REACTIONS TO LIGHT IN VANESSA ANTIOPA 411 



Feeling that the methods, as described above, were scarcely 

 adequate to justify the conclusions drawn, a somewhat similar 

 piece of apparatus was constructed with the important modifica- 

 tions that the light conditions were sharply defined, and that the 

 animals were suspended above the disc by means of a mechanical 

 holder, instead of being held with the hand. The object of this 

 experiment was to ascertain the nature of the stimulus effective 

 in orientation, that is, whether light produces orientation through 

 its continuous action or through a change of intensity. A way 

 of determining this would be to exclude one of these possible 

 methods of stimulation and to test the effect of the other alone. 

 Consequently, if the latter method is to be excluded, the organ- 

 ism should be so held that it would be subjected to no changes 

 of intensity. This condition is met only if the precautions 

 described above, or similar ones, are taken. 



That this condition might be fulfilled and that the direction 

 of movement of the animal might be detected, a circular piece 

 of thin black card board, 10 cm. in diameter, was suspended in 

 a horizontal position by means of a hat pin which was held in a 

 support that offered as little resistance to the easy movement 

 of the pin as possible. To the bottom of the disc a thin cork 

 was glued so that the pin pierced the cork as well as the disc. 

 The butterflies were suspended above the disc by means of a 

 holder which clamped the wings firmly together. The holder 

 was then adjusted so that the insects faced the center of the disc, 

 and were at such a distance above the disc that they could just 

 touch it with their feet, and yet not get a firm grip upon it. 

 A Nernst glower was so situated that the rays from it struck 

 the right eye of the animal at right angles to the long axis of the 

 body. By means of screens the beam of light was made so small 

 that the area of a cross section of it was but slightly larger than 

 the surface of the illuminated eye. In line with the glower and 

 the head of the animal a mirror was so placed that it reflected 

 the shadow of the head of the animal down upon the table which 

 supported the whole apparatus, as is shown in figure 20. A screen 

 was placed around the image so that as little light as possible 

 might be reflected into the room. Under these conditions the 



