The nocturnal wolf spider (top, left; its retina, right) has eight eyes, four of which have a mirror, or tapetum, which reflects light back through the retina. Animal Eyes by M. F. Land and D.-E. Nilsson (Oxford University Press, £24.95, $45; pbk) discusses such adaptations and many more, including the visual equipment of the horsefly (below, left) and butterfly (right).