192 The Animal Mind 



trated : the animal closes its shell when a shadow is moved so 

 as to fall on its eye spots in rapid succession (360). Generally 

 speaking, the simple invertebrate eye, however, is adapted to 

 respond to changes in light intensity rather than to moving 

 objects. Plateau found that caterpillars, which have only 

 simple eyes, could see moving objects no better than those at 

 rest (333), and Willem was inclined to think snails saw rest- 

 ing objects better than moving ones (441). On the other 

 hand, the compound eye is specially formed to be affected by 

 moving 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 (21). 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 movements. 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, con- 

 cludes that "visual perception of movement" is best devel- 

 oped in the Lepidoptera (moths and butterflies), Hymen- 

 optera (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 (335). 



It is possible that response to a moving stimulus received 

 through the eye may be accompanied by spatial perception 

 of movement, although if the eye is compound, the experience 

 must differ from our own visual movement perception. 



