Central origin of the efferent neurons projecting to the eyes of Limulus polyphemus

Opsin Repertoire and Expression Patterns in Horseshoe Crabs: Evidence from the Genome of Limulus polyphemus (Arthropoda: Chelicerata)

Horseshoe crabs are xiphosuran chelicerates, the sister group to arachnids. As such, they are important for understanding the most recent common ancestor of Euchelicerata and the evolution and diversification of Arthropoda. Limulus polyphemus is the most investigated of the four extant species of horseshoe crabs, and the structure and function of its visual system have long been a major focus of studies critical for understanding the evolution of visual systems in arthropods. Likewise, studies of genes encoding Limulus opsins, the protein component of the visual pigments, are critical for understanding opsin evolution and diversification among chelicerates, where knowledge of opsins is limited, and more broadly among arthropods. In the present study, we sequenced and assembled a high quality nuclear genomic sequence of L. polyphemus and used these data to annotate the full repertoire of Limulus opsins. We conducted a detailed phylogenetic analysis of Limulus opsins, including using gene structure and synteny information to identify relationships among different opsin classes. We used our phylogeny to identify significant genomic events that shaped opsin evolution and therefore the visual system of Limulus We also describe the tissue expression patterns of the 18 opsins identified and show that transcripts encoding a number, including a peropsin, are present throughout the central nervous system. In addition to significantly extending our understanding of photosensitivity in Limulus and providing critical insight into the genomic evolution of horseshoe crab opsins, this work provides a valuable genomic resource for addressing myriad questions related to xiphosuran physiology and arthropod evolution.

Opsins in Limulus eyes: characterization of three visible light-sensitive opsins unique to and co-expressed in median eye photoreceptors and a peropsin/RGR that is expressed in all eyes

The eyes of the horseshoe crab Limulus polyphemus have long been used for studies of basic mechanisms of vision, and the structure and physiology of Limulus photoreceptors have been examined in detail. Less is known about the opsins Limulus photoreceptors express. We previously characterized a UV opsin (LpUVOps1) that is expressed in all three types of Limulus eyes (lateral compound eyes, median ocelli and larval eyes) and three visible light-sensitive rhabdomeric opsins (LpOps1, -2 and -5) that are expressed in Limulus lateral compound and larval eyes. Physiological studies showed that visible light-sensitive photoreceptors are also present in median ocelli, but the visible light-sensitive opsins they express were unknown. In the current study we characterize three newly identified, visible light-sensitive rhabdomeric opsins (LpOps6, -7 and -8) that are expressed in median ocelli. We show that they are ocellar specific and that all three are co-expressed in photoreceptors distinct from those expressing LpUVOps1. Our current findings show that the pattern of opsin expression in Limulus eyes is much more complex than previously thought and extend our previous observations of opsin co-expression in visible light-sensitive Limulus photoreceptors. We also characterize a Limulus peropsin/RGR (LpPerOps1). We examine the phylogenetic relationship of LpPerOps1 with other peropsins and RGRs, demonstrate that LpPerOps1 transcripts are expressed in each of the three types of Limulus eyes and show that the encoded protein is expressed in membranes of cells closely associated with photoreceptors in each eye type. These finding suggest that peropsin was in the opsin repertoire of euchelicerates.

What the clock tells the eye: lessons from an ancient arthropod

Circadian changes in visual sensitivity have been observed in a wide range of species, vertebrates, and invertebrates, but the processes impacted and the underlying mechanisms largely are unexplored. Among arthropods, effects of circadian signals on vision have been examined in most detail in the lateral compound eye (LE) of the American horseshoe crab, Limulus polyphemus, a chelicerate arthropod. As a consequence of processes influenced by a central circadian clock, Limulus can see at night nearly as well as they do during the day. The effects of the clock on horseshoe crab LE retinas are diverse and include changes in structure, gene expression, and rhabdom biochemistry. An examination of the known effects of circadian rhythms on LEs shows that the effects have three important outcomes: an increase in visual sensitivity at night, a rapid decrease in visual sensitivity at dawn, and maintenance of eyes in a relatively low state of sensitivity during the day, even in the dark. All three outcomes may be critically important for species' survival. Specific effects of circadian rhythms on vision will certainly vary with species and according to life styles. Studies of the circadian regulation of Limulus vision have revealed that these effects can be extremely diverse and profound and suggest that circadian clocks can play a critical role in the ability of animals to adapt to the dramatic daily changes in ambient illumination.

