The number of retinula cells in an ommatidium vary among arthropod species. Damselflies (Zygoptera) have only four retinula cells per ommatidium (Mazokhin-Porshnyakov, 1969), as do beetles (coleoptera) (Meyer-Rochow, 1975). There are eight retinula cells in each ommatidium of dragonflies (odonata) and in flies (diptera). However, bees generally have nine, one of which is a smaller, eccentric cell (Laughlin, 1975; Gribakin, 1975). The horseshoe crab, Limulus, a chelicercate, has ten retinula cells plus a large eccentric cell per ommatidium (Bullock and Horridge, 1965). The silkworm moth, Bombyx mori, also has ten retinula cells plus an eccentric cell (Mazokhin-Porshnyakov, 1969). Figure 5.1-3 shows schematically a longitudinal section and corresponding cross sections of an ommatidium of a worker bee. Structures are identified in the figure caption. There are three major anatomical types of retinula cell defined in this figure by Gribakin (1975). Note the dense cross hatching at the center of the ommatidium where the retinula cells meet. This is the rhabdom region of the retinula cells, where photon trapping and transduction leading to retinula cell depolarization occurs. Figure 5.1-4 illustrates schematically cross sections through a Romalea (grasshopper) ommatidium. Distally, near the lens, there are six ommatidia. A section near the base of the ommatidium shows that two eccentric retinula cells without rhabdoms are present. All eight retinula cells send axons centrally. In electron micrographs, rhabdoms appear as closely packed, dense, parallel tubules, about 50 to 60 nm in diameter. Figure 5.15 shows an electron micrograph of a transverse section of a bee's ommatidium showing rhabdoms. In this proximal section, only seven retinula cells are seen; the eighth cell lies deeper in the ommatidium (Gribakin, 1975).
Grouped rhabdom tubules have been shown to act as dielectric waveguides, similar to modern optical fibers used in telecommunications. The rhabdoms contain visual pigments that trap photons, which, in turn, initiate chemical reactions that lead to retinula cell depolarization. In a given ommatidium, certain retinula cells have visual pigments that have peak sensitivities at different wavelengths, giving the animal "color vision." Snyder (1975) has shown that in diptera (flies), there is both physical and electrophysiological evidence for retinula cells with three distinct spectral sensitivities. He claims that rhabdoms of cells 1 to 6 have green peak sensitivity, the rhabdom of cell 7 peaks in the violet or near ultraviolet (UV), and the rhabdom of cell 8 may have a yellow-green peak. The UV sensitivity of retinula cell 7 is mostly the result of the physical dimensions of its rhabdom tubules. The rhabdom of retinula cell 8 lies under that of cell 7, so blue light is presumably filtered out before impinging on the cell 8 rhabdom (Snyder, 1975).
A retinula cell is a nonlinear analog sensor for light. It responds to increases in light intensity by a graded depolarization of its resting potential that propagates down its axon to its lamina cartridge. Figure 5.1-6 shows a montage of typical, direct-coupled, intracellularly recorded retinula cell depolarization responses to light flashes of increasing intensity. The eye of a bee was used. Note that dynamics of
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