Spatially Determined Reactions 223 



state of "hydratation" or desiccation it responds to the 

 attraction or repulsion of the larger screens with fatal 

 uniformity (80). 



Plateau attempted to test the responses of certain Dip- 

 tera to the size of an opening admitting light, by placing 

 them in a dark room, into which light entered from two 

 sources. One was a single orifice large enough to let the 

 insects out ; the other was covered with a net whose meshes 

 were too fine to allow them to pass. The amount of light 

 from the two sources could be made equal. When this 

 was done, the insects, which were positively phototropic, 

 sought the two equally often ; if the light from either was 

 made more intense, they went to that one. Plateau con- 

 cluded both that the flies could not see the netting and that 

 the area of the light source did not affect them (592). 

 On the other hand, Parker found that the mourning-cloak 

 butterfly did discriminate areas, flying to the larger of two 

 sources of equally intense light (537). 



This method of testing the image-forming power of an 

 animal's eyes has been elaborated by L. J. Cole. He sub- 

 jected animals with decided positive or negative photot- 

 ropism to the influence of two lights made equally intense 

 but differing in area, one coming through a piece of ground 

 glass 41 cm. square, the other a mere point. Eyeless 

 animals, the earthworm, for example, reacted equally 

 often to each light. Animals whose eyes from their struc- 

 ture have been judged capable of perceiving merely the 

 direction of light rays, such as the planarian Bipalium, 

 confirmed the argument from structure by showing little 

 more discrimination than the eyeless ones. On the other 

 hand, animals with well-developed compound or camera 

 eyes, for example certain insects and frogs, did distinguish 

 between the lights, going, if positively phototropic, toward 



