The oriental hornet has built-in "solar cells" that generate electricity from sunlight—a first in the animal kingdom, according to a new study.Scientists already knew that the hornet species, for unknown reasons, produced electricity inside its exoskeleton, according to study leader Marian Plotkin of Tel-Aviv University.
Plotkin's late mentor Jacob Ishay made the discovery after observing that the insect is active when the sun is most intense—unusual for hornets.
Plotkin and colleagues recently went a step further by examining the structure of the hornet's exoskeleton to find out how the electricity is produced.
Their research revealed that pigments in the hornet's yellow tissues trap light, while its brown tissues generate electricity. Exactly how the hornets use this electricity is still not entirely understood, Plotkin noted.
(See "Implanted Fuel Cell Powered by Rat's Body Fluids.")
"When I was running my experiment, people told me it was never going to work," she said. "I'm so happy at the results."
While solar cells using human-made substances are usually 10 to 11 percent efficient at generating electricity, the hornet's cells are only 0.335 percent efficient. For instance, the hornet still gets the vast majority of its energy from food.
But that's hardly the point, Plotkin said.
"We've seen solar harvesting in plants and bacteria, but never before in animals."
Hornet Pigment a Solar Power Source
The team found that many of the hornet's brown tissues contain melanin, the pigment that protects human skin cells by absorbing damaging ultraviolet light and transforming it into heat.
(Related: "Bio-Computer Created Inside Living Cell.")
A structural analysis of the brown tissues also uncovered grooves that capture light by channeling rays into the tissues and breaking them apart into smaller rays.
The brown tissues "are a lot like a light trap—only one percent of the light that strikes is reflected away," said Plotkin, whose study appeared in the December issue of the journal Naturwissenschaften.
The hornet's yellow tissues contained the obscure pigment xanthopterin, which gives butterfly wings and mammal urine their color. (Read about a urine battery that turns pee into power.)
When the team isolated xanthopterin in a liquid solution, and then placed the solution inside a solid solar cell electrode, a type of conductor. When the scientists shed light on the electrode, the pigment in the solution generated electricity.
"Fabulous" Hornet Study Needs Comparison
Entomologist Chris Lyal at London's Natural History Museum called the study a "fabulous investigation."
"I'd love to see a comparison with the [exoskeleton] structure of other hornets that do not appear to be gathering energy from the sun. In theory, other hornets should have exoskeleton layers that look very different," said Lyal, who was not involved in the study.
It's also possible other insects have similar electricity-generating abilities, Lyal added.
"For instance, I remember coming across the Apollo butterfly in the Pyrenees, which basks in the sun before flying—presumably absorbing solar radiation," he said.
"I wonder how different the hornet tissues actually are from those of that butterfly."
