Not flying blind: a comparative study of photoreceptor function in flying and non-flying cockroaches.

Flying is often associated with superior visual performance, as good vision is crucial for detection and implementation of rapid visually guided aerial movements. To understand the evolution of insect visual systems it is therefore important to compare phylogenetically related species with different investments in flight capability. Here, we describe and compare morphological and electrophysiological properties of photoreceptors from the habitually flying green cockroach and the American cockroach , which flies only at high ambient temperatures. In contrast to , ommatidia in were characterized by two-tiered rhabdom, which might facilitate detection of polarized light while flying in the dark. In patch-clamp experiments, we assessed the absolute sensitivity to light, elementary and macroscopic light-activated current and voltage responses, voltage-activated potassium (K) conductances, and information transfer. Both species are nocturnal, and their photoreceptors were similarly sensitive to light. However, a number of important differences were found, including the presence in of a prominent transient K current and a generally low variability in photoreceptor properties. The maximal information rate in was one-third higher than in , owing to a substantially higher gain and membrane corner frequency. The differences in performance could not be completely explained by dissimilarities in the light-activated or K conductances; instead, we suggest that the superior performance of photoreceptors mainly originates from better synchronization of elementary responses. These findings raise the issue of whether the evolutionary tuning of photoreceptor properties to visual demands proceeded differently in Blattodea than in Diptera.