Biofluorescence of the Monarch Butterfly

By Chris Clemens


As a biologist, my interest in entomology predates my fascination with fluorescent minerals by decades, so it was only natural that when my wife Kristine and I found some monarch butterfly eggs last summer, I would examine them for fluorescence under UV at their various stages of development into adult butterflies. All butterflies (and moths) undergo 4 primary stages of development through the process of metamorphosis: 1.) egg, 2.) larva, 3.) pupa, and 4.) adult. The idea for this project was conceived when I found a nearly fully developed monarch caterpillar, and on a whim checked it for fluorescence under the high output beam of a Convoy S2+ long wave ultraviolet (UV) flashlight. To my surprise, the larva was fluorescent, and thus this project was born.

This study begins with a mountain bike ride last summer through the Grand Kankakee Marsh in northwestern Indiana. As we were riding along the trail, my wife and I noticed a female monarch butterfly making the rounds from plant to plant through the local population of milkweed, laying eggs on the undersides of the leaves. Milkweeds of the genus Asclepias are the preferred food plant for the monarch caterpillar, and due to their content of cardenolide glycosides, a family of cardiotoxic steroids, they confer to the monarch larvae and adult butterflies poor taste and toxicity, therefore providing a defense against potential predators that might otherwise eat them. We stopped and collected several milkweed leaves with eggs attached and took them home. The following photograph (Figure 1) shows a close-up view of one of the eggs.

Figure 1. A single monarch butterfly egg attached to the underside of a milkweed leaf, seen under visible light.

For the purpose of determining the fluorescent response at each stage of the life cycle, the monarch specimens were examined under long wave UV (365nm) produced by a Convoy S2+ LED flashlight. Figure 2, following, shows the fluorescent response of the same egg seen in Figure 1.

Figure 2. The same egg seen in Figure 1, viewed under long wave UV. Note the dim red fluorescent response of the milkweed leaf.

As seen in Figure 2, above, both the monarch egg and milkweed leaf are only dimly fluorescent under long wave UV. Interestingly, the milkweed leaf shows dim red fluorescence, likely due to red emission from the chlorophyll.

Within several days of finding the eggs, they hatched into tiny monarch larvae, each only a couple of millimeters long. Having a garden planted with native plants in our back yard, including several milkweed species, we had an abundant supply of food plants and were well equipped to raise the hatchling monarch caterpillars. After four molts over the course of approximately 10 days, the monarch caterpillars were full grown. Figure 3, following, shows one of the full grown larva.

Figure 3. Full grown monarch caterpillar, seen under visible light on its host milkweed plant.

At this stage of the study, it was time for another observation under UV light. Figure 4, following, shows the fluorescent response of the caterpillar under long wave UV.

Figure 4. Fluorescent response of the same monarch caterpillar shown in Figure 3, viewed under long wave UV light.

In contrast to the dim fluorescence of the egg, the monarch caterpillar showed a much brighter fluorescent response under long wave UV. The white and yellow bands showed the brightest fluorescence, while the black bands were not fluorescent.

Within 24 hours of taking the photographs of the fully developed larvae, they attached themselves by their rear legs and a pad of silk to the stem of the milkweed plant, hung upside down and shed their skins as caterpillars for the last time. The pupal stage emerged, and quickly hardened into beautiful green chrysalises adorned with metallic gold and black spots. The following photograph (Figure 5) shows three chrysalises.

Figure 5. Three monarch butterfly chrysalises on a single milkweed stem, seen under visible light.

Ever since I saw my first monarch chrysalis as a child, they have reminded me of beautiful little green Christmas tree ornaments. Little did I know back then that they also offered beauty under wavelengths of light unseen by the human eye. It was time for another observation under UV light. Figure 6, following, shows the fluorescent response of the chrysalises under long wave UV.

Figure 6. The fluorescent response of the three monarch butterfly chrysalises under long wave UV.

Under long wave UV, the chrysalises showed ghostly blue/white fluorescence. Interestingly, the junctions of the chrysalis segments were highlighted by the brightest response, revealing structural details not easily seen under visible light.

After nearly two weeks, the chrysalises hatched into fully developed, adult monarch butterflies. While hanging from the empty shells of their chrysalises, the adult butterflies pumped fluid into their wings, allowing them to expand and harden over the next several hours. The following picture (Figure 7) shows the adult butterflies a few hours after hatching.

Figure 7. The three adult monarch butterflies have emerged from their chrysalises. Seen under visible light.

Our three monarchs would soon be ready to fly, so it was time for the final observation under long wave UV. Unlike humans, who cannot see ultraviolet light, the eyes of insects can see into the UV region of the spectrum. Because of this, the adult butterflies were highly reactive to the beam of the Convoy long wave UV flashlight, and were difficult to photograph. Figure 8, following, shows my best attempt to photograph the fluorescent response of the butterflies under UV.

Figure 8. The fluorescent response of the three adult monarch butterflies under long wave UV. The center butterfly can be seen reacting to the UV source, producing a blurred image.

Under long wave UV, the white spots on the bodies and undersides of the wings produced a bright white fluorescent response while the larger orange areas fluoresced a dim to moderate orange color. The black areas were not fluorescent. Interestingly, the empty chrysalis shells fluoresced a blue/purple color. The following two photographs show close-up detail of one of the adult butterflies seen under visible light (Figure 9) and long wave UV (Figure 10).

Figure 9. Close-up detail of a single adult monarch butterfly clinging to its empty chrysalis shell.

Figure 10. Close-up detail of the fluorescent response of the adult butterfly under long wave UV.

After the last set of photographs were taken, the adult butterflies were released outside to take their first sips of flower nectar and to start their long migration south to Mexico for the winter.

Biofluorescence occurred in the monarch butterfly during all stages of its life cycle except the egg. The mechanism of fluorescence cannot be known with certainty without further analysis to determine the responsible fluorophore(s); however, it may be related to the cardenolide glycosides stored in the bodies of the caterpillar and adult butterflies, as some of these compounds are known to fluoresce light blue under long wave UV. If this is the case, it might explain why the eggs are not significantly fluorescent, as they are produced prior to the consumption of the milkweed food plant from which the cardenolide glycosides are obtained. Alternately, the fluorescence might be activated by a component of the outer cuticle of the larva and chrysalis, such as chitin, which produces blue/white fluorescence in some arthropods. In the adult butterflies, the white fluorescent response of the spots is likely associated with some component of the white wing and body scales. Whether the observed fluorescence plays a biological role in the life of the monarch butterfly, or is simply a biochemical coincidence, is not known. It would be interesting to expand this study to include other species of butterflies, moths, and other insects to determine how widespread the phenomenon of biofluorescence is within the insect world.

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