Humans can distinguish the so-called primary colors of light (red, green, and blue) allowing us to see various colors. But how many colors can the butterfly distinguish? Scientists have been able to examine the facets of a swallowtail’s eye by inserting electrodes into them. What has been discovered is that the swallowtail can distinguish as many as six color elements (ultraviolet, purple, blue, green, red, and the wideband light from red to purple). It has the ability to see a world of rich colors and, most notably, it can discern ultraviolet light, unlike humans. In fact, many flowers have an area close to where their nectar is that absorbs ultraviolet light. It is not hard to see why it is extremely important for a butterfly to be able to see ultraviolet in order to identify the location of nectar.

What is surprising, however, is that individually the facets are not able to perceive all six color elements. It was found that each facet can see several color elements. This was a remarkable discovery, as it had been believed that all the facets were identical.

It has been found that a single facet of a swallowtail’s eye is comprised of nine cells, as shown in the diagram below. The size of each cell varies according to its depth below the surface of the facet. Seen in cross-section, they appear as beautiful as flowers. The six color elements are perceived using these nine cells.

Light which has entered through the surface of the facet passes down the cylindrical rhabdome that extends through the middle of the facet. As the light passes down the rhabdome, the cells surrounding the rhabdome absorb the light and perceive the color. In other words, each of the nine cells from the top to the bottom of the facet senses a different color.

As adjacent facets are identical in structure, they were thought to see the same color. However, experiments have shown that adjacent facets have different properties. Why is it that they differ?

The breakthrough came after a discovery by Professor Arikawa and his team in 1997: the facets possess individual characteristics.

When viewed externally, the eye of the swallowtail appears to be black. However, light that passes down the facet’s rhabdome and is reflected back from the bottom will travel back up the rhabdome and emerge from the eye. Examining this light, known as “eye shine,” reveals which colors are absorbed by the eye. It was discovered that the swallowtail’s eye shine varies from facet to facet, and that there are three kinds of colors: deep pink, pale pink, and yellow.

The facet’s color varies according to the color of the pigment surrounding its rhabdome, which can be either red, pale pink (red and fluorescent), or yellow. All facets have the same structure. Thus, the facets of varied characteristics can be said to be created by the wearing of three different colored spectacles.

The function of the pigment is to absorb the light of different colors before the light passing through the rhabdome is absorbed by the cells. Facets of different pigmentation perceive different colors. For example, a pale pink facet recognizes purple, while a yellow facet identifies green.

The clever thing is that the cells that enclose the rhabdome recognize different colors according to their depth below the surface of the facet. While cells that are close to the surface of a red facet recognize ultraviolet light, cells that are deeper recognize red, since light is gradually absorbed as it passes down the rhabdome.

With the three different facets, the swallowtail thus has the ability to recognize six color elements. Its so-called colored spectacles (in the shape of the pigment) enable it to see a world rich in color.

Incidentally, it was British physician and physicist Thomas Young, who, in the 19th century, identified the fact that the human eye’s perception was based on the so-called three primary colors. Can the swallowtail then really have the ability to distinguish six color elements based on the six primary colors?

Professor Arikawa asserts that the swallowtail makes use of four primary colors. Although it possesses the ability to distinguish six color elements, for some reason it does not appear to use its ability to distinguish purple or the wideband light. The swallowtail is highly sensitive to the wideband light. This ability might be useful for seeing at dusk, while being able to see purple might be connected to some other information other than nectar. It is not yet known why these cells should remain unused. In the wake of the discovery of “colored spectacles,” further discoveries are anticipated.

It was probably the fact that Professor Arikawa himself is partially red color blind which led to his fascination with how the butterfly sees the world. Professor Arikawa’s studies represent the first step toward answering the larger question of how humans and other creatures see the world.