Spatially Determined Reactions 221 



near the nest were moved to the opposite side. Turner 

 made a similar observation, and Santschi suggests that 

 the compound eye may perceive the direction of light by 

 acting as a kind of sundial. He was able to make ants re- 

 verse their course when he altered the light direction by 

 the use of mirrors. 



65. Methods of Investigating the Visual Image 



Various methods of solving the problem as to the nature 

 and accuracy of an animal's visual images have been used. 

 One method consists in a study of the sense-organ itself, 

 removed from the body. For example, Petrunkevitch 

 (575) has thus investigated the sense of sight in spiders. 

 These animals do not have the compound eye, but a num- 

 ber of simple eyes placed in groups. By a careful measure- 

 ment of the possible minimal angles of vision in two spiders, 

 Phidippus and Lycosa, the conclusion is reached that 

 while a creeping insect about one square centimeter in 

 size would be to the human eye so clearly visible at a dis- 

 tance of three metres that its species could be recognized, 

 it would be only an indefinite moving speck to the eye 

 of Phidippus and wholly beyond the range of vision of 

 Lycosa. 



Again, inferences are drawn as to the visual powers of 

 animals from miscellaneous peculiarities of behavior. 

 Thus Petrunkevitch (576) reports that a male spider of 

 the species Dysdera crocata,m the courting stage, "watched" 

 the movements of the end of a hatpin with which the ob- 

 server was breaking clumps of earth, and when the move- 

 ment ceased the spider approached the spot and scratched 

 it with his front legs. The sight of a female spider digging 

 had the same effect upon him, so evidently the visual 