Opsin1-2, G(q)α and arrestin levels at Limulus rhabdoms are controlled by diurnal light and a circadian clock

Dark and light adaptation in photoreceptors involve multiple processes including those that change protein concentrations at photosensitive membranes. Light- and dark-adaptive changes in protein levels at rhabdoms have been described in detail in white-eyed Drosophila maintained under artificial light. Here we tested whether protein levels at rhabdoms change significantly in the highly pigmented lateral eyes of wild-caught Limulus polyphemus maintained in natural diurnal illumination and whether these changes are under circadian control. We found that rhabdomeral levels of opsins (Ops1-2), the G protein activated by rhodopsin (G(q)α) and arrestin change significantly from day to night and that nighttime levels of each protein at rhabdoms are significantly influenced by signals from the animal's central circadian clock. Clock input at night increases Ops1-2 and G(q)α and decreases arrestin levels at rhabdoms. Clock input is also required for a rapid decrease in rhabdomeral Ops1-2 beginning at sunrise. We found further that dark adaptation during the day and the night are not equivalent. During daytime dark adaptation, when clock input is silent, the increase of Ops1-2 at rhabdoms is small and G(q)α levels do not increase. However, increases in Ops1-2 and G(q)α at rhabdoms are enhanced during daytime dark adaptation by treatments that elevate cAMP in photoreceptors, suggesting that the clock influences dark-adaptive increases in Ops1-2 and G(q)α at Limulus rhabdoms by activating cAMP-dependent processes. The circadian regulation of Ops1-2 and G(q)α levels at rhabdoms probably has a dual role: to increase retinal sensitivity at night and to protect photoreceptors from light damage during the day.

Spike firing pattern of output neurons of the Limulus circadian clock

The lateral eyes of the horseshoe crab (Limulus polyphemus) show a daily rhythm in visual sensitivity that is mediated by efferent nerve signals from a circadian clock in the crab's brain. How these signals communicate circadian messages is not known for this or other animals. Here the authors describe in quantitative detail the spike firing pattern of clock output neurons in living horseshoe crabs and discuss its possible significance to clock organization and function. Efferent fiber spike trains were recorded extracellularly for several hours to days, and in some cases, the electroretinogram was simultaneously acquired to monitor eye sensitivity. Statistical features of single- and multifiber recordings were characterized via interval distribution, serial correlation, and power spectral analysis. The authors report that efferent feedback to the eyes has several scales of temporal structure, consisting of multicellular bursts of spikes that group into clusters and packets of clusters that repeat throughout the night and disappear during the day. Except near dusk and dawn, the bursts occur every 1 to 2 sec in clusters of 10 to 30 bursts separated by a minute or two of silence. Within a burst, each output neuron typically fires a single spike with a preferred order, and intervals between bursts and clusters are positively correlated in length. The authors also report that efferent activity is strongly modulated by light at night and that just a brief flash has lasting impact on clock output. The multilayered firing pattern is likely important for driving circadian rhythms in the eye and other target organs.

Opsin co-expression in Limulus photoreceptors: differential regulation by light and a circadian clock

A long-standing concept in vision science has held that a single photoreceptor expresses a single type of opsin, the protein component of visual pigment. However, the number of examples in the literature of photoreceptors from vertebrates and invertebrates that break this rule is increasing. Here, we describe a newly discovered Limulus opsin, Limulus opsin5, which is significantly different from previously characterized Limulus opsins, opsins1 and 2. We show that opsin5 is co-expressed with opsins1 and 2 in Limulus lateral and ventral eye photoreceptors and provide the first evidence that the expression of co-expressed opsins can be differentially regulated. We show that the relative levels of opsin5 and opsin1 and 2 in the rhabdom change with a diurnal rhythm and that their relative levels are also influenced by the animal's central circadian clock. An analysis of the sequence of opsin5 suggests it is sensitive to visible light (400-700 nm) but that its spectral properties may be different from that of opsins1 and 2. Changes in the relative levels of these opsins may underlie some of the dramatic day-night changes in Limulus photoreceptor function and may produce a diurnal change in their spectral sensitivity.

