Compound eyes

The most sophisticated insect visual organ is the compound eye. Virtually all adult insects and nymphs have a pair of large, prominent compound eyes, which often cover nearly 360 degrees of visual space.

The compound eye is based on repetition of many individual units called ommatidia (Fig. 4.10). Each ommatidium resembles a simple stemma: it has a cuticular lens overlying a crystalline cone, which directs and focuses light onto eight (or maybe 6-10) elongate retinula cells (see transverse section in Fig. 4.10). The retinula cells are clustered around the longitudinal axis of each ommatidium and each contributes a rhab-domere to the rhabdom at the center of the ommatid-ium. Each cluster of retinula cells is surrounded by a ring of light-absorbing pigment cells, which optically isolates an ommatidium from its neighbors.

The corneal lens and crystalline cone of each omma-tidium focus light onto the distal tip of the rhabdom from a region about 2-5 degrees across. The field of view of each ommatidium differs from that of its neighbors and together the array of all ommatidia provides the insect with a panoramic image of the world. Thus, the actual image formed by the compound eye is of a series of apposed points of light of different intensities, hence the name apposition eye.

The light sensitivity of apposition eyes is limited severely by the small diameter of facet lenses. Crepuscular and nocturnal insects, such as moths and some beetles, overcome this limitation with a modified optical design of compound eyes, called optical superposition eyes. In these, ommatidia are not isolated optically from each other by pigment cells. Instead, the retina is separated by a wide clear zone from the corneal facet lenses, and many lenses co-operate to focus light on an individual rhabdom (light from many lenses super-imposes on the retina). The light sensitivity of these eyes is thus greatly enhanced. In some optical superposition eyes screening pigment moves into the

Fig. 4.10 Details of the compound eye: (a) a cutaway view showing the arrangement of the ommatidia and the facets; (b) a single ommatidium with an enlargement of a transverse section. (After CSIRO 1970; Rossel 1989.)

clear zone during light adaptation and by this means the ommatidia become isolated optically as in the apposition eye. At low light levels, the screening pigment moves again towards the outer surface of the eye to open up the clear zone for optical superposition to occur.

Because the light arriving at a rhabdom has passed through many facet lenses, blurring is a problem in optical superposition eyes and resolution generally is not as good as in apposition eyes. However, high light sensitivity is much more important than good resolving power in crepuscular and nocturnal insects whose main concern is to see anything at all. In the eyes of some insects, photon-capture is increased even further by a mirror-like tapetum of small tracheae at the base of the retinula cells; this reflects light that has passed unabsorbed through a rhabdom, allowing it a second pass. Light reflecting from the tapetum produces the bright eye shine seen when an insect with an optical superposition eye is illuminated in the flashlight or car headlight beam at night.

In comparison with a vertebrate eye, the resolving power of insect compound eyes is rather unimpressive. However, for the purpose of flight control, navigation, prey capture, predator avoidance, and mate-finding they obviously do a splendid job. Bees can memorize quite sophisticated shapes and patterns, and flies and odonates hunt down prey insects or mates in extremely fast, aerobatic flight. Insects in general are exquisitely sensitive to image motion, which provides them with useful cues for avoiding obstacles and landing, and for distance judgment. Insects, however, cannot easily use binocular vision for the perception of distance because their eyes are so close together and their resolution is quite poor. A notable exception is the praying man-tid, which is the only insect known to make use of binocular disparity to localize prey.

Within one ommatidium, most studied insects possess several classes of retinula cells that differ in their spectral sensitivities; this feature means that each responds best to light of a different wavelength. Variations in the molecular structure of visual pigments are responsible for these differences in spectral sensitivity and are a prerequisite for the color vision of flower visitors such as bees and butterflies. Some insects are pentachromats, with five classes of receptors of differing spectral sensitivities, compared with human di- or trichromats. Most insects can perceive ultraviolet light (which is invisible to us) allowing them to see distinctive alluring flower patterns visible only in the ultraviolet.

Light emanating from the sky and reflected light from water surfaces or shiny leaves is polarized, i.e. it has greater vibration in some planes than in others. Many insects can detect the plane of polarization of light and utilize this in navigation, as a compass or as an indicator of water surfaces. The pattern of polarized skylight changes its position as the sun moves across the sky, so that insects can use small patches of clear sky to infer the position of the sun, even when it is not visible. In like manner, an African dung beetle has been shown to orientate using polarized moonlight in the absence of direct sighting of the moon, perhaps representing a more general ability amongst nocturnal insects. The microvillar organization of the insect rhabdomere makes insect photoreceptors inherently sensitive to the plane of polarization of light, unless precautions are taken to scramble the alignment of microvilli. Insects with well-developed navigational abilities often have a specialized region of retina in the dorsal visual field, the dorsal rim, in which retinula cells are highly sensitive to the plane of polarization of light. Ocelli and stemmata also may be involved in the detection of polarized light.

4.4.5 Light production

The most spectacular visual displays of insects involve light production, or bioluminescence. Some insects co-opt symbiotic luminescent bacteria or fungi, but self-luminescence is found in a few Collembola, one hemipteran (the fulgorid lantern bug), a few dipteran fungus gnats, and a diverse group amongst several families of coleopterans. The beetles are members of the Phengodidae, Drilidae, some lesser known families, and notably the Lampyridae, and are commonly given colloquial names including fireflies, glow worms, and lightning bugs. Any or all stages and sexes in the life history may glow, using one to many luminescent organs, which may be located nearly anywhere on the body. Light emitted may be white, yellow, red, or green.

The light-emitting mechanism studied in the lampyrid firefly Photinus pyralis may be typical of luminescent Coleoptera. The enzyme luciferase oxidizes a substrate, luciferin, in the presence of an energy source of adenosine triphosphate (ATP) and oxygen, to produce oxyluciferin, carbon dioxide, and light. Variation in

ATP release controls the rate of flashing, and differences in pH may allow variation in the frequency (color) of light emitted.

The principal role of light emission was argued to be in courtship signaling. This involves species-specific variation in the duration, number, and rate of flashes in a pattern, and the frequency of repetition of the pattern (Fig. 4.11). Generally, a mobile male advertises his presence by instigating the signaling with one or more flashes and a sedentary female indicates her location with a flash in response. As with all communication systems, there is scope for abuse, for example that involving luring of prey by a carnivorous female lampyrid of Photurus (section 13.1.2). Recent phylo-genetic studies have suggested a rather different interpretation of beetle bioluminescence, with it originating only in larvae of a broadly defined lampyrid clade, where it serves as a warning of distastefulness (section 14.4). From this origin in larvae, luminescence appears to have been retained into the adults, serving dual warning and sexual functions. The phylogeny suggests that luminescence then was lost in lampyrid relatives and regained subsequently in the Phengodidae, in which it is present in larvae and adults and fulfills a warning function. In this family it is possible that light is used also in illuminating a landing or courtship site, and perhaps red light serves for nocturnal prey detection.

Bioluminescence is involved in both luring prey and mate-finding in Australian and New Zealand cave-dwelling Arachnocampa fungus gnats (Diptera: Myceto-philidae). Their luminescent displays in the dark zone of caves have become tourist attractions in some places. All developmental stages of these flies use a reflector to concentrate light that they produce from modified Malpighian tubules. In the dark zone of a cave, the larval light lures prey, particularly small flies, onto a sticky thread suspended by the larva from the cave ceiling. The flying adult male locates the luminescent female while she is still in the pharate state and waits for the opportunity to mate upon her emergence.

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