6 Ommatidial Adaptations for Spatial, Spectral, and Polarization Vision in Arthropods

The arthropods constitute the largest single phylum of the animal kingdom and, with almost a million known species, more than three-quarters of all animal species. The insects and crustaceans alone account for nearly all arthropods, and their compound eyes are thus the most numerous and widespread eye design found on Earth. Not surprisingly, the great adaptability and versatility of arthropods, and their conquest of almost every conceivable habitat, have led to the evolution of a remarkable range of visual specializations, both peripherally in the eyes themselves and centrally in the brain centers responsible for the higher processing of visual information. The compound eyes of insects, crustaceans, and the horseshoe crab Limulus (a xiphosurid chelicerate) are composed of individual optical units known as ommatidia (Fig. 1A). Each of these generally contains one or more lenses (the dioptric apparatus) that capture and focus the incoming light and a number of photoreceptors that together act as an optical waveguide, within which the light propagates and is absorbed. Visual pigment (rhodopsin) molecules, densely packed within the microvillar membranes of the waveguide, convert the absorbed light energy into an electrical signal—the visual response—that is carried by the photoreceptor axons to higher levels in the visual system for further processing. Exactly which qualities of light lead to an electrical signal depends on a number of factors—both morphological and physiological—that are inherent within each ommatidium, and these differ markedly from species to species according to lifestyle and habitat. The sensitivity of the eye...