Typically, wasps and hornets are more active during the early morning as they begin their daily activities. In contrast, the oriental hornet is most active during the middle of the day. It is a social insect that nests underground and correlates its digging activity with the intensity of the sunlight.
As it turns out, there is a good reason why these hornets tend to work in direct sunlight. These hornets have an outer layer (cuticle) that actually allows them to absorb sunlight. The brown and yellow colors of the oriental hornet not only serve to warn potential predators, but also contain pigments that harvest solar energy. The banded sections have multiple layers that get successively thinner and sandwich the pigments. The brown cuticle has about 30 layers while the yellow cuticle has roughly 15. Scientists have found that the outer brown layer is covered in grooves that act almost like gratings that help trap light, allowing the rays to funnel inward for better absorption. The outer yellow layer is covered in oval-shaped bumps that increase effective surface area for absorption. Both of these areas exhibit antireflection and light-trapping properties, enhancing the absorption of light in the cuticle. The role of the layers getting successively thinner is still under investigation.
The sunlight that these hornets capture is likely converted into electrical energy. There exists a voltage between the inner and outer layers of the yellow stripe that increases in response to illumination. The harvested energy may be used in physical activity (digging or flight) and temperature regulation. It even seems to provide enough energy to carry out metabolic functions similar to the liver (producing or filtering enzymes and sugars). The enzymatic activity in these regions has been shown to decrease when the hornet is exposed to light, allowing it to conserve its energy.
This summary was contributed by Leon Wang.
“The complex structure of the cuticle is produced by extracellular secretion from the epidermis. It is constructed as a composite consisting of chitin filaments, structural proteins, lipids, catecholamine derivatives, and minerals. The Oriental hornet cuticle (the exoskeleton) exhibits a brown-yellow pattern…The yellow segments protect the cuticle from potentially harmful solar UV radiation, similar to the role of melanin in the brown color segments of the hornet’s body…The yellow segments contain xanthopterin, which is housed in an array of barrel-shaped granules…the voltage between the hypocuticle and the exocuticle of the yellow stripe showed a negative potential at the hypocuticle with respect to the positive exocuticle. In response to illumination of the yellow stripe, the difference in potentials between light and darkness increases…The fact that the Oriental hornet correlates its digging activity with insolation, coupled with the ability of its cuticular pigments to absorb part of the solar radiation, may suggest that some form of solar energy harvesting is performed in the cuticle.” (Plotkin et al. 2010:1068)
“[T]he epicuticle acts as an antireflective layer…the surface structure has evolved to confer both AR [anti-reflective] and light-trapping properties to the epicuticle, enhancing the absorption of light within the cuticle of the hornet, resulting in more efficient collection of solar energy.” (Plotkin et al. 2010:1073-4)
“[T]his underlying layered structure contributes to the overall reflectance properties of the epicuticle…Light passing through the yellow stripes is absorbed by xanthopterin, which serves as a light-harvesting molecule. The xanthopterin resides in tightly packed yellow pigment granules, which may serve to increase the effective surface area available for light absorption…allow absorption in the UV wavelengths while allowing an increase in the reflectance of higher wavelengths. The ability of xanthopterin to serve as a visible light absorber in a photo electrochemical solar cell is clearly evident from the I–V characteristics of the xanthopterin-sensitized solar cell…diffusion potential across the cuticle, with the inside negative with respect to the outside. Digby has suggested that electrons move through the semiconductive cuticular layer. This process creates calcium carbonate that precipitates in the cuticle…Oriental hornet has evolved a cuticle design to harvest solar energy…the surface structures confer AR and light-trapping properties, enhancing absorption by approximately 5% compared to a flat surface. The xanthopterin pigment found within the cuticle has been proven to be a suitable absorber of light for the harvesting of solar energy by a demonstration of its use in an organic solar cell, with a conversion efficiency of 0.335%.” (Plotkin et al. 2010:1075)
“The findings clearly show that in regions of the hornet’s body where the cuticle is translucent, which in the Oriental hornet are the yellow stripes…there are ‘pockets’ engaged in enzyme activity and glucose analogous to the liver functions and enzymatic activity in other organs of mammals…As for the influence of illumination on this process, in all our tested hornets, the enzymatic activity was higher in the dark than in light…we have clearly ascertained that our hornet’s yellow stripes play a role in its metabolism. In short, the translucent, light-colored stripes comprise a very extensive tissue that actively participates in the metabolic processes of the hornet. This tissue somehow utilizes sunlight and promotes the production of enzymes and glucose and very possibly of other materials as well.” (Plotkin 2009 et al. :134)