216 PRINCIPLES AND CONCLUSIONS. 



are unable to direct themselves in semi-obscurity. Insects with com- 

 pound eyes pay no heed to differences of form existing between two illum- 

 inated openings and are deceived either by excess of luminous intensity 

 or of apparent surface. In short, they do not distinguish the forms of 

 objects at all, or do so very poorly (1885:251). 



Plateau (1888:437) stated that Exner, Notthaft, Carriere, Forel, and 

 Bleuler had all concluded from theoretical considerations, often sup- 

 ported by observations, that the majority of insects see the movements 

 of bodies much better than they do the bodies themselves. He regarded 

 his own experiments as fully confirming this opinion and permitting the 

 following conclusions. The faculty of perceiving the displacement of 

 mobile objects is very well developed in many insects with compound 

 eyes, those best endowed in this respect being the Lepidoptera, Hymen- 

 optera, Diptera, and Odonata. However, the distance at which the move- 

 ments of an object of small volume are perceived does not exceed 2 meters. 

 On the average it is 1.5 meters for the butterflies, 58 cm. for the bees, and 

 68 cm. for the flies. The perception of movement plays a great r61e as 

 the determining cause in the behavior of insects. It explains how, with- 

 out a clear vision of form, the dragon-flies pursue their victims in the 

 air, how different insects fly about among leaves moved by the wind, and 

 why those with slow movement often escape their enemies. On the other 

 hand, insects may be readily touched or caught if the movements are suf- 

 ficiently slow. For them an object that ceases to move merges at once 

 into the vague background. 



Discrimination of form. — As a result of simple experiments with a 

 spider, bee, and fly, Dahl (1889:243) found himself in agreement with 

 Lubbock and Forel as to the ability of insects to distinguish forms, and in 

 opposition to the conclusions of Plateau. He regarded the statement of 

 the latter that insects often deceive themselves by flying to unopened 

 flowers as indicating that they are attracted primarily by color rather 

 than form, and not that they are unable to distinguish forms. He also 

 observed (Knuth, 1898:45) that in spring young bees light on flowers 

 whose nectar is too deep-seated for them and make vain endeavors to reach 

 it, while the older bees merely come near and fly away without landing. 

 This indicates that the latter recognize the flower on sight as one with 

 unavailable honey, since guidance by odor would make it unnecessary 

 for them to approach the flower. 



Vision in honey-bees. — Buttel-Reepen stated that bees do not find 

 their way home when thrown up into the air at dusk, not far from the 

 hive; they make several small circles and fall to the ground completely 

 lost. Their flight is often strongly influenced by dark clouds, especially 

 during the honey-flow, when the approach of a storm sends thousands 

 of bees scurrying back to the hives. In their haste many blunders are 

 made in entering, which would not be true if they were drawn to the proper 

 hive like a magnet by the "unknown force." It was also observed that 

 when they went to the wrong hive, they tried to enter as at their own, 

 even when no entrance was present, showing that they had a memory 

 picture of their hive, though they had mistaken it. He also cited an in- 



