Zoologger: The solar-powered electric hornet (New Scientist)

• 15:00 10 November 2010 by Michael Marshall
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Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: Vespa orientalis

Habitat: throughout south-east Europe, north-east Africa and south-west Asia, and Madagascar

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.

Oriental hornets live in underground colonies. The workers prepare their home by digging into mud and then filling it with hexagonal cells much like those of bees. It has been known since the late 1960s that these insects are much more active in the middle of the day, unlike other wasps which tend to be most active in the early morning.

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.

Busy busy

The colony's workers spend their time clearing soil out of the nest. During the summer months they do much of this hard work in the heat of noon, whenultraviolet radiation is at its strongest. It turns out that the hornets have evolved some ingenious adaptations to harvest sunlight.

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.

So much power

The hornet can definitely transform solar radiation into electricity, as shining light on the cuticle generates a small voltage. But what does the insect do with it?

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.

Alternatively, it might help keep the hornets at a comfortable temperature. They can stay active when air temperatures are as high as 40 °C, and they may do so partly by converting some of the heat energy into electricity which they then store and convert back to heat when conditions became chilly.

The electricity might also give the hornets' wing muscles an extra jolt of energy. Anaesthetised hornets wake up faster, and immediately fly away, if ultraviolet light is shone on them. Solar power indeed.

Journal reference: Naturwissenschaften, DOI: 10.1007/s00114-010-0728-1