Behavioral evidence for within-eyelet resolution in twisted-winged insects (Strepsiptera).

Compound eyes are typically composed of hundreds to thousands of ommatidia, each containing 8-10 receptors. The maximal spatial frequency at which a compound eye can sample the environment is determined by the inter-ommatidial angle. Males of the insect order Strepsiptera are different: their eyes are composed of a smaller number of relatively large units (eyelets), each with an extended retina. Building on a study of Xenos vesparum, we use a behavioral paradigm based on the optomotor response to investigate the possibility that the eyelets of the Strepsiptera Xenos peckii are image-forming units. From anatomical evidence, we hypothesize that spatial sampling in the strepsipteran eye is determined not only by the interactions of widely spaced photoreceptors in different eyelets, but also by the angular separation between groups of closely spaced photoreceptors within eyelets. We compared X. peckii's optomotor response with the predictions of an elementary motion detector (EMD) model consisting of two distinctly different sampling bases. The best match between our empirical results and the model shows that the optomotor response in X. peckii males is determined by both the small (intra-eyelet) and large (possibly inter-eyelet) separations. Our results indicate that the X. peckii eye has sampling bases around 10 degrees and 20 degrees , and that each eyelet could be composed of up to 13 sampling points, which is consistent with previous anatomical findings. This study is the first to use the EMD model explicitly to investigate the possibility that strepsipteran eyes combine motion detection features from both camera and compound eyes.