1. The compound eye of arthropods.

• Insects and other arthropods, such as Limulus, the horseshoe crab, often have a compound eye consisting of ommatidia (Eckert, Fig. 7-35b) (Eckert, Fig. 7-35c). Light is absorbed by the retinular cells, which are sensitive to single photons of light, just as for the receptors of the human retina. Electrical receptor potentials are produced by light absorption, using a G-protein signaling system inside the retinular cells. The "cascade" effect of the chemical signaling system amplifies the reponse of the rhodopsin to light absorption.
• The compound eye doesn't use a lens to form an image on a retina. Rather, it uses the ommatidia like little light pipes, each pointing at a slightly different direction. (Eckert, Fig. 7-34a-1), (Eckert, Fig. 7-34a-2), (Eckert, Fig. 7-34b-1)
• Very small animals, such as insects, can't use a lens type system due to optical diffraction.

Our eye and the eye of the octopus both have a lens which produces an image on a retina which is comprised of an array of photoreceptive cells. This is an effective design for relatively large animals. But, if we tried to scale down this type of eye to fit onto an insect's body, we would have a big problem: The lens opening of the eye would be very small. Light passing through a small opening diffracts (spreads) due to the interference of the light waves. If the pupil is large, there is very little edge compared with the interior of the opening and there is little diffraction. (This in itself is a scaling problem similar to the surface to volume problem.) If we tried to scale down a vertebrate or mollusc eye to fit on an insect, diffraction would cause so much spreading that the image focused on the retina would be very blurry and essentially useless. So, insects make do with a much simpler type of eye. Insect compound eyes have many individual units, called ommatidia, each pointed in a different direction. These are simple detectors which signal how much light is coming from the direction into which the ommatidium is pointed.

The first of the following figures, both redrawn from Optima for Animals by R. McNeill Alexander (Edward Arnold), shows a diagrammatic representation of a compound eye with the individual ommatidia. Two tubular ommatidia are indicated in more detail, each pointing in a different direction as the result of the hemispherical shape of the compound eye. The insect can resolve two points (shown in blue and green and corresponding to the directions that two of the ommatidia are pointing) if these points are not separated by an angular distance, alpha, which is too small. The minimum resolvable distance is a function of two factors: 1) Optical diffraction causes "blurring" which becomes more serious as the diameter of the ommatidium decreases. So, for the least diffractive effect, the ommatidium must be as large as possible. 2) But, each ommatidium acts as a light pipe. All the light entering the pipe is averaged and an electrical signal is produced which indicates how much light enters that particular pipe. If the ommatidium is large in diameter, light will enter from a larger range of angles and the minimum alpha resolvable will increase. The optimum diameter for the ommatidium is where these two effects balance out. For a wavelength 0.5 micrometers (yellow-green), the optimum ommatidium diameter is 27 micrometers, a value very close to that measured in insects. For optimum resolution, the wavelength must be as short as possible. That is why many insects are most sensitive to light in the blue and near ultraviolet spectral regions.



The rhodopsins of different animals absorb in different regions of the spectrum. the Owlfly, sees almost exclusively in the ultraviolet. (Question - why aren't there any organisms that "see" further down into the short wavelengths, say, 200 nm?) (Rhodopsin spectra), Even some insects have color vision. A spingid moth, Deilephila elpenor has three different types of photoreceptors! (Moth spectra)

3. The octopus eye uses a lens imaging system but using a retina with receptor cells in front.

• Why is the vertebrate retina backwards from the cephalopod retina? Surely, the cephalopod retina could have developed the elaborate cross connections of neurons that permit the vertebrate retina to do lateral inhibition (sharpening of edges). One idea is that the vertebrate retina offers an advantage of having blood vessels on both sides of the retina, thereby permitting a greater density of receptor cells and thus greater visual acuity.
• The octopus has a flicker fusion frequency of 70 Hz, 10 times that of our retina!

4. Some cells in the leaf veins and petioles of plants are sensitive to the direction of light, not just its intensity. The pigment is a blue absorbing molecule. But, we don't know very much about the way in which this information is communicated to the pulvinus at the base of the leaf, that allows the leaves of heliotropic plants to twist and turn to orient with respect to the direction of the solar beam.

5. Can individual animal cells "see"? Look here for some speculation.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.