INSECT BEHAVIOR 353 



cniwl downward; in other words, they are positively photo- 

 tropic only so long as they are unfed. Here the kind of pho- 

 totropism is dependent upon nutrition. 



Phototropism may be overruled by chemotropism and influ- 

 enced by conditions of metabolism, as Parker found for the 

 butterfly Vanessa antiopa. In his words : Vanessa antiopa, in 

 bright sunlight, comes to rest with the head away from the 

 source of light, that is, it is negatively phototropic, when the 

 surface on which it settles is not perpendicular or very nearly 

 perpendicular to the direction of the sun's rays. When, how- 

 ever, this surface is perpendicular to the sun's rays the insect 

 settles without reference to the direction of the rays. When 

 feeding or near food [such as running sap] the butterflies do 

 not respond phototropically. 



This negative phototropism is seen only in intense sunlight 

 and after the butterfly has been on the wing, i. e., after a cer- 

 tain state of metabolism has been established. 



V. antiopa creeps and flies toward a source of light, that is, 

 it is positively phototropic in its locomotor responses. Posi- 

 tive phototropism also occurs in intense sunlight, and is not 

 dependent upon any particular phase of metabolism. 



Both negative and positive phototropism in this species are 

 independent of the " heat rays " of sunlight. 



The position assumed in negative phototropism exposes the 

 color patterns of the wings to fullest illumination, and prob- 

 ably has to do with bringing the sexes together during the 

 breeding season. 



To these may be added other important conclusions of 

 Parker's : 



No light reactions are obtained from the butterfly when 

 shadows are thrown upon any part of the body except the 

 head. When one eye is painted black the butterfly creeps or 

 flies in circles with the unaffected eye always toward the cen- 

 ter. When both eyes are painted black all phototropic re- 

 sponses cease and the insect flies upward. Butterflies with 

 normal eyes liberated in a perfectly dark room come to rest 

 24 



