How can butterflies see

This comes in handy for food sources that do not have UV patterning, like rotting meat. Yes, butterflies derive nutrients from deceased animals.

Butterfly wing scales as seen under a microscope. Photo credit: Thomas Eisner. Some people will be surprised to learn that in addition to sensing ultraviolet light, butterflies can also emit ultraviolet light waves through their wings. Their wings are coated with minuscule scales that can reflect different color spectrums, depending on their shape and the angle of light that hits them. As the angle of sunlight shifts, the colors emitted from these scales shift.

These receptors are used to identify ultraviolet markings for use in mating and pollination purposes. A study in Japan of male Swallowtail butterflies found that each class of receptor is stimulated by different colors. For example, some flowers present a visual contrast when pollinating while green light is triggered during egg laying.

Butterflies use light to search for food, find potential mates, recognize predators, and distinguish nectar in flowers. The nectar color guide of the horse chestnut tree will change from yellow to red when nectar is no longer in production.

Butterflies will use these cues to determine which blossoms are capable of pollination. They are also likely to be better than us at telling apart colors that are only slightly different. Some birds change the sensitivity of their cones to different wavelengths in the spectrum by using colored oil droplets inside their eye. These oil droplets work in a similar way to sunglasses with colored lenses.

Some insects with compound eyes also do this [ 2 ]. Butterflies are another example of an animal with complex color vision. Their color vision system appears to have evolved from a three-color system based on UV-, B-, and G-sensing cells, like that of bees.

Over millions of years, the color vision of some butterflies became more complex by adding extra light-sensing cells with different spectral sensitivities, probably to help them find flowers from which to drink nectar. The BB cells got that name because they respond to a wide variety of different wavelengths, rather than one specific color [ 3 ]. We recently studied the common bluebottle butterfly, Graphium sarpedon , and found that it has at least 15 different classes of light-sensing cells in the eye [ 4 ] Figure 3B.

This is the highest number of different kinds of light-sensing cells ever identified in an insect. In the case of the sulfur butterfly, Colias erate , sexual dimorphism is found in the number of red-sensing cells: females have three types that are sensitive to slightly different shades of red, while males only have one type [ 5 ]. You may be asking yourself: why do butterflies need so many types of light-sensing cells? Does having more types result in better color vision?

As we already discussed, most humans have three classes of cones. But some people usually males only have two fully functional cone types, with reduced sensitivity in the third type. We call this condition color blindness [ 6 ]. As we have seen, having just two cone types is enough for color vision.

It may be better to call these people color deficient instead of color blind. If someone color deficient and someone with normal vision view the same scene, they may see rather different things; color deficient people usually have difficulty in telling apart red from green. In other words, having fewer classes of light-sensing cells limits the ability to see colors.

On the other hand, some people have four cone types. This is very rare and is only found in females. These people are thought to see a lot more colors than people with regular vision. The extra cone allows them to see additional colors that are invisible to most of us [ 7 ]. It seems that more light-sensing cells result in better color vision. So far, the richest variety of light-sensing cells that has been found in the animal kingdom is in the eyes of reef-dwelling mantis shrimp, which have up to 16 types Figure 3D [ 5 ].

One might assume that these mantis shrimp have better color vision than any other animal. But in reality, their color vision turns out to be surprisingly poor [ 8 ]. Humans can distinguish between light wavelengths that differ by as little as 1 nm. Invisible patterns are revealed on this black-eyed Susan and Cleopatra butterfly in ultraviolet light.

To the human eye, many butterflies appear the same, but the butterflies themselves can often identify each other quite easily using ultraviolet markings. Shown above is the yellow Cleopatra butterfly Gonepteryx cleopatra. The male and female little sulphur butterflies Eurema lisa differ only in the ultraviolet region, with the males being strongly ultraviolet reflective and the females non-reflective in ultraviolet.

In contrast, butterflies that are palatable to birds display significant differences in appearance. The ultraviolet patches on some butterflies are directionally iridescent, so that they appear to flicker in flight.

This flickering is thought to have an important role in butterfly behavior and communication. Butterflies tend to avoid the color green when feeding, but are attracted to it during egg laying.