Visual efference neuromodulates retinal timing: in vivo roles of octopamine, substance P, circadian phase, and efferent activation in Limulus

Efferent nerves coursing from the brain to the lateral eye of the horseshoe crab, Limulus polyphemus, increase its nighttime sensitivity to light. They release octopamine, which produces a categorical increase of photoreceptor response duration in vitro. Analogous in vivo timing effects on the electroretinogram (ERG) were demonstrated when octopamine was infiltrated into the eye of an otherwise intact animal; nighttime ERGs were longer than daytime ERGs. Related effects on the ERG were produced by daytime electrical stimulation of efferent fibers. Surprisingly, in a departure from effects predicted solely from in vitro octopamine data, nighttime ERG onsets were also accelerated relative to daytime ERG onsets. Drawing on earlier reports, these remarkable accelerations led to an examination of substance P as another candidate neuromodulator. It demonstrated that infiltrations of either modulator into the lateral eyes of otherwise intact crabs increased the amplitude of ERG responses but that each candidate modulator induced daytime responses that specifically mimicked one of the two particular aspects of the timing differences between day- and nighttime ERGs: octopamine increased the duration of daytime ERGs and substance P infiltrated during the day accelerated response onset. These results indicate that, in addition to octopamine's known role as an efferent neuromodulator that increases nighttime ERG amplitudes, octopamine clearly also affects the timing of photoreceptor responses. But these infiltration data go further and strongly suggest that substance P may also be released into the lateral eye at night, thereby accelerating the ERG's onset in addition to increasing its amplitude.

Visual efference in Limulus: in vitro temperature-dependent neuromodulation of photoreceptor potential timing by octopamine and substance P

Efferents from the brain of Limulus course toward its lateral eye and release octopamine and substance P into it. These neurotransmitters have previously been found to act as neuromodulators in this visual system by altering the size of its responses to light. We report here that both also modulate the timing of the receptor potentials (RPs) evoked by brief light flashes and that these timing effects are temperature dependent. Specifically: We extend our previous report that octopamine prolongs ambient RPs in a categorical fashion and here demonstrate that it does the same at colder temperatures. Categorical means that a given RP is either clearly prolonged in a dramatic fashion or its duration is otherwise unremarkable. Octopamine also accelerates the onsets of RPs when they are evoked by weak flashes under cold temperatures. Contrariwise, substance P accelerates RPs at all temperatures and this acceleration dramatically reduces the sluggishness that is otherwise typically present at low temperatures. Quantitative analysis of intensity-response functions also demonstrated that light sensitivity under substance P is significantly augmented. The plain temporal antagonism between these two modulators demonstrates that the visual system of Limulus possesses a well-poised mechanism which could be used to adjust the timing of its neural processing to interface well with the temporal characteristics of those visual stimuli that are currently present.

The eyes of Limulus polyphemus (Xiphosura, Chelicerata) and their afferent and efferent projections

The visual system of the American horseshoe crab Limulus polyphemus (L. polyphemus) is an important preparation for studying the photoresponse, the circadian modulation of the photoresponse and visual information processing. Given its unique position in phylogeny the structure of its visual system also informs studies of the relationships among arthropods and the characteristics of eurarthropods. Much has been learned about the organization of the relatively simple L. polyphemus visual system, but much remains to be discovered. This review summarizes current knowledge of the structure of L. polyphemus eyes and the organization of their afferent and efferent projections and points to important unanswered questions.

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crab Limulus polyphemus Linnaeus, 1758 (Chelicerata, Xiphosura)

