Rugged, obstruction-free, mirror-lens combination for panoramic imaging.

We present a new combination of lenses and reflective surfaces for obstruction-free wide-angle imaging. The panoramic imaging system consists of a reflective surface machined into solid Perspex, which together with an embedded lens, can be attached to a video camera lens. Unlike vision sensors with a single mirror mounted in front of a camera, the view in the forward direction (i.e., the direction of the optical axis) is not obstructed. Light rays contributing to the central region of the image are refracted at a centrally positioned lens and at the Perspex enclosure. For the outer image region, rays are reflected at a mirror surface of constant angular gain machined into the Perspex and coated with silver. The design produces a field of view of approximately 260 degrees with only a small separation of viewpoints. The shape of the enclosing Perspex is specifically designed in order to minimize internal reflections.

Responses of blowfly motion-sensitive neurons to reconstructed optic flow along outdoor flight paths.

The retinal image flow a blowfly experiences in its daily life on the wing is determined by both the structure of the environment and the animal's own movements. To understand the design of visual processing mechanisms, there is thus a need to analyse the performance of neurons under natural operating conditions. To this end, we recorded flight paths of flies outdoors and reconstructed what they had seen, by moving a panoramic camera along exactly the same paths. The reconstructed image sequences were later replayed on a fast, panoramic flight simulator to identified, motion sensitive neurons of the so-called horizontal system (HS) in the lobula plate of the blowfly, which are assumed to extract self-motion parameters from optic flow. We show that under real life conditions HS-cells not only encode information about self-rotation, but are also sensitive to translational optic flow and, thus, indirectly signal information about the depth structure of the environment. These properties do not require an elaboration of the known model of these neurons, because the natural optic flow sequences generate--at least qualitatively--the same depth-related response properties when used as input to a computational HS-cell model and to real neurons.

Movement-induced motion signal distributions in outdoor scenes.

The movement of an observer generates a characteristic field of velocity vectors on the retina (Gibson 1950). Because such optic flow-fields are useful for navigation, many theoretical, psychophysical and physiological studies have addressed the question how ego-motion parameters such as direction of heading can be estimated from optic flow. Little is known, however, about the structure of optic flow under natural conditions. To address this issue, we recorded sequences of panoramic images along accurately defined paths in a variety of outdoor locations and used these sequences as input to a two-dimensional array of correlation-based motion detectors (2DMD). We find that (a) motion signal distributions are sparse and noisy with respect to local motion directions; (b) motion signal distributions contain patches (motion streaks) which are systematically oriented along the principal flow-field directions; (c) motion signal distributions show a distinct, dorso-ventral topography, reflecting the distance anisotropy of terrestrial environments; (d) the spatiotemporal tuning of the local motion detector we used has little influence on the structure of motion signal distributions, at least for the range of conditions we tested; and (e) environmental motion is locally noisy throughout the visual field, with little spatial or temporal correlation; it can therefore be removed by temporal averaging and is largely over-ridden by image motion caused by observer movement. Our results suggest that spatial or temporal integration is important to retrieve reliable information on the local direction and size of motion vectors, because the structure of optic flow is clearly detectable in the temporal average of motion signal distributions. Ego-motion parameters can be reliably retrieved from such averaged distributions under a range of environmental conditions. These observations raise a number of questions about the role of specific environmental and computational constraints in the processing of natural optic flow.

Butterfly wing colours: scale beads make white pierid wings brighter.

The wing-scale morphologies of the pierid butterflies Pieris rapae (small white) and Delias nigrina (common jezabel), and the heliconine Heliconius melpomene are compared and related to the wing-reflectance spectra. Light scattering at the wing scales determines the wing reflectance, but when the scales contain an absorbing pigment, reflectance is suppressed in the absorption wavelength range of the pigment. The reflectance of the white wing areas of P. rapae, where the scales are studded with beads, is considerably higher than that of the white wing areas of H. melpomene, which has scales lacking beads. The beads presumably cause the distinct matt-white colour of the wings of pierids and function to increase the reflectance amplitude. This will improve the visual discrimination between conspecific males and females.

Burrow surveillance in fiddler crabs. I. Description of behaviour.

