Backgrounds and the evolution of visual signals

Color signals which mediate behavioral interactions across taxa and contexts are often thought of as color 'patches' - parts of an animal that appear colorful compared to other parts of that animal. Color patches, however, cannot be considered in isolation because how a color is perceived depends on its visual background. This is of special relevance to the function and evolution of signals because backgrounds give rise to a fundamental tradeoff between color signal detectability and discriminability: as its contrast with the background increases, a color patch becomes more detectable, but discriminating variation in that color becomes more difficult. Thus, the signal function of color patches can only be fully understood by considering patch and background together as an integrated whole.

Retinal organization and visual abilities for flower foraging in swallowtail butterflies

Papilio butterflies' ability to forage for flowers relies upon multiple visual cues such as color, brightness, and motion. Papilio learns the color of rewarding flowers and detects it at a distance. Its color vision is based on four photoreceptor classes: UV, blue, green, and red, providing sensitive wavelength discrimination. These four receptor classes also contribute to the perception of brightness and polarization. Papilio's motion vision is based on a different set of receptors: green, red, and broad band. This implies that two visual pathways exist in Papilio. The contribution of several receptor classes not only for chromatic vision but also achromatic vision likely enhances the butterfly's ability to detect flowers in complex visual environments.

A comparative analysis of colour preferences in temperate and tropical social bees

The spontaneous occurrence of colour preferences without learning has been demonstrated in several insect species; however, the underlying mechanisms are still not understood. Here, we use a comparative approach to investigate spontaneous and learned colour preferences in foraging bees of two tropical and one temperate species. We hypothesised that tropical bees utilise different sets of plants and therefore might differ in their spontaneous colour preferences. We tested colour-naive bees and foragers from colonies that had been enclosed in large flight cages for a long time. Bees were shortly trained with triplets of neutral, UV-grey stimuli placed randomly at eight locations on a black training disk to induce foraging motivation. During unrewarded tests, the bees' responses to eight colours were video-recorded. Bees explored all colours and displayed an overall preference for colours dominated by long or short wavelengths, rather than a single colour stimulus. Naive Apis cerana and Bombus terrestris showed similar choices. Both inspected long-wavelength stimuli more than short-wavelength stimuli, whilst responses of the tropical stingless bee Tetragonula iridipennis differed, suggesting that resource partitioning could be a determinant of spontaneous colour preferences. Reward on an unsaturated yellow colour shifted the bees' preference curves as predicted, which is in line with previous findings that brief colour experience overrides the expression of spontaneous preferences. We conclude that rather than determining foraging behaviour in inflexible ways, spontaneous colour preferences vary depending on experimental settings and reflect potential biases in mechanisms of learning and decision-making in pollinating insects.

Generation and Evolution of Neural Cell Types and Circuits: Insights from the Drosophila Visual System

The Drosophila visual system has become a premier model for probing how neural diversity is generated during development. Recent work has provided deeper insight into the elaborate mechanisms that control the range of types and numbers of neurons produced, which neurons survive, and how they interact. These processes drive visual function and influence behavioral preferences. Other studies are beginning to provide insight into how neuronal diversity evolved in insects by adding new cell types and modifying neural circuits. Some of the most powerful comparisons have been those made to the Drosophila visual system, where a deeper understanding of molecular mechanisms allows for the generation of hypotheses about the evolution of neural anatomy and function. The evolution of new neural types contributes additional complexity to the brain and poses intriguing questions about how new neurons interact with existing circuitry. We explore how such individual changes in a variety of species might play a role over evolutionary timescales. Lessons learned from the fly visual system apply to other neural systems, including the fly central brain, where decisions are made and memories are stored.

Coral reef fish perceive lightness illusions

Visual illusions occur when information from images are perceived differently from the actual physical properties of the stimulus in terms of brightness, size, colour and/or motion. Illusions are therefore important tools for sensory perception research and from an ecological perspective, relevant for visually guided animals viewing signals in heterogeneous environments. Here, we tested whether fish perceived a lightness cube illusion in which identical coloured targets appear (for humans) to return different spectral outputs depending on the apparent amount of illumination they are perceived to be under. Triggerfish (Rhinecanthus aculeatus) were trained to peck at coloured targets to receive food rewards, and were shown to experience similar shifts in colour perception when targets were placed in illusory shadows. Fish therefore appear to experience similar simultaneous contrast mechanisms to humans, even when targets are embedded in complex, scene-type illusions. Studies such as these help unlock the fundamental principles of visual system mechanisms.

Innate preference and learning of colour in the male cotton bollworm Helicoverpa armigera

We investigated the ability of colour discrimination and learning in male adults of nocturnal cotton bollworm moth, Helicoverpa armigera, under a dim light condition. The naïve moths preferred blue and discriminated the innately preferred blue from several shades of grey indicating that the moths have colour vision. After being trained for 2 days to take nectar at yellow, an innately non-preferred colour, moths learned to select yellow over blue. The choice distribution between yellow and blue changed significantly from that of naïve moths. However, the dual choice distribution of the trained moths was not significantly biased to yellow: the preference for blue is robust. We also tried to train moths to grey, which was not successful. The limited ability to learn colours suggests that H. armigera may not strongly rely on colours when searching for flowers in the field, although they have basic property of colour vision.