Despite ongoing interest into the architecture, biochemistry, and physiology of the visual systems of the xiphosuran Limulus polyphemus, their ontogenetic aspects have received little attention. Thus, we explored the development of the lateral eyes and associated neuropils in late embryos and larvae of these animals. The first external evidence of the lateral eyes was the appearance of white pigment spots-guanophores associated with the rudimentary photoreceptors-on the dorsolateral side of the late embryos, suggesting that these embryos can perceive light. The first brown pigment emerges in the eyes during the last (third) embryonic molt to the trilobite stage. However, ommatidia develop from this field of pigment toward the end of the larval trilobite stage so that the young larvae at hatching do not have object recognition. Double staining with the proliferation marker bromodeoxyuridine (BrdU) and an antibody against L. polyphemus myosin III, which is concentrated in photoreceptors of this species, confirmed previous reports that, in the trilobite larvae, new cellular material is added to the eye field from an anteriorly located proliferation zone. Pulse-chase experiments indicated that these new cells differentiate into new ommatidia. Examining larval eyes labeled for opsin showed that the new ommatidia become organized into irregular rows that give the eye field a triangular appearance. Within the eye field, the ommatidia are arranged in an imperfect hexagonal array. Myosin III immunoreactivity in trilobite larvae also revealed the architecture of the central visual pathways associated with the median eye complex and the lateral eyes. Double labeling with myosin III and BrdU showed that neurogenesis persists in the larval brain and suggested that new neurons of both the lamina and the medulla originate from a single common proliferation zone. These data are compared with eye development in Drosophila melanogaster and are discussed with regard to new ideas on eye evolution in the Euarthropoda.

Mechanisms controlling the sensitivity of the Limulus lateral eye in natural lighting

Electroretinograms were recorded from the horseshoe crab compound eye using a high-intensity light-emitting diode and a whole-eye seawater electrode. Recordings were made from both lateral eyes in natural daylight or in continuous darkness with the optic nerve intact or cut. Recordings from two eyes of the same animal in different conditions facilitated direct comparisons of the effects of diurnal lighting and circadian efferent activity on the daily patterns of sensitivity of the eye. Structural changes appear to account for about half of the total electroretinogram excursion. Circadian input begins about 45 min in advance of sunset and the nighttime sensitivity returns to the daytime values 20 min after sunrise. When the optic nerve is cut, the nighttime sensitivity shows exponential decay over the next 5 or 6 days, consistent with a light-triggered structural light adaptation process unopposed by efferent input. Our results suggest that two mechanisms mediate the increase in lateral eye sensitivity at night-physiological dark adaptation and circadian efferent input. Three mechanisms appear to be involved in mediating the decrease in lateral eye sensitivity during daylight-physiological light adaptation, a continuous structural light adaptation process, and a separate light-triggered, efferent-primed structural light adaptation process.

The phylogenetic significance of crustacean optic neuropils and chiasmata: a re-examination

Recent molecular data challenge the traditional hypotheses of arthropod phylogeny founded on morphologic characters. In this discussion, the structure of the visual systems in Pterygota (Hexapoda) and Decapoda (Malacostraca, Crustacea) is an important argument. Although many components of their visual systems depict structural homology, differences exist between Pterygota/Decapoda on the one side and Branchiopoda (Entomostraca) on the other in that the latter do not have a third optic neuropil or optic chiasmata. Therefore, the goals of the current study were to explore whether the third optic neuropils in Pterygota and Decapoda are homologous, to examine the formation of the first two optic neuropils and the chiasmata in Crustacea, and to compare these processes with Pterygota. For this purpose, five species of entomostracan and malacostracan crustaceans were analyzed by examination of serial sections, fluorescence labeling with phallotoxins, and anti-histamine immunohistochemistry. We found that the chiasmata of Decapoda and Pterygota are characterized by striking similarities regarding both the level of individually identifiable classes of neurons and ontogenetic mechanisms, which are clearly different from those in Branchiopoda. Furthermore, the third optic neuropil of Decapoda and Pterygota, the lobula, shares an ontogenetic protocerebral origin and an innervation by corresponding sets of histamine-immunoreactive neurons, suggesting homology of the lobula in these two groups. In conclusion, the characteristics of the visual system are in conflict with the traditional classification of Arthropoda. Instead, they support a sister-group relationship of Hexapoda and Malacostraca, as suggested by some of the molecular studies.