When defending resources, animals need to reliably detect and identify potential competitors. Animals that live at high population densities would be expected to be efficient in this aspect of resource defence since the time lost in false alarms could be substantial and the failure of identifying a competitor could be very costly. How does an animal decide whether another animal is or is not a threat to a resource or a territory? Fiddler crabs [Uca vomeris (McNeill)] operate from burrows that they guard and defend vigorously against other crabs. The crabs live in dense populations, with many animals inhabiting one square metre of mudflat. We describe here the behavioural responses of foraging crabs to repeated presentations of small crab-like dummies approaching their burrows. We explore the relationship between the probability and the timing of burrow defence responses, the crab's behavioural state, and the visual appearance and direction of approach of the dummies. We find that the probability of response of resident crabs is independent of the relative position of crab and dummy but is strongly affected by the dummy's position and movement direction relative to the crab's burrow. The critical stimuli are the dummy's distance from the crab's burrow and whether the dummy is moving towards the burrow or not. The response distance (dummy-burrow distance) increases with the crab's own distance from the burrow, indicating that the crabs modify their assessment of threat depending on their own distance away from the burrow. Differences in dummy size and brightness do not affect the probability or the timing of the response. We discuss these results in the context of fiddler crab social life and, in a companion paper, identify the visual and non-visual cues involved in burrow defence.

Burrow surveillance in fiddler crabs. II. The sensory cues.

Using crab-like dummies, we have shown previously that fiddler crabs [Uca vomeris (McNeill)] defend their burrows against intruders in a burrow-centred frame of reference. The crabs respond whenever an intruder approaches to within a certain distance of the burrow entrance, and this distance is independent of the approach direction. We show here that the crabs combine information from the path integration system on the location of their invisible burrow and visual information on the retinal position of an intruder to make this allocentric judgement. Excluding all alternative visual cues, we propose that the crabs employ a small set of matched visual filters to determine the relationship between a crab-like object and the invisible burrow. To account for the constantly varying distance between the crabs and their burrows, the state of the path integrator may select the appropriate one of these retinal 'warning zones'. We have shown before that burrow-owning fiddler crabs are extremely responsive to potential burrow snatchers, which we simulated with crab-like dummies moving across the substratum towards the burrow of residents. The crab's decision to respond to these dummies depends mainly on the spatial arrangement between itself, its burrow and the approaching dummy. The most important factor predicting response probability is the dummy's distance from the crab's burrow: the crabs are more likely to respond the closer the dummy approaches the burrow. The dummy-burrow distance not only determines the overall response probability but also the timing of burrow defence responses (i.e. when the crabs decide to react). Most interestingly, this response distance is independent of the dummy's direction of approach to the burrow. In addition, the crabs respond earlier to a dummy approaching their burrow if they themselves are further away from it, indicating that knowledge of their own distance from the burrow has an influence on their decision to respond. These results raise a number of interesting issues, which are the focus of this paper, regarding the cues and the information used by the crabs in burrow surveillance.

Signals from 'crabworld': cuticular reflections in a fiddler crab colony.

Fiddler crabs inhabit intertidal sand- and mudflats, where they live in dense colonies and are active on the surface during low tide. They exhibit a rich behavioural repertoire, with frequent interactions between animals in the context of territorial and mating activities. Male fiddler crabs have one massively enlarged and conspicuously coloured claw, which they use in waving displays and in fights with other males. The crabs carry their eyes on long, vertically oriented stalks high above the body and, as a consequence, see the bodies of conspecifics in the ventral visual field, below the local visual horizon, and against the mudflat surface as background. We filmed events in a colony of Uca vomeris with a normal video camera and an ultraviolet-sensitive camera placed at the eye height of an average crab, approximately 2-3 cm above ground. We also used a spectrographic imager and linear polarized filters to analyse the cues potentially available to the animals for detecting, monitoring and possibly identifying each other. Areas of high contrast in mudflat scenes include specular reflections on the wet cuticle of crabs that are horizontally polarised. Besides specular reflections, some parts of the cuticle generate high-contrast signals against the mudflat background, both at wavelengths between 400 and 700 nm, and in the ultraviolet region between 300 and 400 nm. Uca vomeris can be very colourful: the different parts of the large claw of the male are white, orange or red. The carapace colours of both males and females can range from a mottled yellowish green brown, to a brilliant light blue. White and blue colours contrast starkly with the mudflat background, especially in the ultraviolet wavelengths. Under stress, the blue and white colours can change within minutes to a duller and darker blue or to a dull white.

