PHOTORECEPTION 167 



axons from several ommatidia. Each bundle in the lamina becomes 

 associated with the axon of a giant monopolar cell, with which most 

 of the retinal fibres synapse, and the axon of the centrifugal cell whose 

 cell body lies in the medulla externa. The units ('optic cartridges' of 

 Power) in the lamina thus consist of a long retinal fibre, a number of 

 short retinal fibres, a giant monopolar fibre, and a centrifugal fibre. 

 These fibres now enter the outer chiasma. Fibres from the anterior 

 ommatidia cross to the posterior region of the medulla externa; 

 fibres from the posterior ommatidia cross to the anterior area of the 

 medulla externa; fibres from the middle ommatidia pass directly 

 through the chiasma without crossing. All fibres from the lamina 

 ganglionaris end in the medulla externa. At the present state of our 

 knowledge of the eye there is little point in pursuing the complexities 

 of the optic lobe beyond this level. 



The Function of the Retinal Cells 



It is generally assumed that the photosensitive pigment is located in 

 the retina cells. As yet there is no direct evidence for this. Prior to 1957 

 all attempts to isolate a visual pigment failed (Goodwin and Srisukh, 

 1949; Wald and Burg, 1957; Wolken, 1957; Wolken, Mellon, and 

 Contis, 1957), but Goldsmith (1958 a) was finally able to isolate 

 retinene from the heads of honeybees. It had the specific absorption 

 peak 664 mii. Since no retinene was obtained from parts of the body 

 other than the head and since retinene previously has been found only 

 in eye tissues, Goldsmith concluded that it is the chromophore of a 

 visual pigment located in the compound eyes and/or ocelli. He found a 

 pigment that possesses a wavelength maximum at about 440 mu.. On 

 bleaching in light the maximum moves to 370 m[i, probably represent- 

 ing the formation of retinene (Goldsmith, 1958 b). Spectral sensitivity 

 measurements of the eyes of honeybees reveal several peaks at diff*er- 

 ent wavelengths, one at 440 mjx (compound eye of drone). This is the 

 only light-sensitive pigment containing retinene that has been ex- 

 tracted from insect eyes thus far. Bowness and Wolken (1959) isolated 

 from the house fly a yellow pigment with a spectral absorption maxi- 

 mum at 437 mil when unbleached and at 440-446 and 350-360 my. 

 when bleached, but neither vitamin A nor retinene was detected in this 

 pigment. The work on colour vision suggests that there are others with 

 maxima at different wave lengths. 



M 



