Structural organization of xanthine crystals in the median ocellus of a member of the ancestral insect group Archaeognatha

Biogenic purine crystals function in vision as mirrors, multilayer reflectors and light scatterers. We investigated a light sensory organ in a primarily wingless insect, the jumping bristletail Lepismachilis rozsypali (Archaeognatha), an ancestral group. The visual system of this animal comprises two compound eyes, two lateral ocelli, and a median ocellus, which is located on the front of the head, pointing downwards to the ground surface.We determined that the median ocellus contains crystals of xanthine, and we obtained insights into their function. To date, xanthine biocrystals have only been found in the Archaeognatha. We performed a structural analysis, using reflection light microscopy, cryo-FIB-SEM, microCT and cryo-SEM. The xanthine crystals cover the bottom of a bowl-shaped volume in the median ocellus, in analogy to a tapetum, and reflect photons to light-sensitive receptors that are spread in the volume without apparent order or preferential orientation. We infer that the median ocellus operates as an irregular multifocal reflector, which is not capable of forming images. A possible function of this organ is to improve photon capture, and by so doing assess distances from the ground surface when jumping by determining changes in the intensity and contrast of the incident light.     

Remodeling of the Nauplius Eye into the Adult Ocelli during Metamorphosis of the Barnacle, Balanus amphitrite hawaiiensis

The planktonic barnacle larva has a single median ocellus (nauplius eye), while the adult possesses two distinct sets of photoreceptors; a pair of lateral ocelli and a single median ocellus. The nauplius eye of the cypris larva of Balanus amphitrite hawaiiensis is composed of 14 visual cells grouped into three components (a pair of lateral components and a single ventral component) surrounding two centrally located pigment cells; each lateral component consists of 5 visual cells and the ventral component, 4 visual cells. In each component, the rhabdom is made up of apposing microvilli arising directly from the neighboring visual cell bodies.
During metamorphosis into the adult form, the three components of the median ocellus become separated. Each lateral component migrates laterally on the mantle and is remodeled into the adult lateral ocellus, losing two visual cells but gaining new pigment and tapetum cells in the process. The ventral component remains in the mid portion and becomes the adult median ocellus without fundamental modification in composition. The visual cells in both ocelli undergo a marked increase in volume and form many finger-like dendrites. Rhabdomes are made up of interdigitating microvilli arising from the the dendrite tips.     

The organization of honeybee ocelli: Regional specializations and rhabdom arrangements

We have re-investigated the organization of ocelli in honeybee workers and drones. Ocellar lenses are divided into a dorsal and a ventral part by a cusp-shaped indentation. The retina is also divided, with a ventral retina looking skywards and a dorsal retina looking at the horizon. The focal plane of lenses lies behind the retina in lateral ocelli, but within the dorsal retina in the median ocellus of both workers and drones. Ventral retinula cells are ca. 25 μm long with dense screening pigments. Dorsal retinula cells are ca. 60 μm long with sparse pigmentation mainly restricted to their proximal parts. Pairs of retinula cells form flat, non-twisting rhabdom sheets with elongated, straight, rectangular cross-sections, on average 8.7 μm long and 1 μm wide. Honeybee ocellar rhabdoms have shorter and straighter cross-sections than those recently described in the night-active bee Megalopta genalis. Across the retina, rhabdoms form a fan-shaped pattern of orientations. In each ocellus, ventral and dorsal retinula cell axons project into two separate neuropils, converging on few large neurons in the dorsal, and on many small neurons in the ventral neuropil. The divided nature of the ocelli, together with the particular construction and arrangement of rhabdoms, suggest that ocelli are not only involved in attitude control, but might also provide skylight polarization compass information.     