The optomotor response and spatial resolution of the visual system in male Xenos vesparum (Strepsiptera).

The Strepsiptera are an enigmatic group of parasitic insects whose phylogenetic relationships are hotly debated. Male Strepsiptera have very unusual compound eyes, in which each of a small number of ommatidia possesses a retina of at least 60 retinula cells. We analysed the optomotor response of Xenos vesparum males to determine whether spatial resolution in these eyes is limited by the interommatidial angle or by the higher resolution potentially provided by the extended array of retinula cells within each ommatidium. We find that the optomotor response in Strepsiptera has a typical bandpass characteristic in the temporal domain, with a temporal frequency optimum at 1-3 Hz. As a function of spatial wavelength, the optomotor response is zero at grating periods below 12 degrees and reaches its maximum strength at grating periods between 60 degrees and 70 degrees. To identify the combination of interommatidial angles and angular sensitivity functions that would generate such a spatial characteristic, we used motion detection theory to model the spatial tuning function of the strepsipteran optomotor response. We found the best correspondence between the measured response profile and theoretical prediction for an irregular array of sampling distances spaced around 9 degrees (half the estimated interommatidial angle) and an angular sensitivity function of approximately 50 degrees, which corresponds to the angular extent of the retina we estimated at the centre of curvature of the lens. Our behavioural data strongly suggest that, at least for the optomotor response, the resolution of the strepsipteran compound eye is limited by the ommatidial sampling array and not by the array of retinula cells within each ommatidium. We discuss the significance of these results in relation to the functional organisation of strepsipteran compound eyes, their evolution and the role of vision in these insects.

A glimpse into crabworld.

Almost all known arthropod compound eyes exhibit regional variations of resolving power, absolute light, spectral and polarisation sensitivity which are likely to be matched to the probability of significant events and the availability of cues in the visual world. To understand the signal processing requirements that have led to the evolution of matched sensory and neural filters, we thus need a detailed description of the input signals to a visual system and of the tasks to be performed under natural operating conditions. We report here on the first steps we took in an attempt to reconstruct an animal's specific visual world with emphasis on the motion domain. Fiddler crabs (genus Uca) live in burrows on sand- and mudflats and are active during low tide. They carry their eyes on long, vertically oriented stalks and use vision to detect predators and conspecific signals generated by males waving one massively enlarged claw. The crabs sit on the ground plane of a flat world, where significant events are most likely to occur in a narrow band around the horizon. We recorded scenes in a crab colony with a video camera at crab eye height. The salience of relevant features in the spatial, spectral and polarisation domains was analysed in digitised video images and short sequences of film were processed by a two-dimensional network of motion detectors at various spatial scales. The output of the network provides us with histograms of the direction and strength of motion signals in various spatio-temporal frequency bands. We discuss our results in terms of detection problems, predictability of events, global vs local information content and higher level motion processing involved in intraspecific communication.

Structure and function of learning flights in ground-nesting bees and wasps

Bees and wasps perform systematic flight manoevres when they leave their nest or a foodplace, during which they acquire or update their visual memory of the goal location. In a typical learning flight, the insect backs away from the goal in a series of arcs that are roughly centred on the goal. The mean rate of turning is rather constant and tends to balance the angular speed at which the arc is described. As a result, the insect views the goal at relatively fixed retinal positions in its left and right visual field, depending on flight direction. The general direction in which the insect backs away from the goal and the transition from one arc segment to the next are influenced by the local scene and by compass cues. Insects returning to the goal repeat some of the flight manoeuvres of their preceding learning flights. Their orientation in space and the retinal positions at which they view nearby landmarks are similar. One important function of learning flights appears to be the acquisition of visual depth information. We review the consequences of the structure of learning flights for visual information processing and discuss how they may relate to the acquisition of a visual representation and the task of pinpointing the goal.

The variation of resolution and of ommatidial dimensions in the compound eyes of the fiddler crab Uca lactea annulipes (Ocypodidae, Brachyura, Decapoda)

We studied variations in the optical properties of the compound eyes of Uca lactea annulipes using in vivo optical and histological techniques. The distribution of resolving power in the eyes of this fiddler crab species is typical for arthropods that inhabit flat environments: the eyes possess a panoramic equatorial acute zone for vertical resolution and a steep decrease of resolution away from the eye equator in the dorsal and ventral visual fields. The dimensions of the cellular components of the ommatidia vary accordingly: in the equatorial part of the eyes, facets are larger, and crystalline cones and rhabdoms are longer than in the dorsal and ventral parts of the eyes. Along the eye equator, horizontal resolution is low compared with vertical resolution and varies little throughout the visual field. The eyes of Uca lactea annulipes are unusual in that the gradient of vertical anatomical and optical resolution is steeper in the dorsal than in the ventral visual field. We interpret this difference as indicating that the information content of the world as seen by the crabs differs above and below the horizon line in specific and predictable ways.

