Biological bifocal lenses with image separation

Abstract

Almost all animal eyes follow a few, relatively well-understood functional plans. Only rarely do researchers discover an eye that diverges fundamentally from known types. The principal eye E2 of sunburst diving beetle (Thermonectus marmoratus) larvae clearly falls into the rarer category. On the basis of two different tests, we here report that it has truly bifocal lenses, something that has been previously suggested only for certain trilobites. Our evidence comes from (1) the relative contrast in images of a square wave grating and (2) the refraction of a narrow laser beam projected through the lens. T. marmoratus larvae have two retinas at different depths behind the lens, and these are situated so that each can receive its own focused image. This is consistent with a novel eye organization that possibly comprises "two eyes in one." Moreover, we find that in contrast to most commercial bifocal lenses, the lens of E2 exhibits asymmetry, which results in separation of the images both dorsoventrally and rostrocaudally within the layered retina. Visual contrast might thus be improved over conventional bifocal lenses because the unfocused version of one image is shifted away from the focused version of the other, an organization which could potentially be exploited in optical engineering.