4.4. Insect vision
Excepting a few blind subterranean and endoparasitic species, most insects have some sight, and many possess highly developed visual systems. The basic com- ponents needed for vision are a lens to focus light onto photoreceptors — cells containing light-sensitive molecules — and a nervous system complex enough to process visual information. In insect eyes, the photoreceptive structure is the rhabdom, comprising several adjacent retinula (or nerve) cells and consisting of close-packed microvilli containing visual pigment. Light falling onto the rhabdom changes the configuration of the visual pigment, triggering a change of electrical potential across the cell membrane. This signal is then transmitted via chemical synapses to nerve cells in the brain.
Comparison of the visual systems of different kinds of insect eyes involves two main considerations: (i) their resolving power for images, i.e. the amount of fine detail that can be resolved; and (ii) their light sensitivity, i.e. the minimum ambient light level at which the insect can still see. Eyes of different kinds and in different insects vary widely in resolving power and light sensitivity and thus in details of function.
The compound eyes are the most obvious and familiar visual organs of insects but there are three other means by which an insect may perceive light: dermal detection, stemmata, and ocelli. The dragonfly head depicted in the vignette of this chapter is dominated by its huge compound eyes with the three ocelli and paired antennae in the center.