An animal's vision depends on terrain features that limit the amount and distribution of available light. Approximately 10,000 years ago, vinegar flies (Drosophila melanogaster) transitioned from a single plant specialist into a cosmopolitan generalist. Much earlier, desert flies (D. mojavensis) colonized the New World, specializing on rotting cactuses in southwest North America. Their desert habitats are characteristically flat, bright, and barren, implying environmental differences in light availability. Here, we demonstrate differences in eye morphology and visual motion perception under three ambient light levels. Reducing ambient light from 35 to 18 cd/m² causes sensitivity loss in desert but not vinegar flies. However, at 3 cd/m², desert flies sacrifice spatial and temporal acuity more severely than vinegar flies to maintain contrast sensitivity. These visual differences help vinegar flies navigate under variably lit habitats around the world and desert flies brave the harsh desert while accommodating their crepuscular lifestyle.

The tiny brains of insects presumably impose significant computational limitations on algorithms controlling their behavior. Nevertheless, they perform fast and sophisticated visual maneuvers. This includes tracking features composed of second-order motion, in which the feature is defined by higher-order image statistics, but not simple correlations in luminance. Flies can track the true direction of even theta motions, in which the first-order (luminance) motion is directed opposite the second-order moving feature. We exploited this paradoxical feature tracking response to dissect the particular image properties that flies use to track moving objects. We find that theta motion detection is not simply a result of steering toward any spatially restricted flicker. Rather, our results show that fly high-order feature tracking responses can be broken down into positional and velocity components - in other words, the responses can be modeled as a superposition of two independent steering efforts. We isolate these elements to show that each has differing influence on phase and amplitude of steering responses, and together they explain the time course of second-order motion tracking responses during flight. These observations are relevant to natural scenes, where moving features can be much more complex.

With a great variety of shapes and sizes, compound eye morphologies give insight into visual ecology, development, and evolution, and inspire novel engineering. In contrast to our own camera-type eyes, compound eyes reveal their resolution, sensitivity, and field of view externally, provided they have spherical curvature and orthogonal ommatidia. Non-spherical compound eyes with skewed ommatidia require measuring internal structures, such as with MicroCT (µCT). Thus far, there is no efficient tool to characterize compound eye optics, from either 2D or 3D data, automatically. Here we present two open-source programs: (1) the ommatidia detecting algorithm (ODA), which measures ommatidia count and diameter in 2D images, and (2) a µCT pipeline (ODA-3D), which calculates anatomical acuity, sensitivity, and field of view across the eye by applying the ODA to 3D data. We validate these algorithms on images, images of replicas, and µCT eye scans from ants, fruit flies, moths, and a bee.