no ommatidial cavity is present. In the left eye rhabdomeres R 3, 5 and 6 first twist clockwise along their longitudinal axes, while rhabdomeres R1, 2, 4 and 8 twist counterclockwise. Opposite twisting is observed in the right eye. The twist rate varies along the length of the rhabdomeres. In a middle region of 60 m, within which the direction of twist does not change, the maximal twist rates are approximately 2°–5°/m in R1–6 and even higher in R 8. In a proximal region, the direction of twist is reversed, but the initial orientation of the microvilli not reestablished. Both the cross-sectional shape of the rhabdomeres and their geometric arrangement in the retinula change along with the twisting. It is substantiated that the rhabdomeric twist is not due to artifactual deformation.Supported by the Deutsche Forschungsgemeinschaft (SFB 4: E 2)The authors thank Dr. I. de la Motte for providing the material used in this study, Prof. H. Altner for critical discussion and Dr. M. Burrows for his attentive linguistic corrections     

Structure of the visual system of the two-spotted spider-mite, Tetranychus urticae

The pair of eyes on each side of Tetranychus urticae consists of fifteen retinular cells, six pigment cells, six corneal cells, and one vitreous cell. Five rhabdomeres lie beneath the anterior lens, twelve beneath the posterior lens. The pigment and corneal cells appear to determine the positions at which rhabdomeres occur. The volumes of all rhabdomeres have been measured. A pair of fibrous masses are associated with tendons at the dorsal apodemes; the nerves of these ‘apodemal organs’ join the optic nerves en route to the brain. There are a pair of optic neuropiles, each surmounted by a large secretory cell.     

Ommatidial structure in relation to turnover of photoreceptor membrane in the locust

Summary In the compound eye of the locust, Locusta, the cross-sectional area of the rhabdoms increases at dusk by 4.7-fold due to the rapid assembly of new microvillar membrane, and decreases at dawn by a corresponding amount as a result of pinocytotic shedding from the microvilli. The rhabdoms at night have more and longer photoreceptor microvilli than rhabdoms during the day. The orientations of the six rhabdomeres that comprise the distal rhabdom also change. The density of intramembrane particles on the P-face of the microvillar membrane, putatively representing mostly rhodopsin molecules, or aggregates thereof, does not change.An alteration in the size of the ommatidial field-stop, produced by the primary pigment cells, is concomitant with the change in rhabdom size. At night the increase in size of the field-stop must widen the angular acceptance of a rhabdom, increasing the capture of photons from an extended field. Conversely, during the day, when photons are more abundant, its decrease must narrow the acceptance angle, increasing angular resolution. Because of the presence of this field-stop, the optics of the ommatidium would not be greatly affected if the rhabdom were to remain always at its night size. It is argued, therefore, that the variable-size rhabdom must have resulted from some demand other than that of light/dark adaptation.Changes in size and organisation of the rhabdoms in response to various light regimes indicate that: (1) Rapid shedding of photoreceptor membrane is induced by the onset of light, but shedding also occurs slowly in darkness during the day. (2) Microvillar assembly is initiated by the onset of darkness, but also occurs at the normal time of dusk without a change in ambient lighting, provided there has been some light during the day. Therefore, both shedding and assembly of microvillar membrane are affected by the state of illumination, but also appear to be under some endogenous control.     

Role of Spastin in Apical Domain Control along the Rhabdomere Elongation in Drosophila Photoreceptor

Background

Mutations in spastin are the most common cause of hereditary spastin paraplegia, a neurodegenerative disease. In this study, the role of spastin was examined in Drosophila photoreceptor development.

Methodology/Principal Findings

The spastin mutation in developing pupal eyes causes a mild mislocalization of the apical membrane domain at the distal section, but the apical domain was dramatically reduced at the proximal section of the developing pupal eye. Since the rhabdomeres in developing pupal eyes grow from distal to proximal, this phenotype strongly suggests that spastin is required for apical domain maintenance during rhabdomere elongation. This role of spastin in apical domain modulation was further supported by spastin''s gain-of-function phenotype. Spastin overexpression in photoreceptors caused the expansion of the apical membrane domain from apical to basolateral in the developing photoreceptor. Although the localizations of the apical domain and adherens junctions (AJs) were severely expanded, there were no defects in cell polarity.

Conclusions/Significance

These results strongly suggest that spastin is essential for apical domain biogenesis during rhabdomere elongation in Drosophila photoreceptor morphogenesis.     

The Ventral Photoreceptor Cells of Limulus : I. The microanatomy

The ventral photoreceptor cells of Limulus polyphemus resemble the retinular cells of the lateral eyes both in electrical behavior and in morphology. Because of the great size of the ventral photoreceptor cells they are easy to impale with glass capillary micropipettes. Their location along the length of the ventral eye nerve makes them easy to dissect out and fix for electron microscopy. Each cell has a large, ellipsoidal soma that tapers into an axon whose length depends upon the distance of the cell from the brain. The cell body contains a rich variety of cytoplasmic organelles with an especially abundant endoplasmic reticulum. The most prominent structural feature is the microvillous rhabdomere, a highly modified infolding of the plasmalemma. The microvilli are tightly packed together within the rhabdomere, and quintuple-layered junctions are encountered wherever microvillar membranes touch each other. Glial cells cover the surface of the photoreceptor cell and send long, sheet-like projections of their cytoplasm into the cell body of the photoreceptor cell. Some of these projections penetrate the rhabdomere deep within the cell and form quintuple-layered junctions with the microvilli. Junctions between glial cells and the photoreceptor cell and between adjacent glial cells are rarely encountered elsewhere, indicating that there is an open pathway between the intermicrovillous space and the extracellular medium. The axon has a normal morphology but it is electrically inexcitable.     

