The solar cell in hornet cuticle: nanometer to micrometer scale

This paper focuses on structures observed by scanning and transmission electron microscopy and atomic force microscopy upon and within the abdominal cuticle of the Oriental hornet, Vespa orientalis. Taken together, these structures form an 'organ' whose function is the absorption ef sunlight and storage of the resultant electrical energy. In principle, this organ resembles a solar cell. The gaster of the hornet is cone-shaped and comprised of the following components: (i) structures protruding from the epicuticular surface, which are arranged lengthwise and generally parallel to one another like furrows; (ii) these furrows overlie straight, terrace-like flat surfaces, which are shaped like imperfect polygons, each about 100 microm2 in area; (iii) each terrace abuts an area that is positioned lower than it, the juncture between the two is comprised of a stripe, which is vertical to the topmost polygonal flat and perpendicular to the furrows; (iv) between the juncture stripes, at distances of 10-100 microm apart, there are smooth flat surfaces reminiscent of a parabolic mirror of about 20 microm2, each possessing a rounded, eccentric area recessed to a depth of about 7.5 microm (i.e. a heat sinking housing), underneath which can be seen the upper part of the peripheral photoreceptor (PP); (v) cuticular lamellae, about 30 in number, create a pore between them in the PP region, fold in vertical fashion and enwrap the entire PP down to its broad base which is immersed in a yellow bacterial layer and where the cuticular layers are very thin. Presumably the light energy impinges vertically upon the individual terraced flat surfaces while the hornet is in flight, creating such situations for a fraction of a second at a time. The insolation is absorbed more intensively in the furrows, owing to their shape. The picked up solar energy is now transmitted as electric charges from the furrows to the conduction stripes between abutting terraces, and thence to: (a) the rhodopsin pigment in the PP (the smaller moiety), or (b) the numerous cuticular layers, depending on the energy level of the picked up photons (the larger moiety), and (c) finally to the bacterial layer for storage in its walls. The latter region will be a p-type as compared to the energy stored in the layers of brown cuticle. The cuticle thus was found to contain many PPs but within the cuticle they are dispersed in contrast to the situation in the compound eye, where the many photoreceptors are concentrated.