Each summer, scores of sunny yellow dragonflies transform into vibrant scarlet flyers. Behind this showy makeover scientists have discovered a simple chemical reaction every bit as elegant as the spectacle itself.
The discovery—made by a team of biologists in Japan—reveals both the pigments and mechanism behind this colorful change, a process they describe in a study released yesterday in Proceedings of the National Academy of Sciences.
"Because dragonflies are diurnal [awake during the day] and have a better sense of vision than smell or sound, body color is essential for partner recognition," says lead author Ryo Futahashi, a biologist at Japan's National Institute of Advanced Industrial Science and Technology. Futahashi, who has been fascinated by dragonflies since he was a child, explains that the red dragonfly in particular has a powerful cultural significance in Japan. The timing of their maturation coincides with late summer, making them a popular poetic symbol of autumn.
Futahashi and colleagues studied two dragonfly genera, Crocothemis and Sympetrum, in which males shift from a sunny yellow to scarlet when they reach sexual maturity. In addition to Japan, dragonflies of either genus can be observed in various parts of Europe, Africa, Asia, Australia and North America. Their color shift—referred to by biologists as nuptial coloration—signals that a male is on the market for a mate. There's an added advantage to the transformation: Young males are spared the hardship of territorial squabbles. Mature males ignore the yellow youths as nonthreatening.
To decipher exactly what causes the change, the scientists collected specimens from six species. Collecting and analyzing epidermis samples from the abdomens of these dragonflies, they identified the color source as ommochrome pigments, responsible for color in many insects and crustaceans.
Next, they chemically induced color change by microinjecting solution into a live dragonfly's abdomen. They found that at the heart of the mechanism was a simple chemical redox reaction. In redox reactions the charge of chemical components becomes more positive through oxidation or more negative through reduction, removing or adding electrons, respectively. An injection of ascorbic acid (vitamin C) triggered reduction, which transformed yellow dragonflies to red. An injection of sodium nitrite, causing oxidation, reversed the effect.
The researchers observed that through reduction they could not only redden immature males, they could also transform females into the scarlet hue. In the wild, gynandromorphic dragonflies, which have both male and female characteristics, display a patchwork of red and yellow when they mature. These observations have led the researchers to suspect that a hormonal cue acting at the cellular level triggers a reduction reaction in the maturing male.
Futahashi speculates that there may even be an additional benefit to this change. The altered oxidative state in mature males may actually have extra antioxidant protection, offering a natural defense against the sun's UV radiation. He adds that this is the first time a redox reaction has been identified behind this kind of color change in insects—although a similar process occurs in maturing fruit such as tomatoes.
"Dragonflies are among those amazing animals that captivate naturalists for numerous reasons, including their aerial acrobatics and their striking colors—which notoriously vanish in most museum specimens," says Bruce Hammock, an entomologist at the University of California, Davis, who was not connected to the study. Hammock has examined a similar chemical change in the American bird grasshopper. He praises the study for its elegant design, which starts with a basic observation, explores through experimentation and incorporates sophisticated chemical techniques.