Ultraviolet light and puparial weight as factors in the autogeny of the fleshfly, Sarcophaga falculata

Light conditions and the puparial weight were assessed as factors that affect autogeny in Sarcophaga falculata. The frequency and degree of autogeny differed in flies originating from large or small puparia. The limitation of puparial weight range did not prevent the variation in the stage of autogeny. Autogeny was low in flies kept in complete darkness and was very high in uv irradiated flies. Removal of the ocelli before u.v. irradiation prevented most of the increase in autogeny, and it is assumed that the ocelli are the main transmitter of the u.v. effect on oögenesis. Neurosecretion was found within the cells of the ocellar nerve of adult flies. It is possible that this neurosecretion plays a rôle in the u.v. induced effect on autogeny.     

昆虫单眼的结构和功能

大多数昆虫的视觉器官除了复眼外还有一些简单的小眼,称为单眼。昆虫成虫和半变态类若虫的单眼称为背单眼,位于头顶两复眼之间。背单眼在数目和结构上都有较大变化,但基本结构包括角膜晶体、一层角膜生成细胞(覆盖在角膜晶体上)、视网膜(由大约1000个感光细胞构成,视类群而不同)。背单眼对弱光比较敏感,但在图像感知方面的作用并不显著;它是一种“激发器官”,可以增加复眼的感知能力。全变态昆虫的幼虫既没有复眼也没有背单眼,但在其头部两侧有些类似复眼小眼的侧单眼。侧单眼的结构也与小眼相似,包括角膜,晶体和由一些视网膜细胞组成的视杆。侧单眼是完全变态类昆虫幼虫仅有的感光器官,与复眼一样,它们可以感知颜色、形状、距离等等。     

Role of moth ocelli in timing flight initiation at dusk

ABSTRACT. The role of ocelli in timing flight initiation of cabbage looper moths, Trichoplusia ni (Hübner) (Noctuidae), at dusk was studied under simulated sunset conditions using a multichannel actograph. The mean time of flight initiation was determined for control, sham (sham/occluded, sham ablated, unilateral ablated) and experimental (ocellus occluded, ocellus ablated moths). Ocellus occlusion delayed flight initiation on the first day following treatment but was less effective on the subsequent days. Ocellus ablation also delayed flight initiation on the first day, and also produced pronounced delays on subsequent days. In studies where the sunset was advanced 1 h, control sham ocellus occluded and unilaterally ocellus ablated moths responded to the advance, but bilaterally ocellus ablated moths did not. These results indicate that moths make use of input from the ocelli in determining the threshold light intensity for flight and in making adjustments to small light-phase changes.     

Ultrastructural investigations of presumed photoreceptive organs in two Saccocirrus species (polychaeta,saccocirridae)

In addition to the pigmented ocelli, four different types of photoreceptor-like organs without shading pigment have been found in Saccocirrus papillocercus and S. krusadensis. The sensory cells of these presumed ocelli are either ciliary or rhabdomeric with ciliary rudiments. With the exception of the multicellular type-2 ocelli they are bicellular consisting of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the sensory elements. In the type-2 ocellus, 7 supportive cells form an ovoid cavity leaving openings through which dendritic processes of an equal number of sensory cells enter the cavity. The pigmented ocelli possess an ocellar cavity communicating with the exterior through a pore in the eyecup, ciliary rudiments in both sensory and supportive cell, and additional non-photoreceptive sensory cells in the opening of the eyecup. The sensory organs show characteristic differences between the two species, such as presence or absence of a particular type of ocellus (type 2 is absent in S. krusadensis, type 3 in S. papillocercus), number of cilia in type-4 ocelli, density of microvilli, number of non-photoreceptive sensory cells in the pore of the pigmented ocellus, etc. These differences provide important characters which can be used for discrimination either of species or of subgeneric taxa in Saccocirrus. The phylogenetic significance of the different photoreceptive organs is discussed.     

