456 s^THE PHOTOTROPISM OF THE MOURNING-CLOAK BUTTERFLY, 



and hibernate till the next spring. The individuals on which most of my observa- 

 tions were made were hibernated ones from the region about Cambridge, Massachu- 

 setts, though I have also studied the reactions of butterflies of the summer brood. 



II. OBSERVATIONS AND^DEDUCTIONS. 



The observations that led me to study the phototropism of V. antiopa was .made 

 in a piece of ^open woodland on a bright sunny day toward the end of March. On 

 this occasion several of these butterflies that had hibernated were seen flitting 

 about, and it was observed that when they settled on the ground their orientation 

 with reference to the sun's rays was extremely exact. A straight stick held verti- 

 cally at an appropriate point cast a shadow that fell exactly on the length of the butter- 

 fly's body, but the butterfly instead of being oriented with the head toward the sun, 

 as might have been expected, took up its position with the head away from the source 

 of light, that is, it was negatively phototropic (Plate XXXIII, Figs. 1, 2). This un- 

 usual orientation at once attracted my attention and led me to study further the 

 reactions of this species to light. 



Subsequent observations made on more than eighty of these butterflies demon- 

 strated that this method of orientation was almost invariable. Whenever a butter- 

 fly alighted on a piece of level ground in full sunlight, it oriented accurately with the 

 head away from the sun. Now and then an individual in settling would be thrown 

 slightly out of position by some accidental irregularity of the ground, but such an 

 individual invariably readjusted itself to an exact orientation before coming finally 

 to rest. In one Instance I observed a butterfly that repeatedly settled with the head 

 some thirty degrees to the right of the position of exact orientation, but this indi- 

 vidual invariably turned its body into correct orientation immediately after alighting. 

 As I could discover no constant irregularity on the surfaces on which it settled, 

 I assumed that the initial divergence was due to some peculiarity in the butterfly's 

 organization, such as a distorted leg or other defect, but the insect was so wary that 

 I was unable to catch it to ascertain if this was the correct explanation. 



So constant was the orientation of these butterflies in full sunlight that any 

 divergence from the usual conditions was easily noted. What seemed at first sight to 

 be an exceptional position was noticed often when a butterfly settled on the vertical 

 trunk of a tree in sunlight. Under such conditions the axis of its body was held ver- 

 tical and the head was pointed downward. This inverted position had long been 

 familiar to me, but I had never thought of an explanation of it till I observed the 

 accurate orientation of this species on horizontal surfaces. If on level ground the 



