460 THE PHOTOTROPISM OF THE MOURNING-CLOAK BUTTERFLY, 



end, they flew or crept toward the end which was at the moment illuminated. In 

 other words, they were as positively phototropic as the individuals previously tested 

 in the laboratory. But when they were placed on the floor of the laboratory, in the 

 bright sunshine, they oriented negatively with as much accuracy and precision as 

 they had shown in the woods. In fact, when they did not fly from the sunlit floor 

 to the window, it was very interesting to see them creep over the floor in the direction 

 of the sun and, just before stopping, make a turn of 180°, coming to rest with the head 

 directed away from the sun. It thus was clear that while in their flight and creeping 

 they were positively phototropic, in their resting position they were negative; and 

 further, that although their positive phototropism could be demonstrated either in 

 weak or in strong light, their negative phototropism was observable only in strong 

 sunlight. 



Other arthropods are known that show also this double phototropism with uniform 

 intensity of light. Thus the young king-crab, Limulus, according to Loeb ('93, p. 98), 

 swims toward the light but creeps away from it, even when the intensity remains 

 unchanged; and Cole (:01, p. 202) has discovered an interesting though not wholly 

 parallel instance in the pycnogonid Anoplodactylus, which either swims backward 

 or creeps forward toward a light. These double forms of response make the case of V. 

 antiopa appear less exceptional. 



Having ascertained that V. antiopa may orient negatively to bright sunlight 

 in the laboratory, I was puzzled to know why the animals with which I had first experi- 

 mented had not yielded similar results. On reflection the only marked difference 

 that I could think of between the specimens that had shown negative phototropism 

 in the laboratory and those that had not done so was that the first lot had been kept 

 overnight in the laboratory, while the second lot had been experimented upon directly 

 on arriving indoors. I therefore captured more individuals, that I had seen orienting 

 negatively in the sunlight, and kept them in a box overnight in the laboratory. On 

 testing them the next morning in sunlight, I found that, though they flew and crept 

 toward the light, they did not orient negatively in the sunlight as they had done the 

 day before. In these respects they were precisely like the first lot with which I had 

 worked. An obvious difference between these individuals and those that were still 

 in the woods was that the former had not had the vigorous exercise of flight, and I 

 therefore determined to try whether exercise would bring them into a state in which 

 negative orientation would take place. I took to the back of a large room a butterfly 

 that did not orient negatively in the sunlight, and let it fly some fifty feet to a distant 

 window. This I repeated five times, after which the insect, on being placed in the 

 sunlight on the floor, crept a short distance toward the sun and then, turning through 



