Rhabdomeric twist in bees ? Artefact or in vivo structure?

Polarisation signals: a new currency for communication

Most polarisation vision studies reveal elegant examples of how animals, mainly the invertebrates, use polarised light cues for navigation, course-control or habitat selection. Within the past two decades it has been recognised that polarised light, reflected, blocked or transmitted by some animal and plant tissues, may also provide signals that are received or sent between or within species. Much as animals use colour and colour signalling in behaviour and survival, other species additionally make use of polarisation signalling, or indeed may rely on polarisation-based signals instead. It is possible that the degree (or percentage) of polarisation provides a more reliable currency of information than the angle or orientation of the polarised light electric vector (e-vector). Alternatively, signals with specific e-vector angles may be important for some behaviours. Mixed messages, making use of polarisation and colour signals, also exist. While our knowledge of the physics of polarised reflections and sensory systems has increased, the observational and behavioural biology side of the story needs more (and more careful) attention. This Review aims to critically examine recent ideas and findings, and suggests ways forward to reveal the use of light that we cannot see.

Three-dimensional ultrastructural organization of the ommatidium of the minute parasitoid wasp Trichogramma evanescens

Existing information on insect compound eyes is mainly limited to two-dimensional information derived from histological or ultrathin sections. These allow a basic description of eye morphology, but are limited in z-axis resolution because of the section thickness or intervals between sections, so that accurate volumetric information cannot be generated. Here we use serial-sectioning transmission electron microscopy to present a 3-D reconstruction at ultrastructural level of a complete ommatidium of a miniaturized insect compound eye. Besides the general presentation of the three dimensional arrangement of the different cell types within the ommatidium, the reconstruction allowed volumetric measurements and numerical analyses to be undertaken, revealing new insights into the number, size and distribution of cell organelles in insect ommatidia. Morphological features that can be related to miniaturization, namely the dimensions and displacement of nuclei, reduction of average pigment granule volume and loss of pigment granules in the terminals of the cone cells, the impact of metabolic activity of cell types on miniaturization, as well as maintenance of rhabdomere volume and limits to its miniaturization, are all discussed.

The visual system of the Australian 'Redeye' cicada (Psaltoda moerens)

We investigated the functional anatomy of the visual system in the Australian 'Redeye' cicada Psaltoda moerens, including compound eyes and ocelli. The compound eyes have large visual fields, about 7500 ommatidia per eye and binocular overlaps of 10-15° in the frontal and of 50-60° in the dorsal visual field. The diameters of corneal facet lenses range between 22 and 34 μm and the lenses are unusually long with up to 100 μm in some eye regions. In the posterior part of the eyes, the hexagonal facet array changes to a square lattice. The compound eyes are of the eucone apposition type with 8 retinular cells contributing to a fused rhabdom in each ommatidium. The red eye colour is due to the pigment granules in the secondary pigment cells. We found a small Dorsal Rim Area (DRA), in which rhabdom cross-sections are rectangular rather than round. The cross-sections of DRA rhabdoms do not systematically change orientation along the length of the rhabdom, indicating that microvilli directions do not twist, which would make retinular cells in the DRA polarization sensitive. The three ocelli have unusual lenses with a champagne-cork shape in longitudinal sections. Retinular cells are short in the dorsal and ventral part of the retinae, and long in their equatorial part. Ocellar rhabdoms are short (<10 μm), positioned close to the corneagenous layer and are formed by pairs of retinular cells. In cross-section, the rhabdomeres are 2-5 μm long and straight. The red colour of ocelli is produced by screening pigments that form an iris around the base of the ocellar lens and by screening pigments between the ocellar retinular cells. We discuss the organization of the compound eye rhabdom, the organization of the ocelli and the presence of a DRA in the light of what is known about Hemipteran compound eyes. We note in particular that cicadas are the only Hemipteran group with fused rhabdoms, thus making Hemiptera an interesting case to study the evolution of open rhabdoms and neural superposition.

Life as a cataglyphologist--and beyond

Rüdiger Wehner's lifelong research activities centered on Cataglyphis have rendered these thermophilic desert ants model organisms in the study of animal navigation. The present account describes how the author encountered Cataglyphis and established a study site at Maharès, Tunisia; how he increasingly focused his research on the neuroethological analysis of the ant's navigational toolkit; and finally, how he extended these studies to thermophilic desert ants in other deserts of the world, to Ocymyrmex in southern Africa and Melophorus in central Australia. By including aspects of functional morphology, physiology, and ecology in his research projects, he has favored-and advocated-an organism-centered approach. Beyond "cataglyphology," he was engaged in substantial teaching both at his home university in Zürich and overseas, writing a textbook, running a department, and working as a Permanent Fellow at the Institute for Advanced Study in Berlin.

