INSECT BEHAVIOR 289 



This self -preservative " instinct " is purely a response to light. The cater- 

 pillars are positively photo tropic, and as the horizontal components of 

 the surrounding light neutralize each other, only the light from above is 

 effective as a stimulus to orientation. After feeding, however, the 

 larvae are no longer positively photo tropic and crawl downward; in 

 other words, they are positively photo tropic only so long as-they are 

 unfed. Here the kind of phototropism is dependent upon nutrition. 



Phototropism may be overruled by chemotropism and influenced 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, 

 however, 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 certain state of metab- 

 olism has been established. 



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

 tively phototropic in its locomotor responses. Positive 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 independ- 

 ent of the "heat rays" of sunlight. 



The position assumed in negative phototropism exposes the color 

 patterns of the wings to fullest illumination, and probably 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 center. When both eyes are painted black all 

 phototropic responses cease and the insect flies upward. Butterflies 

 with normal eyes liberated in a perfectly dark room come to rest near 

 the ceiling. This upward flight in both cases is due to negative geotrop- 

 ism, not to phototropic activity. 



V. antiopa does not discriminate between lights of greater or less 

 intensity provided they are all of at least moderate intensity and of 

 approximately equal size. V. antiopa does discriminate between light 



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