478 ALFRED O. GROSS 



one taken for the next two tests, et cetera. This use of both 

 lights ehminated from the results such errors as might arise 

 from defects in the apparatus, odors, diffuse light, et cetera. 



The paths plotted by the larvae under the stimulus of blue, 

 green, yellow and red respectively are shown in figures 1 to 4 

 inclusive. Under such conditions the larvae oriented and moved 

 away from the light on a course approximately parallel to the 

 direction of the rays. It is apparent from these experiments 

 that the blow-fl}^ larva is responsive to each of the colored lights 

 when of an intensity used in these investigations. x\s far as I 

 was able to determine from these simple tests, however, the larvae 

 showed no appreciable difference in their response to the several 

 lights. 



Opposed lights of equal intensity and of the same spectral 

 quality were then used in testing the larvae. When two lights 

 were used, the larva was allowed first to orient definitely under 

 the influence of one light and after this had been accomplished 

 the other light was thrown on. The position of the larva at the 

 time the second light was turned on was indicated on the record 

 in order to know over what part of the course it was under the 

 influence of the two fights. As the larva changed its position 

 the slate or the record paper, on which it was crawling, was 

 shifted a corresponding amount to keep the animal in the center 

 of the illuminated area and equi-distant from the two sources 

 of light. In general the larvae turned at right angles to the 

 direction of the rays when they were exposed to both lights. 

 Figures 5 to 8 inclusive are records of larvae tested with pairs of 

 blue, green, yellow and red lights respectively. Here again I was 

 unable to perceive any marked and consistent difference in the 

 responses of the larvae to the first three of the above lights. In 

 these cases the angle marking the change in the course was 

 generally sharp and more or less abrupt. When red lights were 

 used the angle in the path made by the larvae was seldom so 

 abrupt but there was more usually a uniform and prolonged 

 curve as shown in figure 8. However, even in red light, if the 

 larva is permitted enough time in which to crawl, it eventually 

 orients so that its median plane comes to lie at right angles to 



