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TRUE COLOR VISION, COLOR CONSTANCY AND
POLARIZATION IN BUTTERFLIES
Entomology Biology 454
Summer Quarter 2005
Kathryn Filutowski
ABSTRACT
Butterflies in the genus Papilio have been used extensively to study their
physiology in terms of color vision. It has been found that Papilio have true color vision
and use it primarily in search for food, they possess color constancy except under
extremely saturated illumination, and they can discriminate between vertically and
horizontally polarized light.
INTRODUCTION
Butterflies (Lepidoptera) are one of the few bugs that people do not mind having
in their gardens, because they look absolutely brilliant. What is even more fascinating is
that butterflies have color vision that surpasses that of humans, because they are capable
of seeing ultraviolet light (360-588nm spectrum). Not only can they see ultraviolet light
but it has been proven that the swallowtail butterfly (Papilio xuthus) has true color vision.
Light has three qualities: phase, wavelength (or frequency) and e-vector
orientation or polarization angle (Kelber et al., 2001). Phase is not used by any animal, as
far as we know, but both wavelength and polarization angle are known to be exploited by
a variety of animals (Kelber et al., 2001). The wavelength spectrum of light is used
predominantly for object detection and recognition. The most vital organ that is
responsible for these visual sensory functions in the butterfly is the compound eye. The
compound eyes of the adult swallowtail butterfly are made up of hundreds of facets
called ommatidia each with a light-sensitive structure beneath. The species Papilio
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xuthus has at least 3 different types of ommatidia, in a random distribution. In each
ommatidium, nine photoreceptors contribute microvilli to the rhabdom (Arikawa et al.,
1997). Each ommatidia is directed at a slightly different angle from the others enabling
the butterfly to see in virtually every angle simultaneously (Bryson, “Butterfly Vision”).
If an animal is able to discriminate two lights using the wavelength spectrum
independently of stimulus intensity, it is said to have true color vision (Menzel, 1979). A
basic physiological requirement for color vision is for a species to possess a set of
different spectral receptor types in their retina. Papilio xuthus contains at least five types
of these spectral receptors (thus having a pentachromatic visual system) that include
ultraviolet, violet, blue, green and red wavelength regions (Arikawa et al., 1997). If an
animal can discriminate between two stimuli by means of the polarization angle
independently of stimulus intensity, it is said to have true polarization vision (Nilsson and
Warrant, 2000). This paper will focus on two studies that determined physiologically that
swallowtail butterflies use color vision in searching for food, possess color constancy,
and possess polarization dependent color vision; and finally, the adaptational values that
these systems have for the butterflies.
RESULTS
Color vision is the ability to discriminate visual stimuli solely on the basis of their
chromatic content irrespective of their brightness (Goldsmith, 1990). Even though the
basic requirement for color vision is multiple types of spectral receptors it is not the sole
requirement. Color vision must also be demonstrated by behavioral patterns of butterflies
(Kinoshita et al., 1998).
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Kinoshita et al. determined whether foraging Papilio xuthus use color when
searching for food in nature in an indoor color treatment setup. The results of the
experiment illustrated that of the four colored disks the red and yellow disks were visited
significantly more frequently than the green and blue disks in naïve butterflies, which
must suggest that this color preference must be innate (Kinoshita et al., 1998).1 After
training the butterflies the results showed clearly that butterflies were attracted more to
their respective trained color. The study also tested if the swallowtail butterfly would
respond to the same color stimuli even with reduced brightness, and the results indicated
that most butterflies visited their own trained colored disk.2 It also appears that Papilio
xuthus can learn colors associated with food with red and yellow being the most easily
learned. The mechanism underlying the quick change of color preference must be
important for efficient food searching in the field, where many different kinds of flowers
exist simultaneously. Collectively, Papilio xuthus can discriminate color disks on the
basis of their chromatic content regardless of their brightness.
In another study also conducted by Kinoshita et al. (2000) they tested whether
Papilio xuthus possessed color constancy in an indoor cage that was lined with colored
papers on the floor. They conducted two experiments to test this hypothesis. The first
consisted of training butterflies to feed on one color patch (yellow, red or blue), and they
were then tested to select the correct training patch from a four-color pattern with each
patch consisting of a food reward. The second experiment consisted of training butterflies
to feed on a two-color patch, and the butterflies were then tested to see whether they
could select the correct color under both white light and colored illumination. The results
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2
Data results in Appendix A (top)
Data results in Appendix A (bottom)
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from both experiments illustrated that mostly yellow and red-trained butterflies selected
the correct colored patch except under strongly saturated colored light.3 It was thus
concluded that Papilio xuthus demonstrated some amount of color constancy when
searching for food.
Color constancy enables the butterfly to recognize an object’s color regardless of
the spectral content of the illuminating light (Kinoshita et al., 2000). Since Papilio xuthus
selected the correct color even under colored illumination (which drastically changes the
reflection spectra of the color papers to which the butterflies had been trained) this
indicated that the reflection spectrum of the colored patch, which stimulates the spectral
receptors in the ommatidia viewing the patch, does not solely determine color recognition
(Kinoshita et al., 2000).4 The butterflies somehow convert wavelength information at the
retina into color. How this is done is up to debate but there have been well conjectured
ideas about the topic.