Circadian efferent input to Limulus eyes: anatomy, circuitry, and impact

Much is known about the anatomy of Limulus retinal efferent neurons and the structural and functional consequences of their activation. Retinal efferent axons arise from cell bodies located in the cheliceral ganglia of the brain, and they project out all of the optic nerves. Their unique neurosecretory-like terminals contact all cell types in lateral eye ommatidia, the retinular cells of the median eye, and the internal rhabdom of ventral photoreceptors. Lateral and median rudimentary photoreceptors are also innervated. The activity of the efferents is circadian. They are active during the subjective night and inactive during the subjective day. Activation of the efferents drives dramatic and diverse changes in the structure and function of Limulus eyes and causes the sensitivity and responsiveness of the eyes to light to increase at night. Relatively little is known about the molecular mechanisms that produce these structural and functional changes, but one efferent-activated biochemical cascade has been identified. The biogenic amine octopamine is released from efferent terminals, and an octopamine-stimulated rise in cAMP in photoreceptors, with a subsequent activation of cAMP-dependent protein kinase, mediates many of the known effects of efferent input. A photoreceptor-specific protein, myosin III, is phosphorylated in response to efferent input; this protein may play a role in the efferent stimulated changes in photoreceptor structure and function. Anatomical, biophysical, biochemical, and molecular approaches are now being effectively combined in studies of Limulus eyes; thus, this preparation should be particularly useful for further detailed investigations of mechanisms underlying the modulation of primary sensory cells by efferent input.

Efferent innervation of photoreceptors in spiders

The anterior median (AM) eye of the nocturnal spider Araneus ventricosus showed a marked circadian oscillation of sensitivity, but that of the diurnal spider Menemerus confusus showed no circadian oscillation. The AM eyes of the noct/diurnal spiders Argiope amoena and A. bruennichii have two types of photoreceptor cells with different sensitivities. The more sensitive cells showed a circadian oscillation of sensitivity, but the less sensitive cells did not. The circadian sensitivity change of the eyes was controlled by efferent neurosecretory fibers in the optic nerve. Illuminating the brain increased the frequency of efferent impulses in the optic nerve of Argiope, showing that certain photosensitive neurons are present in the brain. However, it seemed that the cerebral photosensitive neurons may be different from the efferent neurosecretory cells. The response of the cerebral photosensitive neurons increased transiently following diminution of the light intensity striking the eyes. The interaction between the cerebral photosensitive neurons and the eyes seemed to play a role in increasing this response.

Cellular distributions and functions of histamine, octopamine, and serotonin in the peripheral visual system, brain, and circumesophageal ring of the horseshoe crab Limulus polyphemus

The data reviewed here show that histamine, octopamine, and serotonin are abundant in the visual system of the horseshoe crab Limulus polyphemus. Anatomical and biochemical evidence, including new biochemical data presented here, indicates that histamine is a neurotransmitter in primary retinal afferents, and that it may be involved in visual information processing within the lateral eye. The presence of histamine in neurons of the central nervous system outside of the visual centers suggests that this amine also has functions unrelated to vision. However, the physiological actions of histamine in the Limulus nervous system are not yet known. Octopamine is present in and released from the axons of neurons that transmit circadian information from the brain to the eyes, and octopamine mimics the actions of circadian input on many retinal functions. In addition, octopamine probably has major functions in other parts of the nervous system as octopamine immunoreactive processes are widely distributed in the central nervous system and in peripheral motor nerves. Indeed, octopamine modulates functions of the heart and exoskeletal muscles as well as the eyes. A surprising finding is that although octopamine is a circulating neurohormone in Limulus, there is no structural evidence for its release into the hemolymph from central sites. The distribution of serotonin in Limulus brain suggests this amine modulates the central processing of visual information. Serotonin modulates cholinergic synapses in the central nervous system, but nothing further is known about its physiological actions.

A myosin III from Limulus eyes is a clock-regulated phosphoprotein

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.

Histochemical localization of acetylcholinesterase in the lateral eye and brain of Limulus polyphemus: might acetylcholine be a neurotransmitter for lateral inhibition in the lateral eye?

