Fig. 3.

Schematic illustration of the evolution of photoreceptive behaviors on a vertical scale of task complexity, with the position of major functional innovations indicated. The line for directional photoreception is dashed to indicate that class II tasks may become superfluous by the evolution of class III tasks, whereas the other classes remain relevant after a higher class has evolved. Eyes capable of class IV tasks can of course still handle class III tasks. The corresponding receptor/eye morphologies are shown to the right. Note that compound and single chambered eyes are principally different solutions suggesting independent transitions from class II to class III.