The fine structure of a nauplius eye

Summary The nauplius eye of the cyclopoid copepod Macrocyclops albidus has been studied by means of the electron microscope. It is composed of 1 ventral and 2 dorsal ocelli. Each dorsal ocellus consists of a large, pigmented cell, 2 tapetal cells which form a hemispherical cup and are tightly packed with crystals, 9 retinula cells and 5 conjunctival cells. The retinula cells have large masses of endoplasmic reticulum, which can be found in two distinct distributional states, also numerous bodies composed of variously coiled membranes, large amounts of glycogen, mitochondria and scattered neurotubules. The light-sensitive brush borders of these cells are closely coapted and form the irregularly shaped rhabdome. Each of the 9 retinula cells sends an axon by one of three routes to the protocerebrum. In addition, a dendrite emerges from the protocerebrum, enters the ocellus and ends blindly in immediate vicinity to the rhabdome. The observations concerning the structure of the eye made in the present study have been compared to those of light microscopical investigations. Comparison of structure and probable function of the nauplius eye and other arthropod eyes has led to consideration of the probable mode of synaptic transmission between primary and secondary sensory neurons in the ocellus, i.e. between retinula cells and eccentric cell dendrite, and various morphological features that might be of importance in this connection.Supported in part by Postdoctoral Fellowship 41044 and Research Grant G-23972 from the National Science Foundation, and Research Grant HE-005129-04 from the National Institutes of Health to the Oregon Regional Primate Research Center.     

Identification of functional sine oculis motifs in the autoregulatory element of its own gene, in the eyeless enhancer and in the signalling gene hedgehog

In Drosophila, the sine oculis (so) gene is important for the development of the entire visual system, including Bolwig's organ, compound eyes and ocelli. Together with twin of eyeless, eyeless, eyes absent and dachshund, so belongs to a network of genes that by complex interactions initiate eye development. Although much is known about the genetic interactions of the genes belonging to this retinal determination network, only a few such regulatory interactions have been analysed down to the level of DNA-protein interactions. Previous work in our laboratory identified an eye/ocellus specific enhancer of the sine oculis gene that is directly regulated by eyeless and twin of eyeless. We further characterized this regulatory element and identified a minimal enhancer fragment of so that sets up an autoregulatory feedback loop crucial for proper ocelli development. By systematic analysis of the DNA-binding specificity of so we identified the most important nucleotides for this interaction. Using the emerging consensus sequence for SO-DNA binding we performed a genome-wide search and have thereby been able to identify eyeless as well as the signalling gene hedgehog as putative targets of so. Our results strengthen the general assumption that feedback loops among the genes of the retinal determination network are crucial for proper development of eyes and ocelli.     

The Spectral Sensitivities of Single Receptor Cells in the Lateral, Median, and Ventral Eyes of Normal and White-Eyed Limulus

Spectral sensitivity curves can be distorted by screening pigments. We have determined whether this is true for Limulus polyphemus by determining, from receptor potentials recorded using intracellular microelectrodes, spectral sensitivity curves for normal animals and for white-eyed animals (which lack screening pigment). Our results show: (a) In median ocelli, the curve for UV-sensitive receptor cells peaks at 360 nm and does not depend on the presence of screening pigment, (b) The curve for ventral eye photoreceptors is identical to that for retinular cells from the lateral eyes of white-eyed animals and peaks at 520–525 nm. (c) In normal lateral eyes, when the stimulating light passes through screening pigment, the curve indicates relatively more sensitivity in the red region of the spectrum than does the curve for white-eyed animals. Therefore, the screening pigment is probably red-transmitting, (d) In median ocelli, the curve for visible-sensitive cells peaks at 525 nm and is approximately the same whether the ocelli are from normal or white-eyed animals. However, the curve is significantly broader than that for ventral eyes and for lateral eyes from white-eyed animals.     

Intracerebral ocelli in an amphipod: extraretinal photoreceptors of the sandhopper Talitrus saltator (Crustacea, Amphipoda)

Abstract. No morphological clues on the amphipod head indicate the existence of ocelli. However, as in several isopod species studied so far, two rudimentary photoreceptors are integrated into the medio-dorsal part of the brain. This electron microscopical study of the photoreceptors is the first report on the presence of ocelli in amphipods. Each ocellus is made up of 3 receptor cells which contribute to the formation of a photoreceptive surface (the rhabdom) formed by tightly packed microvilli. The rhabdoms are twisted and irregular in outline. Membrane turnover is suggested by the presence of different kinds of lysosomes. Lacking dioptric lenses, these photoreceptors are not likely to be involved in image formation but may function as appraisers of ambient light intensity. Physiological and behavioral studies will, henceforth, have to take into account these unexpected ocelli, which may represent remnants of the naupliar eye.     