THE MICROSTRUCTURE OF THE COMPOUND EYES OF INSECTS

The apposition eyes of two diurnal insects, Sarcophaga bullata (Diptera) and Anax junius (Odonata), have been examined with the electron microscope. In the latter case only the rhabdom is described. The rhabdom of the fly consists of a central matrix and seven rhabdomeres, one for each retinula cell. The rhabdomeres show an ordered internal structure built up of transverse tubes, hexagonal in cross-section. These slender compartments running the width of the rhabdomere are 370 A in diameter. After fixation with osmium tetroxide the walls of the compartments are more electron dense than the interiors. The retinula cells contain mitochondria, and pigment granules smaller than those found in the pigment cells. These granules tend to cluster close behind the membranes which separate the retinula cells from their rhabdomeres. The rhabdom of the dragonfly is a single structure which appears to be composed of three fused "rhabdomeres," each similar to a rhabdomere of Sarcophaga. Reasons are given for believing that the rhabdom may be the site of photoreception, as well as the organ for analyzing plane-polarized light, as suggested by other workers.     

Direction ofE for maximum response of a retinula cell

Summary Except for very special fused rhabdoms, e. g. those with orthogonal microvilli like the worker bee, the direction of the electric vector E of linear polarized light necessary for a maximum response from a retinula cell is not parallel (or perpendicular) to the microvilli of the recorded cell. This is because the rhabdomeres of a fused rhabdom are optically coupled, i. e. the properties of each rhabdomere influence the manner in which light is transmitted down the composite rhabdom structure. A rhabdom is analogous to a non-uniform absorbing optical crystal. Such a crystal has two coordinate (optical) axes along which E remains linear polarized as it propagates. Only when the microvilli of the recorded cell are parallel to one of these axes will the direction ofE for maximum retinula cell response be parallel to the microvilli. The locust-type of rhabdom is used as an example.     

Disruption of insect photoreceptor membrane by divalent ions: Dissimilar sensitivity of light- and dark-adapted mosquito rhabdomeres

Summary The conditions that lead to the formation of myelin figures in rhabdomere microvilli were studied in the larval ocelli of the mosquito Aedes aegypti. These artifacts can result from the addition of divalent ions, such as Ca²⁺, to primary-aldehyde fixatives, but they form subsequently during postfixation with OsO₄. In light-adapted ocelli, myelin figures are concentrated at the proximal ends of the microvilli along the cytoplasmic margin of the rhabdomere. The severity of the artifact is proportional to the ion concentration: scattered myelin whorls are induced by Ca²⁺ concentrations as low as 5 mM; they become abundant at 15 mM to 25 mM, and displace much of the rhabdomere margin at 50 mM. In contrast, even at high concentrations of Ca²⁺ few membrane whorls form in dark-adapted rhabdomeres, and these are mostly located at the distal ends of the microvilli. The differential response of the rhabdomere microvilli in light and darkness does not result from a direct action of light during fixation; it reflects an underlying difference between light- and dark-adapted photoreceptor membranes. We suggest that this differential sensitivity to divalent ions is associated with the shedding of membranes from the rhabdomere, a process that is enhanced by light and reduced in darkness.This work was supported by a grant (BNS 76-18623) from the National Science Foundation     

Organisation of the compound eye of a tipulid fly during the day and night

Summary The ultrastructure of the compound eye of the Australian tipulid fly,Ptilogyna spectabilis, is described. The ommatidia are of the acone type. The rhabdom corresponds to the basic dipteran pattern with six outer rhabdomeres from retinular cells 1–6 (R1-6) that surround two tiered central rhabdomeres from R7 and 8. Distally, for about 8 m, the rhabdom is closed. For the remainder, where the rhabdomere of R8 replaces that of R7, the rhabdom is open, and the rhabdomeres lie in a large central ommatidial extracellular space. In the proximal two thirds of the rhabdom, the central space is partitioned by processes from the retinular cells so that the individual rhabdomeres are contained in pockets.At night the rhabdom abuts the cone cells, but during the day it migrates some 20 m proximally and is connected to a narrow (1–2 m) cone cell tract. This tract is surrounded by two primary pigment cells, which occupy a more lateral position at night and thus act like an iris. Pigment in secondary pigment cells also migrates so as to screen orthodromic light above the rhabdom during the day. Between midday and midnight, the rhabdom changes in length and cross-sectional area as a result of asynchrony of the shedding and synthetic phases of photoreceptor membrane turnover. The effects of these daily adaptive changes on photon capture ability are discussed with regard to the sensitivity of the eye.