In modern arthropods the structural unit (ommatidium) is made of a sequence of functional subunits (see fig. 7). Facing the outside world is the dioptric apparatus consisting of a corneal lens in optical contact with a crystalline cone. Tiny images from a narrow field of view appear at the tips of the cones. Proceeding toward the interior of the eye beyond the cone there are two types of sttuctures. In the so-called apposition eyes (mostly found in diurnal insects, Crustacea, etc.), the photoreceptor, or rhabdom, is long and attached directly to the tip of the crystalline cone. On the other hand, in the superposition eye (found in nocturnal forms like moths, fireflies, etc.) a crystalline fiber, like a light guide, intervenes between the tip of the cone and a shorter rhabdom. A dark pigment may fill the space between cones in the apposition eyes, while in the superposition eye the pigment layer can migrate to surround either the cones or the crystalline fibers, when the eye is dark- or light-adapted respectively. Furthermore, the superposition eyes are generally constructed with very regular radial symmetry. This feature and the pigment migration in the dark-adapted eyes have been interpreted as enabling occurrence of collective phenomena, such as superposition of images or diffraction patterns due to adjacent ommatidia. The interpretation of the visual process in the compound eyes, however, has been the subject of great controversy since the pioneering work of Miiller (1826), who introduced the "mosaic" theory, according to which the compound eye is regarded as an assemblage of directional
Disarticulated exuviae of Agrwstus pisiformis (Wahlenberg), from the Upper Cambrian of Vastergotland, Sweden. (x6.8, RLS coll.)
Dorsal and ventral view of reconstructions of Olenoides serratus (Rominger), a Middle Cambrian trilobite. The models were prepared at the Paleontological Museum, University of Oslo. Ptint from a color slide.
units, each yielding a point element in a mosaiclike reconstruction. Exner (1891) further developed the mosaic theory in his classic study of faceted eyes and introduced the distinction between apposition and superposition eyes. However, most of Exner's models of image formation by the dioptric system of the ommatidia turned out to be wrong. For example, Exner assumed that the crystalline cones in Limulus would form images at their tips due to a radial variation of the refractive index (cylinder lens). This is now known not to be true; in fact, the same images can be obtained from a homogenous scaled-up replica of the Limu-lus cone made of lucite when it is immersed in water (Levi-Setti, Park, and Winston 1973).' Furthermore, Exner ignored the role of the crystalline tracts between cone and rhabdom, present in the so-called superposition eyes. What seems to emerge from modern research is that the term superposition is a misnomer (Horridge 1969), since no superposition of sharp images is ever observed in the plane of the photore-ceptors when the crystalline tracts are in place.
After more than eighty years of research, the only significant distinction found in the types of arthropod eyes is that there are eyes with or without crystalline threads, and, correspondingly, with short or long rhabdoms. What has become increasingly apparent is the role of the neurophysi-ological apparatus in modifying the type of response which could be inferred from purely optical considerations. The lateral inhibitory interaction can alter the effects of image overlap between neighboring ommatidia and sharpen contrast. (For a summary, see, for example, Hartline 1969). A functional distinction between the two types of eyes discussed above is still obscure but may not be as fundamental as originally thought by Exner.
' The shape of each ommatidium of Limulus has been found to be optimized for the maximum collection of light incident within a field of view of aperture angle ±19° from the axis. The relevant phenomenon taking place in such a device is total internal reflection at the interface between the corneal medium, or refractive index n = 1.53, and the fluid external to the cone, of refractive index n = 1.35.
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