Humans have been trying to build solar cells since the late 19th century, but despite over 100 years of work we still don't get very much energy out of them. Our efforts are put to shame by the oriental hornet, which had solar cells long before Homo sapiens first walked the earth.
That may seem an inconsequential lifestyle detail, but the late Jacob Ishay of Tel Aviv University in Israel and his colleagues have shown that it is a clue to something more significant: oriental hornets have solar cells built into their skin.
Marian Plotkin of Tel Aviv University and colleagues used advanced microscopes to examine a hornet's exoskeleton, or cuticle. Most of the cuticle is brown, but a few sections are yellow. These colours mark the hornet as a venomous insect, and therefore best avoided by potential predators, but the pigments are also involved in mopping up solar energy.
Both the brown and yellow cuticles are made up of many layers laid on top of each other, around 30 in the brown and 15 in the yellow. The brown areas contain melanin, a pigment also found in human skin, while the yellow areas contain xanthopterin.
Plotkin found that the brown cuticle is covered with grooves, while the yellow cuticle is covered with oval-shaped lumps. Both absorbed 99 per cent of the visible light falling on them.
What's more, the grooves on the brown cuticle are fairly regularly spaced, Plotkin found. As a result, the cuticle surface mimics a series of slits, which act as a diffraction grating, trapping yet more light inside the cuticle layers.
The team also built a solar cell that successfully used xanthopterin to harvest light.
One possibility is that it might harness the electricity to power enzymes in the yellow cuticle. Last year Ishay and Plotkin showed that enzymes in these regions perform metabolic functions similar to those of livers in mammals, and that they are more active when the insects were exposed to ultraviolet light.