1 82 The Animal Mind 



rest in bright sunlight, orients itself with the head away from 

 the light. When it moves, on the other hand, it flies toward 

 light of any intensity (307). Bohn also has noted that cer- 

 tain butterflies orient themselves when alighted in such a 

 way that the posterior part of the eyes is toward the light. 

 When in this position there is a tendency for the wings to be 

 spread apart, while when the insect is facing the light the 

 wings are closely folded (55). The effect on the wings was 

 noted in Vanessa also, and, it is suggested, may have some 

 function in bringing the sexes together (307). The pomace 

 fly when at rest is not oriented at all. Light exerts upon it 

 merely the effect of stimulating it to movement, a "kinetic," 

 not a directive, effect. When the movement has been started, 

 however, it is directed toward the light : positive phototaxis 

 appears. But owing to the kinetic influence of the light, 

 when the insects have been long exposed to sunlight they tend 

 to come to rest in the more shaded portions, with their heads 

 away from the light, for this is the position in which they 

 are least stimulated to movement. The kinetic effect in- 

 creases with the intensity of the light, but its " directive 

 effect," through which orientation is secured after the move- 

 ment is started, was at least in one case lost under intense 

 light (74). 



The background, finally, sometimes determines the sense 

 of the reaction. Keeble and Gamble found that while the 

 crustacean Hippolyte varians would move toward the light 

 whether it was on a white or black background, Macromysis 

 inermis was negative on a white ground and positive on a 

 black ground (218). 



71. Mutual Influence of Light and Gravity Orientations 



Orientation to light and orientation to gravity are not with- 

 \ put mutual influence in determining the behavior of an 



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