The roles of visual parallax and edge attraction in the foraging behaviour of the butterfly Papilio xuthus.

Several examples of insects using visual motion to measure distance have been documented, from locusts peering to gauge the proximity of prey, to honeybees performing visual odometry en route between the hive and a flower patch. However, whether the use of parallax information is confined to specialised behaviours like these or represents a more general purpose sensory capability, is an open question. We investigate this issue in the foraging swallowtail butterfly Papilio xuthus, which we trained to associate a target presented on a monitor with a food reward. We then tracked the animal\u27s flight in real-time, allowing us to manipulate the size and/or position of the target in a closed-loop manner to create the illusion that it is situated either above or below the monitor surface. Butterflies are less attracted to (i.e. slower to approach) targets that appear, based on motion parallax, to be more distant. Furthermore, we found that the number of abortive descent manoeuvres performed prior to the first successful target approach varies according to the depth of the virtual target, with expansion and parallax cues having effects of opposing polarity. However, we found no evidence that Papilio modulate the kinematic parameters of their descents according to the apparent distance of the target. Thus, we argue that motion parallax is used to identify a proximal target object, but that the subsequent process of approaching it is based on stabilising its edge in the 2D space of the retina, without estimating its distance

Molecular logic behind the three-way stochastic choices that expand butterfly colour vision.

Butterflies rely extensively on colour vision to adapt to the natural world. Most species express a broad range of colour-sensitive Rhodopsin proteins in three types of ommatidia (unit eyes), which are distributed stochastically across the retina. The retinas of Drosophila melanogaster use just two main types, in which fate is controlled by the binary stochastic decision to express the transcription factor Spineless in R7 photoreceptors. We investigated how butterflies instead generate three stochastically distributed ommatidial types, resulting in a more diverse retinal mosaic that provides the basis for additional colour comparisons and an expanded range of colour vision. We show that the Japanese yellow swallowtail (Papilio xuthus, Papilionidae) and the painted lady (Vanessa cardui, Nymphalidae) butterflies have a second R7-like photoreceptor in each ommatidium. Independent stochastic expression of Spineless in each R7-like cell results in expression of a blue-sensitive (Spineless(ON)) or an ultraviolet (UV)-sensitive (Spineless(OFF)) Rhodopsin. In P. xuthus these choices of blue/blue, blue/UV or UV/UV sensitivity in the two R7 cells are coordinated with expression of additional Rhodopsin proteins in the remaining photoreceptors, and together define the three types of ommatidia. Knocking out spineless using CRISPR/Cas9 (refs 5, 6) leads to the loss of the blue-sensitive fate in R7-like cells and transforms retinas into homogeneous fields of UV/UV-type ommatidia, with corresponding changes in other coordinated features of ommatidial type. Hence, the three possible outcomes of Spineless expression define the three ommatidial types in butterflies. This developmental strategy allowed the deployment of an additional red-sensitive Rhodopsin in P. xuthus, allowing for the evolution of expanded colour vision with a greater variety of receptors. This surprisingly simple mechanism that makes use of two binary stochastic decisions coupled with local coordination may prove to be a general means of generating an increased diversity of developmental outcomes

Colour constancy of the swallowtail butterfly Papilio xuthus

We have recently shown that the Japanese yellow swallowtail butterfly Papilio xuthus uses colour vision when searching for food. In the field, these butterflies feed on nectar provided by flowers of various colours not only in direct sunlight but also in shaded places and on cloudy days, suggesting that they have colour constancy. Here, we tested this hypothesis. We trained newly emerged Papilio xuthus to feed on sucrose solution on a paper patch of a certain colour under white illumination. The butterflies were then tested under both white and coloured illumination. Under white illumination, yellow- and red-trained butterflies selected the correctly coloured patch from a four-colour pattern and from a colour Mondrian collage. Under four different colours of illumination, we obtained results that were fundamentally similar to those under white illumination. Moreover, we performed critical tests using sets of two similar colours, which were also correctly discriminated by trained butterflies under coloured illumination. Taken together, we conclude that the butterfly Papilio xuthus exhibits some degree of colour constancy when searching for food

Color discrimination at the spatial resolution limit in a swallowtail butterfly, Papilio xuthus

