Design features of eyes

The lens of an eye gathers light from the environment and usually focuses it as a crisp image onto photoreceptor cells. The greater the amount of light that one photoreceptor receives, the easier it is for it to provide an accurate measure of light intensity. This is particularly important for animals that are active in dim lighting conditions, when the amount of light available is small. There are two main ways of maximising the amount of light that a photoreceptor receives: one is for the eye to have a large lens, which increases the amount of light caught from each part of the visual field; the other is for the photoreceptors themselves to be large, so that each captures light over a relatively large area of the image. But having large photoreceptors is not an advantage if the animal needs to extract detail from the image on its retina, because what is required is for neighbouring photoreceptors to sample small but distinct areas. The greater the number of sampling stations that cover one area of the image, the greater the amount of detail that can be resolved in that area. The way in which an eye is constructed, therefore, involves compromise. The compromise between the need for large photoreceptors, which have good ability to capture available light, and small photoreceptors, to enable the image to be examined in detail, is the most basic. The ability to measure light levels is called sensitivity, and the ability to discriminate fine detail in an image is called resolution. The two are quite distinct requirements, and it is difficult to improve the performance of an optical instrument for one of them without deterioration in the other.

Insects often depend heavily on vision, which they achieve by means of compound eyes. A compound eye consists of many discrete optical units, each of which has its own lens that focuses light from a small part of the environment onto its own small group of photoreceptors (Fig. 4.5). Each unit is called an ommatidium, and its lens is often referred to as a single facet. Large dragonflies have almost 30000 ommatidia in each eye, but most insects have fewer than this - the housefly has 3000 and Drosophila has 700. Compound eyes give the animal a very wide field of view; a fly can see in almost every direction from its head. The size of eye that an insect

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Figure 4.5 The arrangement of cells in the ommatidia of two diurnal insects. (a) A locust (Locusta) ommatidium. Transverse sections of the ommatidium at two levels (on the right) show how the rhabdomeres of individual retinula cells form a single, central, light-collecting rod, the rhabdom. (b) A blowfly (Calliphora) ommatidium. Here, the rhabdomeres of different photoreceptors within an ommatidium remain separate. Seven of the photoreceptors are numbered in the cross-section, on the left. Also shown are pigment cells that surround each ommatidium. (a from Wilson, Garrard & McGiness, 1978; copyright 1978 Springer-Verlag; b from Hardie, 1986; reprinted with permission from Trends in Neuroscience; copyright © 1986 Elsevier Science.)

Figure 4.5 The arrangement of cells in the ommatidia of two diurnal insects. (a) A locust (Locusta) ommatidium. Transverse sections of the ommatidium at two levels (on the right) show how the rhabdomeres of individual retinula cells form a single, central, light-collecting rod, the rhabdom. (b) A blowfly (Calliphora) ommatidium. Here, the rhabdomeres of different photoreceptors within an ommatidium remain separate. Seven of the photoreceptors are numbered in the cross-section, on the left. Also shown are pigment cells that surround each ommatidium. (a from Wilson, Garrard & McGiness, 1978; copyright 1978 Springer-Verlag; b from Hardie, 1986; reprinted with permission from Trends in Neuroscience; copyright © 1986 Elsevier Science.)

can carry around is obviously limited and, in order to achieve good resolution, the size of each ommatidium must be small so that as many as possible can be packed into the eye to provide a reasonable sampling density. However, as the size of a lens is reduced, the image that it produces becomes increasingly blurred, and there is a limit to how small individual ommatidia can be.

The reason why small lenses produce blurred images is that when light passes through an aperture or is bent by passing from one medium to another, some waves are scattered - a property called diffraction. Because of this scattering, light that passes through one part of a lens will spread out and interfere with light that passes through another part of the lens. This mutual interference causes blurring in the image that the lens projects onto its focal plane. The image of a small spot of light is focused by a lens to a diffuse spot called an Airy disk. The width of the Airy disk decreases as the diameter of the lens increases, and so an eye that has a wide lens with a large aperture will be able to work with a sharper image than an eye that has a small lens.

If an eye is to make effective use of a sharply focused image, the width of its photoreceptors must be matched to the width of the Airy disk. If the photoreceptors were wider than the Airy disk, two closely spaced images could not be distinguished from each other. However, interference effects set a lower limit to the width of the rhabdom, the photoreceptor element that catches the light which strikes its end, acting as a light guide. As the diameter of a light guide is reduced towards the wavelength of light, interference effects frustrate total internal reflection and an increasing amount of light travels outside the light guide. The parts of photoreceptors that capture light energy and transduce it into an electrical signal are cylindrical light guides. If a large amount of light is lost from these structures, the performance of the eye is degraded because light travelling outside the receptor element will not be captured by the receptor cell and may even cause cross-talk by entering a neighbouring cell. In view of this, light guides made of living tissue cannot usefully be less than 1 ^m in diameter. Thus, diffraction sets a lower limit both to blurring of the image and to the size of the receptor elements.

