792 LIGHT AND LIFE 



It should be further pointed out that the distribution of color re- 

 ceptors may (45) or may not (57) be correlated with externally 

 visible featines such as the color of the accessory pigmentation. 



The Significance of Sensitivity to Near Ultraviolet Light 



Unlike vertebrates, insects appear to have exploited the shortest 

 wavelengths present in their environment. Because of a filter of 

 ozone in the upper atmosphere, virtually no far ultraviolet light 

 (wavelengths shorter than about 300 m^) reaches the surface of the 

 earth (47) . Not siuprisingly, the cornea of insects is transparent 

 through the near ultraviolet but absorbs strongly below- about 300 

 m;, (23). 



It has been suggested that in some apposition eyes visual acuity may 

 be limited by the diffraction of light (3, 52) . These calculations were 

 for visible wavelengths, but it is possible that 340 m/x might have 

 been more appropriate. However, not enough is known about the 

 anatomical identity and distribution of color receptors in the layer of 

 retinulae to pursue this argument further. 



It is probably posing something of a false issue to ask why insects 

 see in the ultraviolet. The appropriate question is why vertebrates do 

 not. The visual pigments of vertebrates have a significant sensitivity 

 to the near ultraviolet. However, the yellow lenses of vertebrates 

 tend to prevent deep violet and idtraviolet light from reaching the 

 visual cells (54, 55). As Wald (54) has pointed out, this restriction 

 of the visible spectrinii was probably introduced in the vertebrate 

 eye to relieve chromatic aberration, a problem which is not met in 

 compound eyes. 



The high effectiveness of ultraviolet in the action spectrum for 

 phototaxis (see Fig. 1) raises an interesting question. It is cmious 

 that the small amount of ultraviolet light available to insects is 

 apparently such a significant part of their environment. Some clue 

 to the nature of this problem appears in a recognition of the mutual 

 interactions that have occiuTed in the evolution of insects and angio- 

 sperms. As von Frisch has said: "The colors of flowers have been 

 developed as an adaptation to the color sense of their visitors. It is 

 evident that they are not designed for the human eye." 



Lutz (39) suggested that variations in the amount of ultraviolet 

 reflected from floral parts might j^roduce patterns of color which 

 could not be ai^prcciaied l)y the human eye but which might be of 

 the utmost importance to the plant's insect pollinators. Measure- 

 ments of the ultraviolet reflectance of petals have confirmed the 



