Arthropods have two very distinct visual systems (Fig. 5.1)—ocelli and compound eyes (Fig. 5.2). There is, however, a large disparity in the distribution of these devices in the different groups of arthropods. Insects generally have three ocelli and two compound eyes. Arachnids do not have compound eyes but up to eight ocelli. These do not form images. They are photoreceptors that are very sensitive to variations in light. Compound eyes, of various types, form images, distinguish colors, and, in some organisms, are sensitive to linear or circular polarization of light. The spectral sensitivity of arthropod eyes is very broad, ranging from UVB (300 nm) to deep red (720 nm). The ocelli of some organisms exhibit spectral sensitivity in the infrared. The resolution of arthropod visual systems is wide, but generally much lower than that of Homo sapiens. Photosensitive pigments are derivatives of vitamin A, in combination with a protein.