Lobula plate and ocellar interneurons converge onto a cluster of descending neurons leading to neck and leg motor neuropil in Calliphora erythrocephala

Summary In the fly, Calliphora erythrocephala, a cluster of three Y-shaped descending neurons (DNOVS 1–3) receives ocellar interneuron and vertical cell (VS4–9) terminals. Synaptic connections to one of them (DNOVS 1) are described. In addition, three types of small lobula plate vertical cell (sVS) and one type of contralateral horizontal neuron (Hc) terminate at DNOVS 1, as do two forms of ascending neurons derived from thoracic ganglia. A contralateral neuron, with terminals in the opposite lobula plate, arises at the DNOVS cluster and is thought to provide heterolateral interaction between the VS4–9 output of one side to the VS4–9 dendrites of the other. DNOVS 2 and 3 extend through pro-, meso-, and metathoracic ganglia, branching ipsilaterally within their tract and into the inner margin of leg motor neuropil of each ganglion. DNOVS 1 terminates as a stubby ending in the dorsal prothoracic ganglion onto the main dendritic trunks of neck muscle motor neurons. Convergence of VS and ocellar interneurons to DNOVS 1 comprises a second pathway from the visual system to the neck motor, the other being carried by motor neurons arising in the brain. Their significance for saccadic head movement and the stabilization of the retinal image is discussed.     

Ultrastructure and development of the rhabdomeric eyes in Lineus viridis (Heteronemertea, Nemertea)

Nemerteans are undoubtedly members of the Spiralia, although their phylogenetic relationships are still a matter of debate. The apparently acoelomate organization suggests a relationship with the platyhelminths, whereas the blood-vascular system has been interpreted as an equivalent to coelomic cavities of annelids, indicating a close relation between annelids and nemerteans. Like other spiralians, most nemertean species are known to have one or several pairs of rhabdomeric and subepidermally situated eyes when adult. The development of these eyes as well as the mode in which the eyes are multiplied is as yet unknown. This is the first attempt to investigate eye formation in a nemertean. In the heteronemertean Lineus viridis (Müller, 1774) the everse rhabdomeric eyes are located deeply underneath the epidermis and consist of a few pigment cells that form a cup-like structure with interdigitating processes that contain numerous pigment granules. In hatchlings, the optical cavity contains processes of 12 sensory cells, each bearing a single cilium and various microvilli. The perikarya of these cells are located distally from the pigment cup. During further development the number of cells increases. Eye development starts with a small anlage situated underneath the epidermis, irrespective of whether this is the first eye or any additional one. The anlage consists of five unpigmented cells and three dendritic processes, each bearing apical microvilli and a single cilium. There is no evidence for an epidermal origin of the eyes. In L. viridis eye formation resembles that described in platyhelminths in which eyes also develop as cerebral derivatives. Although this result has the potential to influence the discussion on the position of Nemertea, the data have to be interpreted with care, since development of L. viridis is derived within the Nemertea.     

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.     

Die Ultrastruktur der Photorezeptoren von Lithobius forficatus L. (Chilopoda: Lithobiidae)

Zusammenfassung Die Untersuchung der feinstrukturellen Organisation der Ocellen von Lithobius forficatus L. ergab, daß der dioptrische Apparat aus einer lamellenartig gebauten, ungleichseitig bikonvexen Cornealinse besteht. Ein Glaskörper und spezielle Pigmentzellen fehlen. Der Augenbecher wird von 35–110 Sinneszellen gebildet, unter denen 2 morphologisch distinkte Typen unterschieden werden können. Die großen Sehzellen des distalen Bereiches besitzen einseitig inserierende Rhabdomere, die in radiärsymmetrischer Anordnung ein umfangreiches geschlossenes Rhabdom bilden. Der proximale Teil des Augenbechers wird von kleineren, konischen Basalzellen in Form einer undeutlich abgesetzten Retinula eingenommen. Durch enge Verzahnung ihrer zirkumapikal oder zweiseitig angeordneten Mikrovilli entstehen stelzenförmige Doppel- und Mehrfachrhabdomere, die mit dem zentralen Rhabdom in Verbindung stehen. Alle Sehzellen sind durch eine Gliederung in verschiedene Zonen gekennzeichnet. Sie sind bei distalen Rezeptoren senkrecht zur optischen Achse, bei Basalzellen transversal zur Längsachse der Zelle angeordnet. Auf die rhabdomerischen Mikrovilli des Augenzentrums folgt nach außen eine Schaltzone aus Elementen des ER und anderen vesilukären Bildungen. Diese Schaltzone stellt wahrscheinlich eine mit dem Adaptationszustand des Auges korrelierte Funktionsstruktur dar. In der cytoplasmatischen Zone fällt die Zahl verschiedenartiger multivesikulärer Korpuskel neben wenigen großen multilamellären Körpern auf. Die funktionelle Bedeutung des Ocellusaufbaus bei Lithobius wird diskutiert.

