Fig. 1. Compound eye designs. (A) A focal apposition compound eye. Light reaches the photoreceptors exclusively from the small corneal lens located directly above. This eye design is typical of day-active insects. (B) A refracting superposition compound eye. A large number of corneal facets and bullet-shaped crystalline cones collect and focus light – across the clear zone of the eye (cz) – towards single photoreceptors in the retina. Several hundred, or even thousand, facets service a single photoreceptor. Not surprisingly, many nocturnal and deep-sea animals have refracting superposition eyes, and benefit from the significant improvement in sensitivity. Diagrams courtesy of Dan-Eric Nilsson. Adapted from Warrant (Warrant, 2004).