Using X-ray microtomography to uncover the visual world of ancient and historical insects

Animal eyes typically have specialized regions of high resolution and/or sensitivity that enable them to execute behavior effectively within their specific visual habitat. These specializations, and evolutionary changes to them, can be crucial for understanding an animal’s ecology. Techniques for analyzing visual specializations typically require fresh samples and are thus limited to studies on extant species. However, the cornea of invertebrate compound eyes is readily preserved, even in fossils. To compare and quantify vision in specimens from different time periods and habitats, and with different preservation states – fossilized in amber, dried or stored in alcohol – we developed a novel technique that uses X-ray microtomography to create high resolution 3D models of compound eyes from which their sensitivity, spatial resolution, and field of view can be estimated using computational geometry. We apply our technique to understanding how the visual systems of some of the smallest flying insects, fungus gnats, have adapted to different types of forest habitat over evolutionary time (~30 mya to today). Our results demonstrate how such investigations can provide critical insights into the evolution of visual specializations and the sensory ecology of animals in general.