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Ultrastructure of the Photoreceptors of Lithobius forficatus L. (Chilopoda: Lithobiidae)

Summary The ultrastructure of the ocelli of Lithobius forficatus L. was investigated by means of conventional electron microscopy. The dioptric apparatus consists of an unequal biconvex corneal lens which has a lamella-like fine structure. Crystalline cones and special pigment cells are lacking. The eye cup is composed of 35 to 110 sense cells of two different morphological types. The large visual cells of the distal region are characterized by unilaterally inserted rhabdomeres which form in a radial symmetrical arrangement the extended closed rhabdome. The proximal part of the eye cup is occupied by somewhat smaller basal cells of conical shape, showing an indistinct retinula. These cells have numerous microvilli either in the apex region or laterally which interdigitate to form stiltlike double or multiple rhabdomeres in close communication to the central rhabdome. In both types of sense cells particular zones appear because of the characteristic distribution of certain cell elements. They are arranged perpendicularly to the optical axis in the large receptors and vertical in the main axis of the basal cells. Elements of ER and other vesicles constitute a Schaltzone which borders the microvilli of the rhabdomer. This Schaltzone probably is a functional structure correlated to the adaptional state of the eye. The cytoplasmatic zone contains numerous different multivesicular and few large multilamellar bodies. The functional meaning of the organization of the Lithobius ocellus is discussed.

Mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Nonvisual Photoreceptors in Arthropods with Emphasis on Their Putative Role as Receptors of Natural Zeitgeber Stimuli

In various insect and arachnid species, three different types of photoreceptors that do not serve image processing have been discovered and analyzed by means of neurobiological methods: They can be found for example: (1) as lamina and lobula organs (LaOs and LoOs) next to the optic neuropils in the optic lobes of holo‐ and hemimetabolous insects; (2) inside the last ganglia of the cord of the scorpion and a marine midge; and (3) as modified visual photoreceptors in metamorphosized larval stemmata and the lateral eyes of scorpions, which have been compound eyes in fossil scorpion relatives. Immunocytology with various antibodies against proteins of the phototransduction cascade, the rhabdom turnover cycle and neurotransmitters of afferent and efferent pathways, was combined with light‐ and ultrastructural investigations in well‐defined adaptational states, in order to study their photoreceptive function and neuronal wiring. Pilot chronobiological experiments with a newly developed twilight simulating lamp, behavioral studies, and model calculations provide evidence that these photoreceptors may well serve a role in the complex task of detecting time cues out of natural dawn and dusk.

“…Clearly more work will be necessary before truly informed judgements can be made about the functional significance of the diversity in photoreception for entrainment. A first step will be the precise identification of photoreceptors and investigations of the mechanisms of transduction, processing and transmission of temporal information provided by the daily light cycle.…” ()     

Differential control of light-dark adaptation in the ocelli and compound eyes of Triatoma infestans

The adaptation to light of compound eyes in insects has been extensively documented and their adaptive role is well understood. Much less attention has been paid, however, to the control of ocelli sensitivity, a study which could help us to understand the functional role of these simple eyes. We analyzed the dynamic changes in the distribution of screening pigments which occur in the ocelli of the haematophagous bug, Triatoma infestans, when the insects are subjected either to light/dark cycles (LD), to constant darkness (DD) or constant light (LL). We then compared these changes with those occurring in the compound eyes of the same individuals and found that, while compound eyes are subject to the control of an endogenous circadian clock, the adaptation of the ocelli is entirely dependent on environmental illumination. In addition, we have observed that environmental temperature is not involved in the control of screening pigments in either ocelli or compound eyes as a direct stimulus, nor as a Zeitgeber. The existence of a differential control in the components of the dual visual system represents an adaptive advantage in the adjustment of visual sensitivity in insects exposed to quick changes in lighting conditions in their natural habitat. We discuss the implications of our findings with regards to the biology of triatomines and with respect to the general understanding the functional role of insect ocelli.     

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     

Central projections of first-order ocellar interneurons in two orthopteroid insects Acheta domesticus and Periplaneta americana

Summary The central projections of ocellar first-order interneurons in the cricket, Acheta domesticus, and the cockroach, Periplaneta americana, were examined in silver-intensified cobalt preparations. Ten morphologically different types of ocellar interneurons among a total of 44 are recognized in the cricket, and five different types among a total of 26 in the cockroach, indicating that these species have simpler ocellar systems than those described previously in locusts. Ocellar interneurons arborize in the following regions of neuropil in both the cricket and cockroach: the ocellar foci of the posterior protocerebrum, the posterior deutocerebrum, the protocerebral bridge, the ocellar synaptic plexus, ocellar nerves and tracts, and the lobula and medulla of the optic lobes. Ocellar first-order interneurons thus project predominantly to sites where they are likely to synapse with other ocellar and optic-lobe interneurons.