338 



THE ANIMAL KINGDOM 



Figure 17.13. A device for estimating the visual acuity of an arthropod. A drum 

 with internal vertical stripes is rotated slowly around a circular glass dish. If an arthopod 

 inside can distinguish the stripes, it tends to move with them and maintain a fixed rela- 

 tion with the surroundings. 



common method is to take advantage of a "status quo" reflex with which 

 many animals attempt to maintain a constant relation to the environ- 

 ment. The animal is placed in a circular, glass-walled container (Fig. 



17.13) around which is rotated a drum with internal, vertical, black and 

 white stripes. If the animal sees only a mixed gray it remains quiet. If, 

 however, it can distinguish the stripes the rotational impression is very 

 strong and the animal turns or walks in circles to stay with the drum. 

 By varying the stripe width the discriminative limit can be tested. 



Two general conclusions can be derived from such studies: (1) 

 Visual acuity varies according to the excellence of the lens systems in 

 the ommatidia, which admit light through wider incident angles in 

 some arthropods than in others. (2) Acuity also varies inversely with the 

 number of ommatidia. The best arthropods have an acuity about %(, as 

 good as that of man. Most of them are much poorer than this. 



Von Frisch has extended his study of vision to an investigation of 

 the honeybee's ability to discriminate among various shapes. If a group 

 of white cards is placed on the ground with a glass dish on each (Fig. 



17.14) and syrup is placed in only one dish, bees discovering the syrup 

 will load up, return to the hive, and come back for more. Others come 

 too, and soon many may be coming and going. The bees are marked 

 with paint as they feed so that they can be recognized when they return. 

 If all the cards look alike to the bees they alight on all of the dishes. 

 If, however, the card with the syrup is recognizably different, once each 

 bee has found it she will return only to that dish. 



By using cards marked in various ways von Frisch found that bees 

 did not discriminate among squares, circles or triangles (Fig. 17.15), nor 

 did they distinguish two lines from a cross. They did, however, dis- 

 tinguish between solid and open figures, and between one line and two 

 lines. While they did not distinguish between a bar and a solid square. 