Color and polarization vision in foraging Papilio

This paper gives an overview of behavioral studies on the color and polarization vision of the Japanese yellow swallowtail butterfly, Papilio xuthus. We focus on indoor experiments on foraging individuals. Butterflies trained to visit a disk of certain color correctly select that color among various other colors and/or shades of gray. Correct selection persists under colored illumination, but is systematically shifted by background colors, indicating color constancy and simultaneous color contrast. While their eyes contain six classes of spectral receptors, their wavelength discrimination performance indicates that their color vision is tetrachromatic. P. xuthus innately prefers brighter targets, but can be trained to select dimmer ones under certain conditions. Butterflies trained to a dark red stimulus select an orange disk presented on a bright gray background over one on dark gray. The former probably appears darker to them, indicating brightness contrast. P. xuthus has a strong innate preference for vertically polarized light, but the selection of polarized light changes depending on the intensity of simultaneously presented unpolarized light. Discrimination of polarization also depends on background intensity. Similarities between brightness and polarization vision suggest that P. xuthus perceive polarization angle as brightness, such that vertical polarization appears brighter than horizontal polarization.

Immunocytochemical localization of amines and GABA in the optic lobe of the butterfly, Papilio xuthus

Butterflies have sophisticated color vision. While the spectral organization of the compound eye has been well characterized in the Japanese yellow swallowtail butterfly, Papilio xuthus, neural mechanisms underlying its color vision are largely unexplored. Towards a better understanding of signal processing in the visual system of P. xuthus, we used immunocytochemical techniques to analyze the distribution of transmitter candidates, namely, histamine, serotonin, tyramine and γ-aminobutyric acid (GABA). Photoreceptor terminals in the lamina and medulla exhibited histamine immunoreactivity as demonstrated in other insects. The anti-histamine antiserum also labeled a few large medulla neurons. Medulla intrinsic neurons and centrifugal neurons projecting to the lamina showed serotonin immunoreactivity. Tyramine immunostaining was detected in a subset of large monopolar cells (LMCs) in the lamina, transmedullary neurons projecting to the lobula plate, and cell bodies surrounding the first optic chiasma. An anti-GABA antiserum labeled a subset of LMCs and populations of columnar and tangential neurons surrounding the medulla. Each of the four antisera also labeled a few centrifugal neurons that innervate the lobula complex from the central brain, suggesting that they have neuromodulatory roles. A distinctive feature we found in this study is the possibility that tyramine and GABA act as transmitters in LMCs of P. xuthus, which has not been reported in any other insects so far.

Simultaneous brightness contrast of foraging Papilio butterflies

This study focuses on the sense of brightness in the foraging Japanese yellow swallowtail butterfly, Papilio xuthus. We presented two red discs of different intensity on a grey background to butterflies, and trained them to select one of the discs. They were successfully trained to select either a high intensity or a low intensity disc. The trained butterflies were tested on their ability to perceive brightness in two different protocols: (i) two orange discs of different intensity presented on the same intensity grey background and (ii) two orange discs of the same intensity separately presented on a grey background that was either higher or lower in intensity than the training background. The butterflies trained to high intensity red selected the orange disc of high intensity in protocol 1, and the disc on the background of low intensity grey in protocol 2. We obtained similar results in another set of experiments with purple discs instead of orange discs. The choices of the butterflies trained to low intensity red were opposite to those just described. Taken together, we conclude that Papilio has the ability to learn brightness and darkness of targets independent of colour, and that they have the so-called simultaneous brightness contrast.

Evolution of color vision in pierid butterflies: blue opsin duplication, ommatidial heterogeneity and eye regionalization in Colias erate

This paper documents the molecular organization of the eye of the Eastern Pale Clouded Yellow butterfly, Colias erate (Pieridae). We cloned four cDNAs encoding visual pigment opsins, corresponding to one ultraviolet, two blue and one long wavelength-absorbing visual pigments. Duplication of the blue visual pigment class occurs also in another pierid species, Pieris rapae, suggesting that blue duplication is a general feature in the family Pieridae. We localized the opsin mRNAs in the Colias retina by in situ hybridization. Among the nine photoreceptor cells in an ommatidium, R1-9, we found that R3-8 expressed the long wavelength class mRNA in all ommatidia. R1 and R2 expressed mRNAs of the short wavelength opsins in three fixed combinations, corresponding to three types of ommatidia. While the duplicated blue opsins in Pieris are separately expressed in two subsets of R1-2 photoreceptors, one blue sensitive and another violet sensitive, those of Colias appear to be always coexpressed.