The spectral sensitivities of the dorsal ocelli of cockroaches and honeybees; an electrophysiological study

The spectral sensitivities of the dorsal ocelli of cockroaches (Periplaneta americana, Blaberus craniifer) and worker honeybees (Apis mellifera) have been measured by electrophysiological methods. The relative numbers of quanta necessary to produce a constant size electrical response in the ocellus were measured at various wave lengths between 302 and 623 mµ. The wave form of the electrical response (ERG) of the dark-adapted roach ocellus depends on the intensity but not the wave length of the stimulating light. The roach ocellus appears to possess a single photoreceptor type, maximally sensitive about 500 mµ. The ERG's of bee ocelli are qualitatively different in the ultraviolet and visible regions of the spectrum. The bee ocellus has two types of photoreceptor, maximally sensitive at 490 mµ and at about 335 to 340 mµ. The spectral absorption of the ocellar cornea of Blaberus craniifer was measured. There is no significant absorption between 350 and 700 mµ.     

The role of the ocelli in the phototactic behaviour of the haematophagous bug Triatoma infestans

In addition to compound eyes, most adult insects posses two or three simple eyes, the ocelli. The function of these photoreceptors remains elusive in most cases. Triatomine bugs posses two well-developed ocelli, located in a latero-dorsal position, behind the compound eyes. We tested the role of the ocelli in the phototactic behaviour of Triatoma infestans, by measuring the time spent by adult males in the dark half of an experimental arena, which had the other half illuminated. The occlusion of the ocelli or the compound eyes alone had little effect on the phototactic response of the bugs. Only those insects which had both their ocelli and compound eyes occluded showed a significant reduction in their negative response to light. The ability of the ocelli of T. infestans to mediate the phototactic response by themselves (i.e., not through the modulation of compound eyes sensitivity) constitutes the first report on this function in insects.     

Fine structure of the eyes of Pseudoceros canadensis (Turbellaria,Polycladida)

Summary The cerebral and epidermal ocelli of the Müller's larva and the cerebral and tentacular eyes of the adult turbellarian Pseudoceros canadensis were studied by electron microscopy. The right cerebral ocellus of the larva consists of one cup-shaped pigmented cell and three sensory cells that bear microvilli. The left cerebral eye of the larva has the above named cells plus a sensory cell with many cilia. Evolutionary significance is attributed to the presence of both ciliary and microvillar photoreceptors in an eye of a flatworm. The one epidermal ocellus of the larva is composed of two cells: a cup-shaped pigmented one bearing flattened cilia, the presumed photoreceptors, and a cell above the cup that adds a few nonciliary lamellae to the stack of ciliary ones from the pigmented cell. The adult eyes contain only microvillar receptors; cilia were not observed.     

Optical properties of the ocelli of Calliphora erythrocephala and their role in the dorsal light response

The 3 ocelli of the blowfly Calliphora erythrocephala, grouped close together on the top of the head (Fig. 1), have large, extensively overlapping visual fields. Together they view the entire upper hemisphere of the surroundings plus part of the lower hemisphere (Figs. 5, 7). It is shown for the lateral ocelli that despite the underfocussing of the ocellar lens large patterns are imaged on the receptor mosaic. Because of the astigmatism of the lens, patterns in longitudinal orientations are more accurately represented than in others (Fig. 3). Nevertheless, an artifical horizon rotated around the long axis of the animal does not elicit head roll. Likewise, changes of overall brightness in the visual field of the median and one lateral ocellus elicit only weak phasic-tonic dorsal light responses of the animal which supplement the tonic dorsal light responses mediated by the compound eyes (Figs. 9, 10). Our results show that, in Calliphora, the ocelli have little influence on head orientation during flight, and must be assumed to serve other functions.Abbreviations body pitch angle - head-tilt angle - DNOVS descending neuron of the ocellar and vertical cell systems - HR head roll - spatial wavelength - R roll angle - SD standard deviation     

