What does the butterfly Papilio aegeus have in common with a Citroen DS23?

Biological systems, living in an information rich world, exploit invariants whenever they can by way of directed perception, to select from the multiple sources of information that may specify a given stimulus. The author describes a control circuit in an insect visual system capable of detecting the topological invariant, the focus of expansion of optic flow. The system optimizes its transient performance to provide a consistent response to flow field discontinuities. The derived signal is in feedback to the sensors and gates only those regions of the eye that are directed toward the focus of expansion. Its role in object detection is discussed.