496 ALFRED O. GROSS 



to keep in the laboratory in a good \agorous condition new 

 specimens were * secured each time it was desired to make a 

 series of tests. Parts of branches were taken into the dark room 

 from which the contained larvae were not removed until they 

 were required in the experiments. These investigations were 

 made during October and November, a time too late for speci- 

 mens of newly hatched larvae but all the individuals of the first 

 winter exhibited a very strong positive phototropism. 



2. Methods. The apparatus used for these experiments on the 

 leopard-moth larvae was essentially the same as that employed 

 for the larvae of the blow-fly. The paths followed by the larvae 

 were, however, traced with a pencil, since methylene blue could 

 not be used to an advantage because of the waxy nature of the 

 larval integument. The pencil tracings were made by following 

 the course of the animal in such a way as not to interfere with 

 its movements or with the rays of light impinging on them. 

 The larvae when cold did not respond very readily to the light, 

 hence it was frequently found desirable to warm the slate, by 

 means of an electric heater placed beneath the apparatus, to a 

 uniform optimum temperature. 



3. Results. The accurate responsiveness of the larvae to the 

 different colors is demonstrated best in experiments with single 

 colors in which the direction of the light is suddenly reversed. 

 A typical record of the result of such an experiment, in which 

 a larva was tested with blue light, is illustrated by figure 35. 

 On this test the larva was placed at A, in the position indicated 

 by the arrow, in the midst of a beam of light coming from the 

 left. The larva immediately oriented and was permitted to 

 crawl towards the light undisturbed, to the position at B. At 

 this point the direction of the light was suddenly changed, where- 

 upon the larva immediately reversed its course in a direction 

 towards the new source of light at the right. The similar results 

 of many such tests made with each of the colors demonstrate 

 the positive phototropism and the marked responsiveness of the 

 leopard-moth larvae to lights of an intensity and quality used 

 in these experiments. The relative efficiency of the four lights 

 was obviously not determinable from the results of these simple 