Stomatopod eye structure and function: a review

Stomatopods (mantis shrimps) possess apposition compound eyes that contain more photoreceptor types than any other animal described. This has been achieved by sub-dividing the eye into three morphologically discrete regions, a mid-band and two laterally placed hemispheres, and within the mid-band, making simple modifications to a commonly encountered crustacean photoreceptor pattern of eight photoreceptors (rhabdomeres) per ommatidium. Optically the eyes are also unusual with the directions of view of the ommatidia of all three eye regions skewed such that over 70% of the eye views a narrow strip in space. In order to scan the world with this strip, the stalked eyes of stomatopods are in almost continual motion. Functionally, the end result is a trinocular eye with monocular range finding capability, a 12-channel colour vision system, a 2-channel linear polarisation vision system and a line scan sampling arrangement that more resembles video cameras and satellite sensors than animal eyes. Not surprisingly, we are still struggling to understand the biological significance of stomatopod vision and attempt few new explanations here. Instead we use this special edition as an opportunity to review and summarise the structural aspects of the stomatopod retina that allow it to be so functionally complex.

Photoreceptor morphogenesis in the Drosophila compound eye: R1-R6 rhabdomeres become twisted just before eclosion

The photosensitive microvilli of Drosophila photoreceptors R1-R6 are not aligned in parallel over the entire length of the visual cells. In the distal half of each cell, the microvilli are slightly tilted toward one side and, in the proximal half, extremely toward the opposite side. This phenomenon, termed rhabdomere twisting, has been known for several decades, but the developmental and cell biological basis of rhabdomere twisting has not been studied so far. We show that rhabdomere twisting is also manifested as molecular polarization of the visual cell, because phosphotyrosine-containing proteins are selectively partitioned to different sides of the rhabdomere stalk in the distal and proximal sections of each R1-R6 photoreceptor. Both the asymmetrical segregation of phosphotyrosine proteins and the tilting of the microvilli occur shortly before eclosion of the flies, when eye development in all other aspects is considered to be essentially complete. Establishment of rhabdomere twisting occurs in a light-independent manner, because phosphotyrosine staining is unchanged in dark-reared wild-type flies and in mutants with defects in the phototransduction cascade, ninaE(17) and norpA(P24). We conclude that antiphosphotyrosine immunofluorescence can be used as a light microscopic probe for the analysis of rhabdomere twisting and that microvilli tilting represents a type of planar cell polarity that is established by an active process in the last phase of photoreceptor morphogenesis, just prior to eclosion of the flies.

Experimental evidence for spatial learning in cuttlefish (Sepia officinalis)

Laboratory mazes were used to study spatial-learning capabilities in cuttlefish (Sepia offcinalis), using escape for reinforcement. In preliminary observations, cuttlefish in an artificial pond moved actively around the environment and appeared to learn about features of their environment. In laboratory experiments, cuttlefish exited a simple alley maze more quickly with experience and retained the learned information. Similar improvement was not found in open-field mazes or T mazes, perhaps because of motor problems. Cuttlefish learned to exit a maze that required them to find openings in a vertical wall. The wall maze was modified to an arena, and simultaneous discrimination learning and reversal learning were demonstrated. These experiments indicate that cuttlefish improve performance over serial reversals of a simultaneous, visual-spatial discrimination problem.

Neurobiology of polarization vision

Insects can perceive the pattern of polarized light (e-vector pattern) in the sky and use it as a compass. To accomplish this navigational task they employ a specialized part of the retina in which the polarization analysers (ultraviolet receptors) are arranged in a way that mimics, by and large, the distribution of e-vector directions in the sky. By sweeping this 'matched polarization filter's across the sky, the insect translates the complex spatial information provided by the celestial e-vector patterns into rather simple temporal modulations of summed receptor outputs. This mechanism provides a striking example of peripheral coding by matched filtering in sensory systems.

Retrograde labelling of photoreceptors in different regions of the compound eyes of bees and ants

The geometry of retinal receptor arrays and the projection patterns of photoreceptor axons are unravelled in the compound eyes of bees and ants by backfilling large populations of photoreceptors with horseradish peroxidase or Lucifer yellow. Such retrograde labelling techniques are applied in the insect retina for the first time. They brilliantly label populations of specific receptor types within more than 100 ommatidia. Such preparations are obtained in three distinctly different parts of the eye: (1) the part of the retina that is positioned at the uppermost dorsal margin of the eye and specialized for the detection of polarized skylight; (2) the remainder of the dorsal retina; and (3) the ventral retina. As shown by the large populations of labelled photoreceptors, the retinal receptor arrays differ strikingly between different parts of the eye. Furthermore, there exist similarities as well as marked differences between bees and ants. Former hypotheses concerning the location of photoreceptor terminals in the first and second visual neuropil have all been based on a few individual Golgi-labelled photoreceptors. The results presented in this paper, based on retrograde mass impregnations of selected types of photoreceptors, confirm some of the former hypotheses and reject others. The new findings are important for understanding how insects analyse polarized skylight.