464 ^THE PHOTOTROPISM OF THE MOURNING-CLOAK BUTTERFLY, 



eye was covered the insects crept or flew in a circle, with the left side invariably 

 toward the centre; and the reverse took place when the other eye alone was blackened. 

 These circus movements agree with those observed by Holmes (: 01, p. 220) in other 

 positively phototropic arthropods. 



When both eyes were blackened, the butterflies invariably flew upward in irregu- 

 lar circles, sometimes with the right side toward the centre, sometimes with the left. 

 This upward flight has long been known and has been interpreted by some investi- 

 gators as indicating that the general integument of the insect is sensitive to light. 

 V. antiopa, however, did not fly toward the sun, which, when these experiments were 

 tried, was in the west, but flew directly upward. I therefore suspected that this 

 reaction was due not to the light but to negative geotropism, which, as Loeb 

 ('90, p. 53) has shown, is well developed in butterflies. To test this hypothesis I 

 liberated a number of normal individuals of V. antiopa in an absolutely dark room, 

 and after they had come to rest I turned on the light, and almost invariably found 

 them clinging to the ceiling or rafters. I therefore concluded that this species is 

 negatively geotropic and that the upward flight observed after both eyes of the insect 

 had been blackened is a geotropic response, and not a reaction to light. That the 

 skin is not stimulated by light, at least in any observable degree, is shown by the 

 fact that when the wings of a butterfly are clipped so that it cannot fly it orients in 

 creeping and in resting as a normal individual does ; but when, under these conditions, 

 its eyes are blackened, all orientation in reference to light, so far as I can observe, 

 ceases. I therefore believe that the eyes are the organs stimulated when V. antiopa 

 orients to the light, and that its upward flight when both eyes are blackened is a 

 geotropic response. 



Having determined that the eyes are the organs stimulated in the phototropism 

 of V. antiopa, it remains to ascertain something of their use. Like the similar organs 

 of many other insects they are well-developed image eyes. They receive light from a 

 large portion of the surrounding field, and, judging from the studies of Exner ('91), 

 this forms an image of considerable richness in detail. It has already been shown 

 that these butterflies are sensitive to light from various parts of the field. If a shadow 

 is cast on an individual settled in sunlight, it invariably rises within a short time and 

 flies to a neighboring patch of sunlight. This patch is found not through the acci- 

 dental wandering of the butterfly into it, but by the butterfly's taking a direct course 

 to it, precisely as the insect finds a single light window in an otherwise dark room. 

 The directive influence, then, is not the intense sunlight that makes the patch, but 

 the much less intense reflected light radiating from the patch. This must form a 