Robber bees (Lestrimelitta limao) and their host chemical and visual cues in nest defense byTrigona (Tetragonisca) angustula (Apidae: Meliponinae).

The nest of the stingless bee,Trigona (Tetragonisca) angustula, is guarded by bees positioned in the nest entrance and others hovering in front of it. Hovering guard bees track returning foragers sideways along the last 10 cm in front of the nest, but intercept and incapacitate nest intruders by clinging with mandibles to wings and legs. When attacked by the cleptobiotic stingless beeLestrimelitta limao, the colony strengthens its aerial defense with hundreds of additional hoverers. To test our hypothesis that this reaction is due to interspecific chemical communication based on kairomone effects, we presented synthetic cephalic volatiles of both species at the nest entrance and counted the number of bees leaving the nest and taking up hovering positions. We conclude that guard bees recognizeL. limao by the major terpenoids of their volatile cephalic secretions, geranial, neral (=citral) and 6-methyl-5-hepten-2-one; other components may fine-tune this recognition. The effect of chemical stimuli is not significantly enhanced by combination with a dummy ofL. limao. Guard bees, we hypothesize, respond to this kairomone by secreting a species specific alarm pheromone; a major component of this pheromone, benzaldehyde, recruits additional bees to defend the nest.

Tactile localisation: the function of active antennal movements in the crayfish Cherax destructor.

Video recordings and single frame analysis were used to study the function of the second antennae of crayfish (Cherax destructor) as a sensory system in freely behaving animals. Walking crayfish move their antennae back and forth through horizontal angles of 100 degrees and more, relative to the body long axis. At rest, animals tend to hold their antennae at angular positions between 20 and 50 degrees. Movements of the two antennae are largely independent of each other. Before and during a turn of the body the ipsilateral antenna is moved into the direction of the turn. Solid objects are explored by repeatedly moving the antennae towards and across them. Both seeing and blinded crayfish can locate stationary objects following antennal contact. On antennal contact with a small novel object, a moving animal withdraws its antenna and attacks the object. When the antenna of a blinded crayfish is lightly touched with a brush the animal turns and attacks the point of stimulation. The direction taken and the distance covered during an attack can be correlated with: the angle at which the antenna is held at the moment of contact and the distance along the antennal flagellum at which the stimulus is applied. From behavioural evidence we conclude that crayfish use information about the angular position of their antennae and about the position of stimulated mechanoreceptors along the antennal flagellum to locate objects in their environment. We suggest ways in which an active tactile system-like the crayfish's antennae--could supply animals with information about the three-dimensional layout of their environment.

Sexual dimorphism in the visual system of flies: the divided brain of male Bibionidae (Diptera).

The mapping of the compound eyes onto the visual neuropils and the cell types in the lamina and the lobula complex of Bibionidae (Diptera) were studied by means of extracellular cobalt injections and Golgi impregnations. Dorsal and ventral eyes in males map into separate dorsal-and ventral neuropils up to the level of the lobula complex. The dorsal-eye lamina is unilayered, while the ventral-eye lamina in males and the lamina in females are multilayered: layers A and C are invaded by en-passant terminals of long visual fibres, layer B by the terminals of short visual fibres. Long visual fibres have a short and a long terminal in the ventral medulla with terminal specialisations in three distinct layers. Only one type of receptor ending exists in the dorsal medulla, the terminal branches of which are restricted to one layer only. Arrays of contralateral neurones are found in the medial part of the dorsal lobula, which receives input from the zone of binocular vision of the ipsilateral dorsal eye, and in the posterior dorsal lobula and lobula plate. The dorsal lobula plate contains large tangential neurones, the dendritic arborisations of which are revealed by cobalt injection into the thoracic ganglia. The divided brain of male bibionids offers the opportunity to investigate separately the nervous systems involved in sex-specific visually guided flight behaviour and in 'general' visually guided flight control.