Volts and Vespa: Buzzing about Photoelectric Wasps

Oriental hornet (Vespa orientalis)

Wasps on the attack can seem like vicious little war machines: merciless flying Terminators, driven by cold insectoid rage, utterly unstoppable (except by a well-swung roll of newspaper). That comparison to robots may have ratcheted up a notch with a new report in the journal Naturwissenschaften that one species of hornet, Vespa orientalis, seems to be partly solar powered. I love this discovery—but in the wake of the arsenic-eating bacteria fiasco, I’m trying not to let confidence that this weird biology news is true run away with me.

Like the worker castes of all wasps that live underground, those of Oriental hornets diligently expand their burrows by grabbing soil, flying a short distance from the nest to throw it away, then returning for more, perhaps hundreds of times a day. Entomologists have noticed, however, that Oriental hornets dig most feverishly at mid-day, when the sun is most intense, unlike other wasp species that concentrate their labors in the cooler early morning hours. That unusual behavior prompted the late Jacob S. Ishay of Tel-Aviv University to begin years of investigation into whether the hornets had some photoelectric traits that gave them an extra jolt of vitality in sunshine.

As far back as 1991, for example, Ishay found that shining light on the hornets—live, anesthetized or even dead—could produce voltage differences of several hundred millivolts across their hard exoskeletons, which suggested that the cuticle material making up the exoskeletons was effectively an organic semiconductor converting light into electricity. Indeed, Ishay even found that shining ultraviolet light on an anesthetized hornet would wake it up faster, as though the light were recharging the insect. The general conjecture that emerged was that the distinctive bright yellow stripe across the Oriental hornet’s otherwise brown abdomen might be functioning as some kind of solar cell, but the mechanism underlying the effect was unclear.

Surface microstructures of the cuticle in the brown (top) and yellow (bottom) areas of the Oriental hornet's abdomen. (Source: referenced Naturwissenschaften paper by M. Plotkin et al.)

Carrying on Ishay’s work, some of those same colleagues (and others) now think they have found confirming evidence of that hypothesis by looking closely at the structure of the cuticle with an atomic force microscope and other instruments. As they describe in the Naturwissenschaften paper, Ishay’s former student Marian Plotkin of Tel-Aviv University and her team found that the wasp’s brown cuticle consists of an array of tiny layered grooves about 500 nanometers apart and 160 nanometers high. In contrast, the yellow cuticle is made up of small interlocking protrusions, about 50 nanometers high, each bearing a “pinhole” depression. Both these sets of structures make the cuticle antireflective across much of the visible light spectrum: they absorb about 5 percent more light than a comparable flat surface would. Within the cuticle, stacked sheets of the hard polymer called chitin further boost the exoskeleton’s ability to trap light.

The key element may be that the yellow stripe derives its color from the pigment xanthopterin. The researchers demonstrated that in an artificial solar cell, xanthopterin can work as the light-harvesting molecule, converting light into electrical energy with 0.0335 percent efficiency.

Plotkin and her colleagues suspect that within the yellow stripe, xanthopterin activated by the trapped sunlight separates electrical charges across layers of the cuticle; some other biochemical process could then tap this voltage gradient to make energy-rich molecules that the wasp’s muscles or other organs could consume as needed. As Plotkin said to the BBC’s Matt Walker, “We assume that some of the energy is transformed in a photo-biochemical process which aids the hornets with their energy demanding digging activity.”

Of course, what that suggestion immediately brings to mind is the idea that the wasps are photosynthesizing. Photosynthesis, the ability to turn sunlight into biologically useful energetic compounds, is a trait that many of us still reflexively associate with green plants, algae and cyanobacteria, not with animals. That distinction has not strictly held for at least the past couple of years, since the 2008 revelation that the emerald green sea slug, Elysia chlorotica, draws energy from sunlight with the help of chloroplasts and genes picked up from the algae it eats. However, no biochemical mechanism yet identified in these hornets closes the loop and turns the energy in the cuticle voltage gradients into ATP, glucose, glycogen or any other energetic molecule that could fuel cells.

