VANESSA ANTIOPA LINN. 465 



localized spot on each retina of the butterfly, and it is the position of these spots that 

 determines the direction of flight. 



It is my belief that V. antiopa reacts very slightly, if at all, to differences hi the 

 intensity of light of considerable brightness. When a butterfly is liberated midway 

 between two bright lights of different intensities, such as an incandescent lamp of 

 sixteen candle-power and one of fifty candle-power, it is about as likely to fly toward 

 one as toward the other, and the same is true of windows differently illuminated. 

 With relatively strong lights, intensity, then, is a factor that may be varied without 

 noticeably influencing the reactions of the butterfly. 



If, in a room illuminated at one end by an open window and at the other by an 

 incandescent lamp, a butterfly is liberated where the intensity of the light from these 

 two sources is about equal, the insect flies almost invariably toward the window. As 

 the light from the window must make a large spot on the retina and that from the 

 incandescent lamp a small one, it is probable that the size of the spot is the factor 

 that determines the direction of flight. This and the indifference shown by the butter- 

 flies to variations in the intensity of bright light offers, I believe, an explanation for 

 the fact that in bright sunlight the butterflies do not fly toward the sun. To them 

 the sun is only one of the hundreds of spots of bright light about them, and, though 

 the retinal image of the sun must be vastly brighter than those of all other spots, 

 the butterflies do not respond to this difference, but rather to the larger size of the 

 images of the sunlit spots in the woods. I therefore believe that V. antiopa stays 

 near the ground on bright sunny days because its flight is directed by large bright 

 retinal spots rather than by small ones, even though the latter are of vastly greater 

 intensity. 



If the intensity of light plays so insignificant a part in the habits of this species, 

 how does it happen that the butterflies retreat with the setting of the sun and emerge 

 again only with its rising? The explanation of these conditions depends, I believe, 

 upon temperature. These butterflies remain during cool spring nights in places similar 

 to those in which they hibernate in the winter, viz., in openings in stone walls, in old 

 outhouses, in openings under the bark of trees, etc. They retire to these places with 

 considerable regularity, so that in the open woods, where dozens of individuals may 

 have been seen flitting about, all may have disappeared a quarter of an hour later. 

 I have watched their retreat with some care. On a clear afternoon in early April 

 I took my stand in a woodland where many mourning-cloak butterflies were to be 

 seen on the wing. They continued actively flying about till approximately four 

 o'clock, when I began to notice a diminution in their numbers. By a quarter past 

 four not a butterfly was to be seen. During the fifteen minutes from four o'clock 



