136 The Animal Mind 



nest. This latter, containing eggs, was surrounded by col- 

 ored paper, and when a spider had become accustomed to 

 going in and out over the paper, another color was substituted, 

 and a false nest made in another place, surrounded by the 

 original strips of paper. The spider under these circum- 

 stances showed some confusion and tendency to go to the 

 false nest (321). The experiments with Daphnia have, 

 however, suggested a fundamental source of error in experi- 

 ments on the color vision of animals. A human being who 

 is totally color-blind is nevertheless able to discriminate 

 among objects that to a normal eye have different colors, 

 because such objects take on to the color-blind eye different 

 shades of gray. It is always possible, then, unless special 

 precautions are taken, that an animal's apparent discrimi- 

 nations of color may be really brightness discriminations, 

 in some way analogous to those made by the color-blind 

 person. No such precautions were taken in the experiments 

 just described, and the color sense of spiders remains unproved. 

 In blind and blinded myriapods, the family to which the 

 centipede belongs, skin sensitiveness to light is shown (329, 



335)- 



49. Vision in Insects 



The compound eye again occurs in insects, together with 

 ocelli or simple eyes, the latter usually placed in the middle 

 of the head. The respective functions of the two kinds of 

 eyes are not definitely known, though there is a possibility 

 that the ocelli may serve for near vision and for vision in faint 

 light. Plateau, however, finds that insects with the com- 

 pound eyes blinded and the simple eyes intact are unable to 

 see even in faint light, and has but a poor opinion of the 

 usefulness of the latter. Caterpillars, which have only sim- 

 ple eyes, depend, he thinks, chiefly on their long hairs or on 

 their feelers to warn them of the approach of obstacles (332). 



