Every CE has directly beneath it a complex nervous network called the optic lobe. It is tempting to view the OL as analogous to the vertebrate retina, but, in fact, it is far more complex in structure and function than a retina. One might argue that the arthropod OL fulfills the role of the retina and most of the visual parts of the CNS in vertebrates. Inside the OL there are three major dense, highly organized ganglionic layers of neurons. Figure 5.1-8 shows an overview of the relation of the OLs to the eyes and the protocerebrum in the lubber grasshopper, Romalea microptera. Figure 5.1-9 shows an artist's composite drawing of vertical sections through a silver-stained, Romalea OL. Figure 5.1-10 shows a light photomicrograph of a coronal (horizontal) section of a silver-stained Romalea OL. Note the dense neuropile in the transmembrane transmembrane log light intensity_
log light intensity_
FIGURE 5.1-6 Graphs of typical intracellular recordings of retinula cell depolarizations to progressively more intense flashes of 440 nm light to the compound eye of drone bees (Apis m.). Flashes are 200 ms in duration. Note the development of a peak in the retinula cell depolarization waveform as the flash intensity increases. A curious double peak in E evolves into an initial peak plus a sustained depolarization phase. This evolution suggests that two independent kinetic processes may be involved. (Figure drawn from data in Naka and Eguchi, 1962.)
lobula and medulla and the giant nerve cells and neural tracts outside these neural networks.
Figure 5.1-11 is a schematic drawing of the OL of the larva of the dragonfly Aeschna, showing typical interneuron pathways seen by light microscopy on silver-stained sections. Figure 5.1-12A is a another schematic of neurons in the OL of the fly Calliphora. Again, note the tracts and the very complex interconnections in the medulla. In Figure 5.1-12B, the tracts between neuropile masses in the OL of the butterfly Celerio euphorbiae are illustrated. (Both figures from Mazokhin-Porshny-akov, 1969.)
The figures show that nerve fibers from the retinula cells pass in bundles through a basement membrane of the CE to the first ganglionic mass in the OL, the lamina ganglionaris. The lamina, in turn, projects fibers to the most complex ganglion layer, the medulla. From the medulla, some fibers go directly to the protocerebrum; others pass to the third ganglion, the lobula, thence to the protocerebrum. Interestingly, efferent fibers carrying nonvisual sensory information also run from the protocerebrum back to the medulla and lamina, where their signals interact with visual information.
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