By any standards, strepsipterans are weird. The males' twisted wings definitely give them a distinctive look but it's their eyes that are their most distinguishing feature. In general, insects have compound eyes composed of many ommatidia. For example, the similarly sized Drosophila eye is composed of ∼700 ommatidia, each of which includes a lens. However,strepsipterans have raspberry-shaped eyes that contain a remarkably low number of lenses. One species, Xenos peckii, has only 50 lenses per eye, and the area of each lens is 15 times larger than that of fruit flies. This suggests that images can be formed within each unit, a feature that is remarkably different from the typical compound eye, in which each ommatidium samples just a portion of the visual scene, forming a low-resolution mosaic image.
To better understand these fascinating eyes, Elke Buschbeck, Birgit Ehmer and Ron Hoy have been exploring the anatomy and physiology of Xenos peckii's unusual visual system. These researchers had already shown that,in contrast to compound eyes, which have only 8–10 photoreceptors per ommatidium, below each Xenos lens lies a retina with over 100 photoreceptor cells. In this current paper, the team has gone on to describe the optics and physiology of the insect's visual system.
First, the team measured the distance behind each lens at which images were focused and found that it corresponded well to the position of the retina behind the lens. While each tiny Drosophila ommatidium is only capable of sampling a point from an image, which the brain then integrates into a visual mosaic, Xenos's large lenses are capable of much more. The authors' calculations demonstrated that the lenses capture enough light to make image formation possible within each `eyelet', at least when Xenos is active during daylight. Most intriguingly, the optics of this design may also allow for a combination of high sensitivity and resolution that is difficult to achieve in compound eyes.
In addition, other features of Xenos's visual system don't conform to the insect standard. In insects with compound eyes, the lamina, the first visual relay in the central nervous system, is compartmentalized, with no overlap of input from other ommatidia. However, in Xenos, no compartments are seen, and the inputs from each eyelet overlap at the borders. Although this organization is odd, it fits well with the authors' calculations suggesting that images falling on neighboring eyelets may also overlap at the borders. These observations suggest that the images formed by individual eyelets are maintained in the central nervous system.
The origin of strepsipteran eyes, which share architectural features of both compound eyes and image-forming single eyes, remains a mystery. Unfortunately, a resolution to the problem is hampered by disagreement about the group's phylogenetic position, although strepsipterans must have evolved from ancestors with compound eyes. Some of the authors' data suggest that strepsipterans might have passed through a nocturnal phase, possibly resulting in poor visual resolution and altering their eyes' design. If this scenario is correct, the authors suggest that Strepsiptera could have regained higher resolution by improving the resolution within each eyelet rather than by adding more lenses. With such profound structural changes, strepsipteran eyes may provide us with unique insight into the evolution of arthropod eyes.