Microlens arrays in the animal kingdom

The compound eyes of insects and crustaceans are image-forming arrays of up to 30 000 microlenses. There are basically two types of design. In apposition eyes the overall erect image is made up from the centres of the small inverted images produced by each element in the array. However, in superposition eyes the ray paths from many elements combine optically to give a real erect image on the retina. The microlenses themselves make use of both curved surface and graded refractive index optics, and in certain crustacea the lenses are replaced by radial mirrors or lens - mirror combinations.