PHOTORECEPTION 165 



in the rhabdomeres, that is, in the lumina of the microviUi. In Erebus 

 there is an extensive underlying structure, the tapetum, composed of 

 tracheoles which are highly reflective (Fernandez-Moran, 1958). 



The retinal cells may occupy one of the two positions with respect to 

 the crystalline cone (Fig. 6). In apposition eyes the retinal cells lie 

 immediately beneath the crystalline cone; in superposition eyes the 

 retinal cells are situated some distance proximad of the crystalline 

 cone. The intervening space is transparent. In all eyes each ommati- 

 dium is enclosed in pigment cells. The number of pigment cells is not 

 constant, but usually there is a distal set (iris pigment cells) surround- 

 ing the crystalline cone and another set (retinal pigment cells) 

 surrounding the retinal cells. 



At the base of the ommatidia, and marking the proximal limit of the 

 retina, is a fenestrated basement membrane. Through the fenestrae 

 pass the proximal processes, the axons, of the retinal cells. There is 

 usually one axon for each cell. In Erebus, however, there are sometimes 

 nine axons from a single ommatidium. This occurrence indicates that 

 sometimes the axons begin to branch close to the cell body. Some 

 synapse almost immediately with neurons of the optic lobe; others ex- 

 tend a considerable distance into the lobe before synapsing. The 

 presence of two kinds of retinal fibres, long and short, has been known 

 for some time. Hanstrom (1927) had speculated upon the possible 

 physiological differences between the two and likened them to the 

 rods and cones of the vertebrate eye. 



The optic lobes are in fact a part of the brain, but since so many 

 physiological studies of the eye treat them as part of the eye rather 

 than as the central nervous system, a brief description at this point will 

 be helpful. Most of our knowledge of these complicated structures 

 stems from the classical studies of Zawarzin (1914) on the larva of the 

 dragonfly Aeschna, Cajal (1909, 191,8) and Cajal and Sanchez (1915) 

 on the house fly and the blowfly. In addition, there are the accounts of 

 Giinther (1912) and Holste (1923) on Dytiscus, Bretschneider (1921) 

 on the moth Deilephila euphorbiae, and Power (1943) on Drosophila. 

 The general works of Hanstrom (1928) and Snodgrass (1935) should 

 also be consulted. 



The optic lobe (Fig. 85) is divided into three areas: the lamina 

 ganglionaris, usually in immediate contact with the basement mem- 

 brane of the retina; the medulla externa; the medulla interna. 



The lamina ganglionaris is the first or most distal synaptic layer of 

 the optic lobe. It consists of giant monopolar cells, small monopolar 

 cells, short retinal fibres, long retinal fibres, and centrifugal fibres. 



