Eye design is extensively influenced by the physical properties of light and the limits of optics. Therefore, most animal eyes studied so far are variations of a few previously described types. In this project, Dr. Buschbeck will study insect eyes that diverge in fundamental ways from known visual organs. Eyes of the predatory larvae of diving beetles (Thermonectus marmoratus) contain at least two retinas, one with receptor cells oriented perpendicular to the axis of light, and one with receptor cells parallel to that axis. This is in contrast to typical insect eyes, which have only one retina with receptor cells oriented parallel to the axis of light. Based on preliminary findings, it is hypothesized that additional receptors are important to gauge appropriate strike distances by using a novel neural mechanism. In this project, these eyes will be characterized using a combination of histological, physiological, optical and behavioral techniques. Experiments will involve the participation of middle- and high-school teachers, who will be trained in a set of simple experiments that can be directly integrated into their school curricula, fostering inquiry-based learning. A broader impact on society at large is anticipated through an improved understanding of principles of vision that could ultimately lead to powerful new technologies. Several graduate and undergraduate students will be involved in the research, in an ongoing collaboration with the Cincinnati Zoo and Botanical Gardens, in broadly disseminating the results through high-profile journals and national and international meetings, and in local events that include public exhibits and lectures.
Most animal eyes fall into a few organizational categories, such as simple lens eyes or compound eyes. However, much can be learned from studying those eyes that function differently. In this project, we investigated the structural and functional organization of the eyes of predaceous diving beetle larva Thermonectus marmoratus (Coleoptera: Dytiscidae), focusing primarily on their unusual principal eyes. These beetle larvae are extremely successful visual predators, needing 400-700 mosquito larvae (or their equivalent) each before reaching adulthood. Central to our investigations has been the question of how the functional eye organization of the principal eyes has allowed these larvae to be such successful hunters. Unlike the eyes of most insects, these eyes fall in the general category of "camera eyes," which also includes human eyes. In contrast to our eyes, these larval eyes form long tubes, and are characterized by multiple layered retinas that are stacked on top of each other. Some of our key findings are that: (1) Anatomically these larvae have small visual fields, but they routinely behaviorally increase their visual field through scanning. (2) The more proximal retina of the principal eyes (situated furthest from the lens) is UV sensitive, and may be specialized to recognize polarization patterns of their prey. (3) The more distal retina consists of multiple layers of green sensitive photoreceptors that could assist in assessing the distance from prey. In addition (4), our most exciting finding has been that these eyes have truly bifocal lenses, something that never before has been demonstrated within the extant animal kingdom. Unlike most commercial bifocal lenses, the larval lenses exhibit an asymmetry that results in the two images (of the bifocal lens) being separated in two ways: sideways and in depth. This organization presumably allows the larvae to see its prey with improved contrast, a mechanism that could potentially be exploited in optical engineering. This project has led to training opportunities for many students, including three PhD, three MS and 17 undergraduates. Educational efforts included the training of a high school teacher (a project that led to the development and testing of beetle-based teaching modules for high schools), the development of online material and participation in a variety of local outreach activities.
