• The common bluebottle butterfly (Graphium sarpedon luctatius), which lives in South and Southeast Asia as well as parts of Australia, has an eye for color.

    Researchers discovered that the butterfly’s large, compound eyes each have at least 15 different classes of photoreceptors, light-detecting cells similar to the rods and cones in the human eye. Before this discovery, the maximum number of classes known to be in an insect eye was nine.

    Why do common bluebottles need so many classes of photoreceptors? Many insects can see color well with only three classes. Even humans only have three classes of cones, the photoreceptors responsible for color vision in human eyes, but we can see millions of colors.

    Researchers think the butterflies use four classes of photoreceptors for regular color vision. The other eleven, they believe, pick up on very specific stimuli in the butterfly’s view, such as fast-moving objects that might be hard to see against the blue sky or colorful objects, such as nectar-rich flowers, mixed in with other vegetation.

    Recognizing colors as they zoom by is an important life skill for these quick, agile butterflies. They communicate visually using their blue-green iridescent wings. They also need to be able to detect even small variations in the blue-green colors of other butterflies to spot rivals—and potential mates—as they flutter by.

    Scientists don’t yet know what some of the classes of receptors do. But they are certain they must be picking up important information that helps the butterflies survive. One possibility is that they help detect wing movement when the same or similar species gather at a water source on the ground. Further research will one day reveal the answers.

    1. What is surprising about common bluebottle eyes, compared with other insects?

      The common bluebottle has 15 classes of photoreceptors in each eye, compared with nine as the maximum scientists have found in other insects. Scientists are still studying what the butterfly can detect with the help of its extra photoreceptors, in addition to a vast number of colors.

    2. How has the common bluebottle adapted to different habitats?

      The common bluebottle lives in damp rain forests at low elevations, where it can be seen flying above the tree canopy. Here the larvae feed on leaves of laurel trees, which includes the cinnamon tree. The common bluebottle has adapted to land clearing for agriculture because it can feed on cinnamon trees grown on plantations. Also, in the eastern parts of Australia, the common bluebottle is common in the gardens of residential areas and has adapted to drier climates there.

    3. How many different butterfly species are there on Earth?

      Lepidopterists, the scientists who study butterflies and moths, estimate there are 20,000 butterfly species on Earth.

  • Term Part of Speech Definition Encyclopedic Entry
    compound eye Noun

    convex eye form found on many insects consisting of hundreds or thousands of tiny light-sensitive units (ommatidia) serving to focus light on the retina. Also called an arthropod eye.

    cone Noun

    cone-shaped photoreceptor in the retina of the eye, sensitive to color and light.

    detect Verb

    to notice.

    iridescent Adjective

    displaying a wide range of colors that appear to shimmer and change depending on the angle of view or the angle of illumination.

    photoreceptor Noun

    specialized cell that is sensitive to light.

    rod-RETINA Noun

    cylinder-shaped photoreceptor in the retina, sensitive to low light.

    species Noun

    group of similar organisms that can reproduce with each other.

    stimulus Noun

    something that inspires or incites action.