Spatial resolution of insect compound eyes is much coarser than that of humans: a single pixel of the human visual system covers about 0.008° whereas that of diurnal insects is typically about 1.0°. Anatomically, the pixels correspond to single cone outer segments in humans and to single rhabdoms in insects. Although an outer segment and a rhabdom are equivalent organelles containing visual pigment molecules, they are strikingly different in spectral terms. The cone outer segment is the photoreceptor cell part that expresses a single type of visual pigment, and is therefore monochromatic. On the other hand, a rhabdom is composed of several photoreceptor cells with different spectral sensitivities and is therefore polychromatic. The polychromatic organization of the rhabdom suggests that insects can resolve wavelength information in a single pixel, which is an ability that humans do not have. We first trained the Japanese yellow swallowtail butterfly Papilio xuthus to feed on sucrose solution at a paper disk of certain color. We then let the trained butterflies discriminate disks of the training color and grey disks each presented in a Y-maze apparatus. Papilio correctly selected the colored disk when the visual angle was greater than 1.18° for blue, 1.53° for green or 0.96° for red: they appeared to see colors in single pixels to some extent. This ability may compensate their rather low spatial resolution

Colour vision of the foraging swallowtail butterfly Papilio xuthus

This paper demonstrates that foraging summer-form females of the Japanese yellow swallowtail butterfly Papilio xuthus have colour vision. The butterflies were trained to feed on sucrose solution placed on a disk of a particular colour in a cage set in the laboratory. After a few such training runs, a butterfly was presented with the training colour randomly positioned within an array of disks of other colours, but with no sucrose solution. The results indicate that the butterflies learn rapidly to select the training colour reliably among different colours. The training colour was also correctly selected when it was covered with neutral density filters to reduce its brightness, or even when the colour was presented together with disks of a variety of shades of grey. These results demonstrate convincingly, for the first time, that a butterfly has true colour vision

Simultaneous color contrast in the foraging swallowtail butterfly, Papilio xuthus

This study demonstrates that the color vision of foraging Japanese yellow swallowtail butterflies, Papilio xuthus, involves simultaneous color contrast. We trained newly emerged Papilio to select a disk of pale green among a set of differently colored disks presented on a black background. When the same set of disks was presented on blue background, the pale green-trained butterflies selected blue-green. The difference in spectra between pale green and blue green was similar to the spectrum of yellow for human vision, suggesting that blue induces yellow. Similarly, the pale green-trained Papilio selected a more bluish spring green on yellow background. We also trained Papilio with orange disks and tested on a green and violet background. The results showed that green induced violet and vice versa. Taken together, we concluded that simultaneous color contrast of Papilio is similar to the effect of complementary colors in human color vision

Spectral properties of identified polarized-light sensitive interneurons in the brain of the desert locust Schistocerca gregaria

Many migrating animals employ a celestial compass mechanism for spatial navigation. Behavioral experimentsin bees and ants have shown that sun compass navigation may rely on the spectral gradient in the sky as well as onthe pattern of sky polarization. While polarized-light sensitive interneurons (POL neurons) have been identifiedin the brain of several insect species, there are at present no data on the neural basis of coding the spectral gradientof the sky. In the present study we have analyzed the chromatic properties of two identified POL neurons in thebrain of the desert locust. Both neurons, termed TuTu1 and LoTu1, arborize in the anterior optic tubercle andrespond to unpolarized light as well as to polarized light. We show here that the polarized-light response of both types of neuron relies on blue-sensitive photoreceptors. Responses to unpolarized light depended on stimulus position and wavelength. Dorsal unpolarized blue light inhibited the neurons, while stimulation from the ipsilateral side resulted in opponent responses to UV light and green light. While LoTu1 was inhibited by UV light and was excited by green light, one subtype of TuTu1 was excited by UV and inhibited by green light. In LoTu1 the sensitivity to polarized light was at least 2 log units higher than the response to unpolarized light stimuli. Taken together, the spatial and chromatic properties of the neurons may be suited to signal azimuthal directions based on a combination of the spectral gradient and thepolarization pattern of the sky

Rough eyes of the Northeast-Asian wood white, Leptidea amurensis

The Northeast-Asian Wood White Leptidea amurensis (Lepidoptera, Pieridae) belongs to Dismorphiinae, a subfamily of the family Pieridae. We here studied the structure of the compound eye in this species through a combination of anatomy, molecular biology and intracellular electrophysiology, with a particular focus on the evolution of butterfly eyes. We found that their eyes consist of three types of ommatidia, with a basic set of one short, one middle and one long wavelength-absorbing visual pigment. The spectral sensitivities of the photoreceptors are rather simple, and peak in the ultraviolet, blue and green wavelength regions. The ommatidia have neither perirhabdomal nor fluorescent pigments, which modulate photoreceptor spectral sensitivities in a number of other butterfly species. These features are primitive, but the eyes of Leptidea exhibit another unique feature: the rough appearance of the ventral two-thirds of the eye. The roughness is due to the irregular distribution of facets of two distinct sizes. As this phenomenon exists only in males, it may represent a newly evolved sex-related feature

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