One proposal to explain how color constancy works involves the lamina. The
lamina is the first optic neuropil of the compound eye (Shimohigashi et al., 1999). In
Papilio xuthus, there are four types of secondary neurons (L1-L4; L meaning large) in the
lamina. The lamina also consists of units known as cartridges derived from a single
ommatidium (Shimohigashi et al., 1999). The neurons L1 and L3 are restricted to a single
cartridge but L2 and L4 are capable of sending neurons to at least seven adjacent
cartridges (Ribi, 1987). It is therefore thought that butterflies L2 and L4 neurons enable
wavelength information from surrounding ommatidia to be integrated, therefore allowing
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4
Data results Appendix B
Data from experiment 2 see Appendix B1
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butterflies recognition of color during indistinguishable conditions (i.e. foggy, cloudy,
misty, etc.).
In another study (Kebler et al., 2001) it was found that the genus Papilio can also
discriminate between vertically and horizontally polarized light of the same color in the
contexts of oviposition and feeding. For the oviposition experiment, Papilio aegeus were
tested with two stimuli of the same green color and same intensity but different
polarization angles. The results indicated that Papilio preferred horizontally polarized
light over vertically or obliquely polarized light of the same color, and they could also
discriminate between stimuli differing in color. For all measured behavioral reactions,
choice distributions were similar (drumming, curling, and egg laying).5
In the feeding experiment Papilio xuthus were trained with stimuli differing in the
polarization angle of light and color. The results were same as with oviposition in that
Papilio can easily discriminate between two polarized lights that differ only in their angle
of polarization (Kebler et al., 2001). The results from both experiments provided
evidence that that polarization can induce a change in perceived color and that intensity
has an additional, but smaller, influence (Kebler et al., 2001). It was also concluded that
separate color and polarization systems do not exist because butterflies do not contain the
necessary requirements. For a polarization-sensitive system involving R and G receptors,
it is expected that optimum discrimination would be in the red or green range and poor
discrimination in the blue range (Kebler et al., 2001). However, the opposite was true in
butterflies. Color influenced the discrimination of stimuli differing only in polarization
angle and the oviposition tests illustrated that the polarization angle influenced the
5
Data in Appendix C
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choices of colors as well—hence polarization and color are processed in the same visual
pathway.
Insects such as bees, flies, and other Lepidoptera are only weakly sensitive to the
polarization angle of light (Warrant et al., 1999). It is suggested that butterflies of the
genus Papilio preserved polarization sensitivity to the entire eye because it is
behaviorally important in oviposition. Females of the species Papilio lay eggs on shiny
leaves of the Citrus family, and these leaves reflect partially polarized light (Kebler et al.,
2001). To our eyes, the shiny leaves reflect white light and thus have a less saturated
color but the same hue (Wehner and Bernard, 1993). To an approaching Papilio female,
however, the shiny leaves of a Citrus bush should have different colors depending on
their orientation: a horizontally oriented leaf should look greener whereas a vertically
oriented leaf should look more blue-green or reddish (Kebler et al., 2001). Horizontally
oriented leaves should therefore be more attractive to an approaching female, whereas
vertically oriented leaves should be less attractive. Additionally a plant with shiny leaves
should look more colorful than one that does not reflect polarized light, which could also
aid the females in choosing a larval food plant from a distance.
CONCLUSION
The Lepidoptera comprise more than 150,000 species, many of economic
importance (e.g., pollinators, agricultural pests, and silk production), and they have
characteristic biological properties that distinguish them from other insects—including
color vision. In order for flowers to pollinate they need to attract butterflies with specific
colors on their petals, so in this respect it is important to know scientifically how and why
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butterflies are attracted to certain colors of plants. Furthermore, some plants contain
ultraviolet color within the plant itself such as, nectar and pollen. For instance, it has also
been proposed that there may be differences in ultraviolet reflectance as a flower matures,
to prevent competition within a species (Eisner et al., 1969). Some flowers have been
recorded as having fluorescent nectar. The significance of fluorescent nectar is still under
debate, but occurs regularly enough to assume that it is not present merely by chance, and
must have some function (Kevin 1976). It would be interesting to perform a study to see
if ultraviolet light in flowers is used as a tactic to attract butterflies.
In regards to polarized light, it has been found that natural scenes contain highly
polarized light reflected by leaves and other shiny surfaces, making the polarizationdependence of color vision a relevant problem. Since many animals have evolved ways to
get around this problem, there must be advantages to the animals that have retained the
polarization-dependence of color vision (Kebler et al., 2001). Polarization-dependent
color vision may have evolved as a cheap solution to very specific problems (Kebler et
al., 2001). It is necessary to study the light environments of animals in more detail to
understand the possible advantages and consequences of these systems better.
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