In honey bees, which are typical of insects that fly on bright days, most facets are about 25 ^m across, and collect light from a cone with an angle of a little over 1°. To resolve two small spots of light as separate rather than one larger spot, a bee would have to use three ommatidia - one for each spot, with an un-illuminated ommatidium between them. Therefore, bees can distinguish between two objects that are about 3° apart. Compound eyes of species that are active in dim light conditions often have ommatidia that are larger than would be expected to give the best balance between sampling frequency and a sharply focused image (Snyder, Stavenga & Laughlin 1977). Usually, in dim light conditions, photoreceptors also collect light not just from the lens of their own ommatidia, but from surrounding lenses as well, an optical arrangement that is called superposition. Insects and crustacea that are active in bright daylight usually operate with the best resolution that a compound eye can deliver; this type of eye is called an apposition compound eye, and ommatidia are shielded from their neighbours so that each works quite independently.

Some diurnal insects depend on vision for the localisation of prey or sexual partners. A good example is the mantis Tenodera, which depends on vision to locate its prey. In order to achieve good resolution for a task such as this, there is a need to dedicate as many ommatidia as possible to sample the visual environment, and each individual ommatidium must have a crisp image of a small part of the environment. This means that each ommatidium must have a lens that is as large as possible. The size that an insect's eye can grow to is clearly limited, however, and there is a compromise between the number of ommatidia and their size in a particular region of the eye. The mantis eye has evolved so that only a small portion of each eye has the high resolution required to examine potential prey. This region, where resolution is enhanced, is called a fovea. The mantis fovea is functionally equivalent to the more familiar fovea of vertebrate eyes.

The fovea of a mantis eye is a small region at the front of the eye (Fig. 4.6a). Individual ommatidia here have lenses that are 50 ^m in diameter, larger than elsewhere in the eye, and rhabdoms that are 1.5 to 2 ^m across, narrower than elsewhere in the eye, a specialisation to maximise resolution by capturing light over a relatively narrow cone of acceptance. Also, the angle between adjacent ommatidia (0.6°) is narrower than elsewhere in the eye (Fig. 4.6b), and so the surface of the eye is less curved here than elsewhere. These structural features are consistent with physiological properties of ommatidia, measured by recording the responses of photoreceptors with intracellular electrodes. Intracellular recordings show that individual ommatidia accept light from a cone of angle 7°, and this is smaller than the acceptance angle for ommatidia measured elsewhere over the eye. In bright light, acceptance angles, measured in electrophysiological experiments, are very close to the angles between adjacent ommatidia, measured anatomically. In dim light, ommatidia in the fovea generate smaller responses than those elsewhere in the eye, which means that they are less sensitive. This is consistent with their narrow rhabdoms. The mantis eye is thus constructed so that resolution is increased at the expense of sensitivity in one region of the eye, the fovea, and is correspondingly reduced in the rest of the eye.

When a prey-like object appears in its peripheral field of view, a mantis responds with a rapid movement of the head that brings the object's image into the fovea (see Fig. 1.7 p. 16). Subsequent movements of the prey are followed by tracking movements of the head, which hold the image of the prey in the fovea while the body is aligned and brought into range for a raptorial

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Figure 4.6 The fovea of the mantis (Tenodera) eye. (a) Anterior view of the head; the broken circle within each eye indicates the region of inter-omma-tidial angles that constitutes the fovea. (b) Facet diameters (•) and inter-ommatidial angles (A) plotted against position in the eye for the row of facets indicated in (a). Note how facet diameters increase as inter-omma-tidial angles decrease. The acceptance angles of individual ommatidia follow the curve for inter-ommatidial angles very closely. (Redrawn after Rossel, 1979.)

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Figure 4.6 The fovea of the mantis (Tenodera) eye. (a) Anterior view of the head; the broken circle within each eye indicates the region of inter-omma-tidial angles that constitutes the fovea. (b) Facet diameters (•) and inter-ommatidial angles (A) plotted against position in the eye for the row of facets indicated in (a). Note how facet diameters increase as inter-omma-tidial angles decrease. The acceptance angles of individual ommatidia follow the curve for inter-ommatidial angles very closely. (Redrawn after Rossel, 1979.)

strike. There is a large area of overlap in the visual fields of the two eyes in the mantis, so that this insect can use binocular cues to estimate prey distance. The foveae of the two eyes are included in the binocular field, and the central axes of the left and right foveae intersect in the sagittal plane about 4 cm in front of the head. It is evident from watching the prey-catching behaviour that the function of the foveae is close examination of spatial detail of potential prey, while the peripheral eye is chiefly responsible for the detection of novel objects.

In absolute resolving power, insect foveae are about an order of magni tude poorer than human peripheral vision and two orders of magnitude poorer than human foveal vision. However, a major function of high resolution is to enable animals to detect objects at a distance. Larger animals need to detect objects at greater distances than small animals. A small insect may need to react to objects that are only a few centimetres away, whereas the appropriate reaction distance for a large primate may be several metres. The force of this point can be made by multiplying the angular separation between visual receptors by the height of the animal, and then comparing the values obtained from a number of active species. Expressed in this way, resolution turns out to be remarkably uniform over a wide range of animals, from Drosophila to Homo. Thus, compound eyes provide their small owners with resolving power that is as good as that of vertebrates when considered in terms of biological needs rather than physical ideals.

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