By Matt Walker
Editor, Earth News
A unique ability to soak up the sun
The Oriental hornet has a unique ability to harvest solar energy, scientists have discovered.
The large wasp species has a special structure in its abdomen that traps the sun's rays, and a special pigment that harvests the energy they contain.
The discovery helps explain why these hornets have a large yellow stripe across their body and why they become more active as the day gets hotter.
It also changes our understanding of how insect metabolism can work.
Xanthopterin works as a light harvesting molecule transforming light into electrical energy
Entomologist Dr Marian Plotkin
The discovery, reported in the journal Naturwissenschaften, was made by a team of researchers working in Israel and the UK, led by Dr Marian Plotkin of Tel-Aviv University.
Wasps are usually most active in the early morning, when they are around twice as active as at any other point in the day.
Oriental hornets (Vespa orientalis), which range from the Near East to India, are most active in the middle of the day.
Scientists have also long observed that Oriental hornet workers, which dig out nests underground, correlate their digging activity with the intensity of sunlight.
However, it was unclear why these Oriental hornets behave in this way.
That was until one biologist, the late Professor Jacob S Ishay, proposed that the insects may somehow be capable of harvesting solar radiation.
The sheet-like cuticle structure
Dr Plotkin's team has now tested this hypothesis, with remarkable results.
Using an atomic force microscope, they examined the fine structure of the hornet's cuticle, hard layers of which form the insect's outer body, or exoskeleton.
The part of the cuticle coloured brown is made from an array of grooves, with a height of just 160 nanometres.
The structure of the yellow part of the hornet's body is different.
This is made from a series of oval-shaped protrusions, each containing a pinhole-sized depression. Each protrusion is just 50nm tall and interlocks with another.
Further tests revealed what these structures do.
Essentially, say the researchers, they stop light being reflected off the hornet's body. Instead the light is trapped, and harvested for energy.
The yellow pigment harvests light
The brown part of the insect's body has the best anti-reflectance properties, helping to split any sunlight that falls upon it into several beams travelling in different directions.
The cuticle also contains a second thin sheet-like structure, with a series of sheets stacked on top of each other, with decreasing thickness from top to bottom.
Stacked together in every layer are rod-like structures composed of chains of a polymer called chitin. These rods are embedded in a protein matrix.
This intricate structure further serves to trap light within the cuticle, forcing it to bounce between different layers.
Capturing the sun
But the ability of the hornets to harvest solar energy does not stop there.
Within this cuticle is a pigment that actually captures the energy of the sun's rays.
"The pigment melanin gives the hornet its dominant brown colour. The pigment xanthopterin, in the head and abdomen in a form of stripes and bands, gives the Oriental hornet its bright yellow colour," explains Dr Plotkin.
Highly magnified view of the wasp cuticle
"Xanthopterin works as a light harvesting molecule transforming light into electrical energy."
The hornets' ability to convert sunlight in this way could explain why they become more active during the middle of the day, when the light intensity is highest.
"We assume that some of the energy is transformed in a photo-biochemical process which aids the hornets with their energy demanding digging activity," Dr Plotkin told the BBC.
The solar-powered hornets have one further unique claim.
Until now, insects were thought to perform metabolism in an organ known as the fat body, which performs a similar function to the human liver.
Most of the fat body is in an insect's abdomen surrounding the gut, where it can quickly take up absorbed nutrients, though some is scattered elsewhere.
"We have found that the main metabolic activity in the Oriental hornet is actually in the yellow pigment layer," says Dr Plotkin.