Spatially Determined Reactions 207 



"mourning cloak" butterfly, Vanessa antiopa, on coming to 

 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 (537). Bohn also has 

 noted that certain 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 ten- 

 dency for the wings to be spread apart, while when the insect 

 is facing the light the wings are closely folded (82). The 

 effect on the wings was noted in Vanessa also, and, it is 

 suggested, may have some function in bringing the sexes 

 together (537) . 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. But owing to the kinetic influence of 

 the light, when the insects have been long exposed to sun- 

 light they tend to come to rest in the more shaded portions, 

 with their heads away from the light, for this is the posi- 

 tion in which they are least stimulated to movement. 

 The kinetic effect increases with the intensity of the light, 

 but its directive effect, through which orientation is 

 secured after the movement is started, was at least in one 

 case lost under intense light (116). Brundin (106) has 

 suggested that the effect of mechanical stimulation in 

 reversing light reaction may be due to the state of 

 activity it induces. 



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 (396). 



