NO. lO TROPISMS OF LEPIDOPTERA McINDOO 7 



latter could have made such positive statements since only two colors 

 were used. It is easy to ascertain that animals can distinguish wave 

 lengths that we call colors, but it is difficult and perhaps impossible 

 to determine whether the responses are brought about by the quality 

 or quantity of the wave lengths, that is, by actual color or by bright- 

 ness. Our only recourse is to test them and to judge their responses 

 from the human point of view, which proves little or nothing in regard 

 to insects. On this point Mast (p. 362) says: 



Bees and fishes can undoubtedly distinguish different regions in the spectrum. 

 They can be trained to select any of the primary colors of the spectrum by asso- 

 ciating these colors with food. That is, they are positive to (or select) one color 

 at one time and. another at a different time. Just what mechanism is involved in 

 this power of selection is unknown. Whether it is on the basis of brightness or 

 on the basis of color vision or neither is a matter concerning which experimental 

 evidence does not warrant a definite conclusion. Many organisms react to ultra- 

 violet much as they do to visible rays. This is in harmony with the following 

 quotation from Schafer referring to man (1898, p. 1055) : "The invisibility of 

 the infra-red rays is probably due to insensitiveness of the retina, while the 

 ultra-violet rays fail to be seen, partly, at any rate, owing to absorption by the 

 ocular media." 



Washburn (90. pp. 144-159) discusses the problem of visual quali- 

 ties in invertebrates. Certain authors believe that vision as far as 

 color is concerned in the lower animals, particularly insects, is similar 

 to that in totally color-blind people. On this point Washburn (pp. 145, 

 147, 148, 157) says: 



It is therefore of some importance to the problem of color vision in the lower 

 animals to find hoiv strongly the light rays of various wave-lengths affect them. 

 But we must bear in mind that for the lower animals it is impossible to conclude 

 color-blindness from the fact that the brightness values, that is, the effective 

 intensities, of the different colors are what they would be for a color-blind human 

 being. Just this unsafe inference is, however, drawn by certain authorities. . . . 

 It is thus clear that when an animal discriminates between rays of different 

 colors, the discrimination may be based merely on the intensity of the rays, 

 either in themselves or in the effect which they have on the organism, rather than 

 on their wave-length or color. . . . He [Hess] found that the yellow and green 

 rays produce much more effect than the red and violet rays. Since this is true 

 also of the color-blind human eye, he argues that the animals tested are totally 

 color-blind. He holds, in fact, that all invertebrate animals are totally color- 

 blind, on the same evidence. . . . But again we may remind ourselves that it 

 does not follow that because a human being who finds the yellow-green, rather 

 than the yellow, the brightest spectral region, is totally color-blind, therefore an 

 animal, especially an invertebrate animal, the chemical substances in whose eye 

 may have no resemblance to those in the human eye, is color-blind if it shows 

 these reactions to the different regions of the spectrum. 



Mayer and Soule (59) in 1906 determined that the caterpillars of 

 the milkweed butterfly are photopositive to ultraviolet rays, but barely 

 respond to the rays which man sees in the spectrum. 



