A bug with bifocals:
Figure 1. Illustration of the Third-Instar Larvae of Thermonectus marmoratus and Its Principal Eyes (A) Picture of the entire animal. (B) Scanning electron micrograph of the larval head, showing the two large lenses of the principal eyes (E1 and E2) on each side of the head. (C) The gross optical and neural organization of E2. Inserts a and b schematically illustrate the eye organization of the two sections indicated in the scanning-electron-micrograph image. White lines show the approximate visual fields of the retinas. Abbreviations are as follows: PR, proximal retina; DR, distal retina; P, pit of distal retina.
Here is the abstract to the paper linked to above:
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.