510 ALFRED O. GROSS 



factor operating in their initial orientation. If the fly on enter- 

 ing the chamber illuminated by opposing colors of nearly the 

 same wave length (as, for example red and yellow) is headed 

 directly away from the more effective light, the chances are it 

 will remain under the influence of the less potent light, since 

 more of the elements of the compound eyes are effected by this 

 light than by the other. In a case where there is a great differ- 

 ence in the efficiency of the two opposing lights (as, for example, 

 when red and blue, or blue alone is used) the more refrangible 

 rays, though impinging upon relatively few elements of the eye, 

 are so effective that the insect will be oriented towards the blue 

 light even when it enters the cylinder facing away from the more 

 potent light, or in a direction oblique to the direction of the rays. 



The reactions of Periplaneta americana in response to colored 

 lights of equal intensity are remarkable in being positive to blue 

 and negative to green and yellow. It is not an uncommon 

 occurrence to produce a reversal of phototropism among the 

 lower organisms by changing the nature of their environment or 

 by using extreme differences in the intensity of illumination, but 

 I know of no recorded observation in which an animal has been 

 shown to be positive to one monochromatic light and negative 

 to another of the same intensity, but of different wave length. 



Graber ^83), working with a closely allied species of cockroach, 

 Blatta germanica, found that these insects collected on the red 

 side of a two-chambered compartment illuminated respectively 

 with blue and red sheets of glass. From these results one would 

 infer that Blatta is negative to blue, not as Graber ('84, p. 152) 

 interprets a "blauscheues resp. rot-holdes Insect." No one has 

 ever verified with spectral light Graber's results on the cock- 

 roach so the discrepancy in our results is in all probability due 

 to difference in method. The blue glass used by Graber trans- 

 mitted not only the blue rays but red and green, some yellow, 

 and the invisible heat rays. Furthermore Graber did not state 

 in just what manner the aggregation of Blatta took place. 



At the present state of our knowledge, it is difficult to offer 

 an explanation of the reversal of the phototropism of the cock- 

 roach to the different colors. 



