458 THE PHOTOTROPISM OF THE MOURNING-CLOAK BUTTERFLY, 



on which the butterflies alighted, it is possible that the irregularities that he so fre- 

 quently noticed might have been explained. 



On the bright spring morning when I first observed the orientation of the mourn- 

 ing-cloak butterfly, the sun was from time to time obscured by rapidly drifting, well- 

 defined clouds. Of the dozen or more butterflies observed that morning, not a single 

 one exhibited any noticeable irregularity in its orientation in full sunlight. When, 

 however, the sun was under a cloud, the animals settled in any position; but as soon 

 as it shone forth again, all such animals, if not by accident properly oriented, turned 

 at once into the characteristic position. The reaction was extremely striking, espe- 

 cially when three or four butterflies that had settled during an overcast period could 

 be watched simultaneously as sunlight broke upon them; all oriented immediately 

 and accurately, as though they had been quickly brought under the influence of some 

 force like that of a magnet. In fact when the sun was on the meridian the heads of 

 these butterflies pointed with greater precision to the true north than does the needle 

 of a common pocket-compass in this latitude. 



This orientation could be experimentally controlled by an artificially produced 

 shadow. If, while the sun was under a cloud, a hat was held so that when the light 

 burst forth the butterfly would still be in shadow, the insect would remain unoriented, 

 and, if it did not fly before the shadow of the hat was removed, it oriented immediately 

 when the direct sunlight was allowed to fall on it. 



The orientation occurred only when the sun was unobscured. It was remarkable 

 how even a small amount of haziness would interfere with the reaction. With a clear 

 sky, however, the reaction never failed to occur. One may, therefore, conclude that 

 the negative phototropism of V. antiopa is dependent upon strong sunlight. 



To test this phototropism under conditions that I could control, I collected a 

 number of butterflies and carried them to the laboratory alive. From the accuracy 

 of their responses in the field, I believed it would be very easy to show their negative 

 phototropism within doors. When, however, a few were liberated in a black box 

 one end of which had a glass window in it, they immediately flew to the window and 

 acted in other respects like positively phototropic animals. I then carried them 

 to a photographic room the walls and ceiling of which were painted black, and liber- 

 ated them one at a time some twenty feet from the only window by which light entered 

 the room. In every instance they flew toward the window. The course that they 

 took was not a straight one, but they flew as in the field, in irregular curves, the result 

 of which was progress towards the window. I was much surprised that their photot- 

 ropism should be so pronouncedly positive, and it occurred to me that possibly they 

 had been influenced by heat, for the room in which I had experimented was much 



