462 THE PHOTOTROPISM OF THE MOURNING-CLOAK BUTTERFLY, 



will circle around an oriented and expanded individual of either sex, till both fly off 

 together. 



Having determined that both negative and positive phototropism are manifested 

 by V. antiopa in sunlight, it may properly be asked whether these reactions are depend- 

 ent upon the so-called heat-rays or upon the light-rays of the sunlight. To test this 

 I passed a large beam of sunlight through a glass jar having parallel sides three 

 inches apart and containing a saturated solution of alum. This beam, when it fell 

 on the floor, illuminated an area of about two square feet. When the butterflies 

 were placed in this area they both crept toward the sun, and upon coming to rest 

 oriented negatively as they did in ordinary sunlight. Consequently the heat-rays 

 from the sun are not essential for either the positive or the negative reactions of V. 

 antiopa. 



If this species is negatively phototropic only when resting in bright sunlight, 

 and positively phototropic when creeping or flying in light of a considerable range 

 of intensity, it may be asked why it is that these butterflies remain near the ground 

 on a sunny day, why, in other words, they do not fly upward toward the sun. Some 

 marine animals, like the copepod Labidocera (compare Parker, :02), swim upward 

 through the water toward a source of light of moderate intensity. But their positive 

 phototropism is held in check by their inability to pass above the surface of the water. 

 No such barrier holds the butterfly to the earth, and since it, like the copepod, is 

 positively phototropic, one might expect it to fly upwards toward the source of light. 

 An answer to the question why V. antiopa does not do this can be found, I believe, 

 by studying the parts of the butterfly that are stimulated by light. When the sun 

 is shining brightly I have never observed butterflies to alight in a shady spot. If, 

 as they settle toward the ground, they come by accident into a small shadow, they 

 flutter a little farther till they can settle in sunlight. If a butterfly about to alight 

 is cautiously followed with the shadow of a hat or other object, it can be made to 

 flutter a long distance, but can never, in my experience, be brought to settle in the 

 shade when sunny spots are near at hand. If a butterfly oriented in the sunlight is 

 put in shadow by holding some object between it and the sun, it remains quiet a short 

 time, and then invariably flits away. In ten trials of this kind the following time- 

 intervals in seconds were observed between the moment when the shadow was thrown 

 on the butterfly and that when it began to move preparatory to flight: 6, 8, 8, 8, 15, 

 17, 8, 9, 9, 8. That the reactions were due to the shadow produced, and not to the 

 movement of the hand, could be shown in several ways. The hand might be moved 

 freely and quickly, but so long as the shadow cast by it did not touch the animal, even 

 though very close to it, a reaction was almost never observed. When the butterfly 