The distribution of acetylcholinesterase (AChE) in the lateral eye and brain of the horseshoe crab was investigated with histochemical means using standard controls to eliminate butyrylcholinesterase and nonspecific staining. Intense staining was observed in the neural plexus of the lateral compound eye, in the lateral optic nerve, and in various neuropils of the brain. Nerve fibers with moderate to weak staining were widespread in the brain. No somata were stained in either the lateral eye or the brain. The distribution of acetylcholinesterase in the supraesophageal ganglia and nerves of the giant barnacle was also investigated for comparison. Although both the median optic nerve of the barnacle and the lateral optic nerve of the horseshoe crab appear to contain the fibers of histaminergic neurons, only the lateral optic nerve of the horseshoe crab shows AChE staining. Other parts of the barnacle nervous system, however, showed intense AChE staining. These results along with the histochemical controls eliminate the possibility that some molecule found in histaminergic neurons accounted for the AChE staining but support the possibility that acetylcholine might be involved as a neurotransmitter in lateral inhibition in the horseshoe crab retina. Two reasonable neurotransmitter candidates for lateral inhibition, histamine and acetylcholine, must now be investigated.

Octopamine immunoreactive neurons in the fused central nervous system of spiders

Using antisera directed against octopamine (OA), we identified and mapped octopamine-immunoreactive (OA-ir) neurons and their projections in the fused, central ganglion complex of wandering spiders, Cupiennius salei. Labeled cell bodies are concentrated in the subesophageal ganglion complex (SEG) where they are arranged serially in ventral, midline clusters. OA-ir processes from these cells project dorsally. Some neurites end close to segmental septa; others merge into longitudinal tracts connecting the neuromeres. Labeled collaterals leaving these tracts project into peripheral neuropil. In the brain, OA-ir somata were found only in the two cheliceral hemiganglia, where a cluster of 4-5 relatively large cells (soma diameter 25 microns) lies next to a group of small somata (diameter < 10 microns). Neurites originating from the large somata descend into the SEG and merge into longitudinal tracts. The central body of the brain contains profuse ascending projections. Except for fine varicosities that are confined to the roots of nerves, we found no OA-ir fibers leaving the central nervous system (CNS). Within the CNS, however, OA-ir varicosities are concentrated in neuropil and near hemolymph spaces. This distribution suggests that OA acts as a neurotransmitter and/or local neuromodulator at central synapses, while it is also released into the hemolymph and presumably acts hormonally at peripheral sites. Using high-pressure liquid chromatography measurements, the hemolymph was in fact found to contain 12-40 nM of free octopamine.

A circadian clock in the Limulus brain transmits synchronous efferent signals to all eyes

A circadian clock in the brain of the horseshoe crab, Limulus polyphemus, has an important role in the function of the peripheral visual system. At night, the clock transmits neural activity to the lateral, ventral, and median eyes via efferent optic nerve fibers. The activity occurs in synchronous bursts (maximum rate of 2 bursts/s) with individual efferent fibers contributing a single spike in each burst. The circadian efferent activity originates in the protocerebrum. Lateral connections synchronize the efferent activity recorded from the two halves of the protocerebrum, suggesting the existence of bilateral circadian oscillators. Circadian efferent activity survives excision of the brain and isolation of the protocerebrum. We conclude that circadian clock and its complex neural circuitry are fundamental components of the Limulus visual system.

Central projections of Limulus photoreceptor cells revealed by a photoreceptor-specific monoclonal antibody

Studies of lateral, median, and ventral eyes of the chelicerate arthropod Limulus polyphemus (the common American horseshoe crab) are providing important basic information about mechanisms for information processing in the peripheral visual system and for the modulation of visual responses by light and circadian rhythms. The processing of visual information in Limulus brain is less well understood in part because the specific central projections of the various classes of visual neurons are not known. This study describes a mouse monoclonal antibody, 3C6A3, which binds to Limulus photoreceptor cell bodies, their axons, and terminals, but not to any other cell type in the central nervous system. This antibody, and intracellular injection of biocytin, are used to demonstrate the central projections of each type of photoreceptor. Our main conclusions are that: 1) the photoreceptors (retinular cells) of the lateral eye project only to the lamina; 2) the photoreceptors of the lateral rudimentary eye project to both the lamina and medulla; 3) the photoreceptors of the median ocellus project only to the ocellar ganglion; and 4) the photoreceptors of the rudimentary median (endoparietal) eye project to the ocellar ganglion and also into the optic tract. These results, along with previous studies, allow us to infer the projections of the secondary cells. The eccentric cells of the lateral eye project to the lamina, medulla, optic tract, and ocellar ganglion. The arhabdomeral cells of the median ocellus project through the ocellar ganglion and to optic tract to the medulla.