Spatially Determined Reactions 217 



brought about by objects in motion. In the mollusk Pecten 

 varius, a transition from shadow vision to movement vision 

 is illustrated : the animal closes its shell when a shadow 

 is moved so as to fall on its eye spots in rapid succession 

 (628). Generally speaking, the simple invertebrate eye, 

 however, is adapted to respond to changes in light intensity 

 rather than to moving objects. Plateau found that cater- 

 pillars, which have only simple eyes, could see moving ob- 

 jects no better than those at rest (597), and Willem was 

 inclined to think snails saw resting objects better than 

 moving ones (788). On the other hand, the compound 

 eye (see page 219) is specially formed to be affected by mov- 

 ing stimuli. The crayfish will react to anything of fairly 

 good size in motion, but is apparently unable to avoid sta- 

 tionary objects in its path (40) . The poor vision of the com- 

 pound eye for resting objects is shown by the ease with which 

 insects may be captured if the movements of the captor 

 are very slow. They may be readily approached, also, 

 if the movements are all in the line of sight, that is, directly 

 toward the insect, so that successive facets of the compound 

 eye are not affected, as would be the case in lateral move- 

 ments. Let the reader try bringing the hand slowly straight 

 down over a fly, and see how much closer he can come before 

 the fly is disturbed than he can if the hand is moved from 

 side to side. Plateau, from experiments on different orders 

 of insects, concludes that "visual perception of movement" 

 is best developed in the Lepidoptera (moths and butter- 

 flies), Hymenoptera (ants, bees, and wasps), Diptera (flies), 

 and Odonata (dragon-flies) ; that the distance at which 

 movements can be seen does not exceed two metres, and 

 averages 1.5 metres for diurnal Lepidoptera, 58 cm. for 

 Hymenoptera, and 68 cm. for Diptera (599). 



It is possible that response to a moving stimulus received 



