Light is absorbed by rhodopsin and metarhodopsin in invertebrates. Absorption of light transforms metarhodopsin back to rhodopsin. The pigments exist in two stable conformations, and light causes them to switch back and fro from one state to the other. This regeneration is essential because conformational changes in rhodopsin, but not in metarhopsin, leads to phototransduction. Experiments on insect behavior indicate that many insect species can distinguish colors.
Color vision depends on the ability to discriminate between light of different wavelengths.
Such discrimination is possible because compound eyes have photopigments with differing spectral properties. Many insects have two rhodopsins, one with maximum absorption in ultraviolet wavelengths and one with maximum absorption in green wavelengths. Some species have a third pigment that absorbs maximally in the blue region. A few Lepidoptera have four visual pigments. Dorsal ocelli Dorsal ocelli occur in larvae of hemimetabolous insects and in nearly all adults.