BENTLEY CLASS 907 



and ()j() m^. riicre is cviclciuc ol .sc'|)aralc icccpiois lor the l)luc-green 

 ami reel maxima, evidence inchicling the discovery ol a coh^rbliiul fly 

 that lacked the ()M() m^ maximum (Antrum and Stmnpl) . White- 

 eyed mutants, however, possess a single maximmn ol sensitivity, at 

 520 lUfx, and a sensitivity 1000 to 10,000 times as great as that ol 

 wildtype flies. These last-mentioned findings seem unexpected and 

 puzzling, in spite of speculative exjilanations by Autrmn and Stumpf. 



Goldsmith considers the existence ol a Purkinje shift in insects to 

 be unsubstantiated. He also evaluates the "Duplicity Theory," ac- 

 cording to Avhich the insect eye is suj^posed to have receptors varying 

 greatly in sensitivity and threshold, like vertebrate rods and cones. 

 Verdict: no compelling evidence. 



The relation of color vision in insects to the structure of the com- 

 pound eye is at present not clear. The mosaic theory of insect vision 

 has recently been questioned (de Vries and Kuiper) on the basis of 

 measurements indicating that the visual angle of an ommatidium is 

 about 8 degrees, great enough to produce overlapping of visual fields. 

 These authors propose on other evidence that the individual receptor 

 may be the retinula cell rather than the entire ommatidium. But if 

 that be so, convergence at the first synapse must take place, since 

 the number of ganglion cells is about the same as the nvmiber of 

 ommatidia. Also, fibers from different cells of an ommatidium may 

 go to different ganglion cells. (In Limulus, however, the ommati- 

 dium probably does behave as a unit receptor.) There is good evi- 

 dence that in insects the color receptors are not equally distributed 

 throughout the eye. In the backswimmer Notonecta, only the dorsal- 

 posterior ommatidia subserve color vision. In the cockroach Peri- 

 planeta, sensitivity to ultraviolet light is restricted to the dorsal half 

 of the eye, but the entire eye is sensitive to green. 



An interesting evolutionary question relates to the ability of insects 

 to respond to ultraviolet radiation and the inability of vertebrates 

 to do so. The visual pigments of vertebrates are sufficiently sensitive 

 to the near ultraviolet, but their lenses screen out the deep violet 

 and ultraviolet rays. Is this, as Wald has suggested, a device to 

 relieve chromatic aberration, a difficulty not met with in the insect's 

 compound eye? However that may be, it is certain that the evolution 

 of flower colors, pleasing as they may be to us, really has little or 

 nothing to do with vertebrate vision, but reflects the visual capacities 

 of insect visitors; and in the ultraviolet band of the spectrum there 

 lie hidden many color jjatterns and harmonies beyond our ken. 



E. F. MacNichol, jr., M. L. Wolbarsht, and H. G. Wagner have 



