In these acone eyes four flat transparent cells are found in place of the cones. In superposition eyes the space between the corneal lines and the photoreceptors is traversed by crystalline tracts. These are thought to act as wave-guides that direct light to the photoreceptors. The eyes of some insects feature a tracheal tapetum, as shown in a figure. Tracheal tapeta are known from several Lepidoptera.
Small tracheae run parallel with the longer cells of the ommatidia.
It has been suggested that these structures function as interference reflector filters. In bright light, the eye is less sensitive to light, and the eye is regarded as light-adapted. Maximum sensitivity occurs in darker conditions, when the eye is fully dark-adapted. Some insect eyes can change light sensitivity by about three orders of magnitude within a few minutes of changing light conditions. There are a number of mechanisms of adaptation to light and dark conditions.