Homology of the Lateral Eyes of Scorpiones: A Six-Ocellus Model

Scorpions possess two types of visual organs, the median and lateral eyes. Both eyes consist of simple ocelli with biconvex lenses that differ in structure and function. There is little variation in the number of median ocelli across the order. Except for a few troglomorphic species in which the median ocelli are absent, all scorpions possess a single pair. In contrast, the number of pairs of lateral ocelli varies from zero to five across Scorpiones and may vary within species. No attempt has been made to homologize lateral ocelli across the order, and their utility in scorpion systematics has been questioned, due to the variation in number. A recent study examined the number of lateral ocelli among various Asian Buthidae C.L. Koch, 1837 and proposed a “five-eye model” for the family. This model has not been examined more broadly within Buthidae, however, nor compared with the patterns of variation observed among other scorpion families. An eyespot, referred to as an accessory lateral eye, situated ventral or posteroventral to the lateral ocelli, has also been reported in some scorpions. Analysis of its structure suggests it serves a nonvisual function. We present the first comparative study of variation in the lateral ocelli across the order Scorpiones, based on examination of a broad range of exemplar species, representing all families, 160 genera (78%), 196 species (9%), and up to 12 individuals per species. We propose a six-ocellus model for Recent scorpions with four accessory ocelli observed in various taxa, homologize the individual ocelli, and correct erroneous counts in the recent literature. We also investigate the presence of the eyespot across scorpions and discover that it is more widespread than previously recognized. Future work should investigate the genetic and developmental mechanisms underlying the formation of the lateral ocelli to test the hypotheses proposed here.     

Electrical activities of a type of electroretinogram recorded from the ocellus of a jellyfish, Polyorchis penicillatus (Hydromedusae)

An electroretinogram (ERG), evoked by light stimuli, was recorded from ocelli of Polyorchis penicillatus (Hydromedusae). The ERG is a polyphasic response with a positive potential change at the onset of illumination followed by a slower biphasic pulse, and a positive deflection at the cessation of illumination which is followed by a series of high-frequency pulses. The most striking features of the initial pulse are its latency-log intensity relation and the gradation of pulse amplitude with respect to the intensity of the light stimulus and to different wavelengths. Maximum spectral sensitivity lies around 530 nm. Response patterns induced by shadowing and repeated stimulation of light- and dark-adapted ocelli are described. Morphological structures which could give rise to the mass response of the ocellus 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 degrees and 28 degrees, 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.     

Ultrastructure of Presumed Ocelli in Parenterodrilus taenioides (Polychaeta,Protodrilidae) and their Phylogenetic Significance

Abstract Three different types of presumed unpigmented ocelli have been found in the anterior end of Parenterodrilus taenioides, a small gutless interstitial polychaete. The type-1 ocelli are located in the palps and four ocelli have been found along the length of each palp. The type-2 and type-3 ocelli lie close together in the head segment and are located in posterior ganglionic expansions of the brain. There is one pair of the minute type-2 ocelli but at least two pairs of the type-3 organs, which are the largest ocelli. In each ocellus the sensory cells are of the ciliary type and possess two cilia whose plasma membranes branch into numerous microvilli. With the exception of the type-1 ocelli they consist of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the densely packed ciliary branches. The type-1 ocelli consist of a single cell forming an intracellular vacuole (phaosome) which contains less densely packed microvillus-like structures. In particular, the structure of these ocelli is compared with that in other polychaetes, with special emphasis on the remaining genera of the Protodrilida.     

A re-examination of circadian rhythmicity in Carausius morosus

Rhythms of egg laying and locomotory activity in C. morosus show a 12 hr phase shift from L : D 12 : 12 to free-running conditions. Both egg laying and locomotory activity are entrained by a red light : dark 12 : 12 light régime. Egg laying is entrained by L : D 12 : 12 in insects with eyes excised suggesting the presence of an extraocular light receptor.