Insect Vision

Excepting a few blind subterranean and endoparasitic species, most insects have some sight, and many possess highly developed visual systems. The basic components 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 photore-ceptive 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.

4.4.1 Dermal detection

In insects able to detect light through their body surface, there are sensory receptors below the body cuticle but no optical system with focusing structures. Evidence for this general responsivity to light comes from the persistence of photic responses after covering all visual organs, for example in cockroaches and lepidopteran larvae. Some blind cave insects, with no recognizable visual organs, respond to light, as do decapitated cockroaches. In most cases the sensitive cells and their connection with the central nervous system have yet to be discovered. However, within the brain itself, aphids have light-sensitive cells that detect changes in day length - an environmental cue that controls the mode of reproduction (i.e. either sexual or partheno-genetic). The setting of the biological clock (Box 4.4) relies upon the ability to detect photoperiod.

4.4.2 Stemmata

The only visual organs of larval holometabolous insects are stemmata, sometimes called larval ocelli (Fig. 4.9a). These organs are located on the head, and vary from a single pigmented spot on each side to six or seven larger stemmata, each with numerous photo-receptors and associated nerve cells. In the simplest stemma, a cuticular lens overlies a crystalline body secreted by several cells. Light is focused by the lens onto a single rhabdom. Each stemma points in a different direction so that the insect sees only a few points in space according to the number of stemmata. Some caterpillars increase the field of view and fill in the gaps between the direction of view of adjacent stemmata by scanning movements of the head. Other larvae, such as those of sawflies and tiger beetles, possess more sophisticated stemmata. They consist of a two-layered lens that forms an image on an extended retina composed of many rhabdoms, each receiving light from a different part of the image. In general, stemmata seem designed for high light sensitivity, with resolving power relatively low.

4.4.3 Ocelli

Many adult insects, as well as some nymphs, have dorsal ocelli in addition to compound eyes. These ocelli are unrelated embryologically to the stemmata. Typically, three small ocelli lie in a triangle on top of the head. The cuticle covering an ocellus is transparent and may be curved as a lens. It overlies transparent epidermal cells, so that light passes through to an extended retina made up of many rhabdoms (Fig. 4.9b). Individual groups

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