But in 2009, Ishay, Plotkin and their coworkers showed that, unexpectedly, a variety of important metabolic activities seem to center on the yellow abdominal stripes of the Oriental hornets rather than around the fat bodies that normally handle them in insects. (Think about what this means: if the same arrangement applied to humans, our skin would be doing the job of our livers.) Moreover, shining ultraviolet light on the yellow cuticle lowered the hornets’ levels of several enzymes, including creatine kinase, alanine aminotransferase and aspartate transaminase. So the yellow stripes do seem to be responsive to light in ways that affect the insects’ metabolism, which might point the way to how the insects either produce energy molecules or use them more efficiently in strong sunlight.

All this work certainly builds a strong circumstantial case that the Oriental hornets have indeed evolved an organic solar collector—perhaps not photosynthetic in the usual sense, but something similar. The answer they’ve deduced is so interesting, it makes me want to believe it. Ordinarily, I’d be loath to second guess the experts’ analyses and conclusions. And yet I must admit that certain points still perplex me about this discovery and leave me reluctant to embrace it.

First, the scientists’ analysis doesn’t quantify just how much energy a wasp could hope to gather by this mechanism. That 0.0335 percent conversion efficiency may be significant but it sounds a bit paltry beside, say, the 0.1 to 2 percent efficiency that many plants show in converting sunlight to biomass. Of course, the wasps would only be getting an energy boost from the sun, not all their energy. For a wasp busily crawling through tunnels, dragging clods of soil to the surface, flying to dump them and returning, maybe every little bit of energy helps. But how much energy could it even theoretically recapture during its brief time above ground compared to its overall energy expenditure? The antireflective and light-trapping properties of the wasp’s cuticle do seem highly suggestive, but they might have evolved for other, unknown reasons, too.

Second, if exposure to light helps to charge up the hornets’ metabolism, then why did Ishay’s team find that ultraviolet light lowered the hornets’ levels of those metabolically important enzymes below what was seen in the dark? I would think that diminishing the concentrations of creatine kinase, for example, would only interfere with the the insects’ ability to use or regenerate ATP in their cells. Can anyone explain this to me in comments?

Perhaps I’m simply being too cautious. Still… after last week’s study about arsenic-eating bacteria turned out to be rather less than it first appeared, it feels incumbent on all of us writing about unusual biology to be extra circumspect for a while.

Referenced papers:

“Solar energy harvesting in the epicuticle of the oriental hornet (Vespa orientalis)” by M. Plotkin, I. Hod, A. Zaban, S. A. Boden, D. M. Bagnall, D. Galushko and D. J. Bergman. Naturwissenschaften (2010) 97:1067–1076, doi:10.1007/s00114-010-0728-1

“Electrical properties of the oriental hornet (Vespa orientalis) cuticle” by J. S. Ishay, A. H. Abes, H. L. Chernobrov, I. Ishay, and A. Ben-Shalom. Comparative Biochemistry and Physiology Part A: Physiology (1991), 100(2):233-271. doi:10.1016/0300-9629(91)90469-S

“Photovoltaic effects in the Oriental hornet, Vespa orientalis” by J.S. Ishay1, T. Benshalom-Shimony, A. Ben-Shalom and N. Kristianpoller. Journal of Insect Physiology (1992), 38(1):37-48. doi:10.1016/0022-1910(92)90020-E

“Hornet flight is generated by solar energy: UV irradiation counteracts anaesthetic effects” by J. S. Ishay. Journal of Electron Microscopy (Tokyo) (2004), 53(6):623-633. doi: 10.1093/jmicro/dfh077

“Some liver functions in the Oriental hornet (Vespa orientalis) are performed in its cuticle: Exposure to UV light influences these activities” by M. Plotkin, S. Volynchik, D. Itzhaky, M. Lis, D. J. Bergman and J. S. Ishay. Comparative Biochemistry and Physiology Part A (2009), 153 (2): 131-135. doi: 10.1016/j.cbpa.2009.01.016

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