Introduction
Light is perceived by insects through a number of different receptors. Most adult insects and larval hemimetabolous insects normally have a pair of compound eyes, whose structure (Section 22.1) and function in form and motion vision (Section 22.2) are described below in turn. Section 22.3 covers the molecular and physiological function of photoreceptors and mechanism of regulating light sensitivity before explaining the processes of color vision and polarization vision. Adult insects also typically have three single-lens eyes, called ocelli, whose optics and function are described in Section 22.4. Larval holometabolous insects have one or more single-lens eyes, known as stemmata, on the sides of the head (Section 22.5). Some insects also possess epidermal light receptors, and, in some cases, light is known to have a direct effect on cells in the brain (Section 22.6). Magnetic sensitivity aids orientation in at least some insects, and has known interactions with light sensitivity (Section 22.7).
Compound eyes
Occurrence
Compound eyes are so called because they are constructed from many similar units called ommatidia. They are present in most adult pterygote insects and the larvae of hemimetabolous insects, but are strongly reduced or absent in wingless parasitic groups, such as the Phthiraptera and Siphonaptera, and in female coccids (Hemiptera). This is also true of cave-dwelling species. Among termites (Isoptera), compound eyes are greatly reduced or absent from stages that are habitually subterranean, and, although present in winged reproductives, the sensory components of the eyes degenerate during the permanently subterranean reproductive life. Among Apterygota, compound eyes are lacking in some Thysanura, but Lepismatidae have 12 ommatidia on each side. Well-developed compound eyes are present in Archaeognatha. In the non-insect orders of Hexapoda, Collembola have up to eight widely spaced ommatidia, while Protura and Diplura have no compound eyes.