The Senses of Animals. 



291 



stippled with the fine stippling of the ends of the rods and 

 cones. In insect-vision the stippling is far coarser, and the 

 image is produced on different principles. 



In the vertebrate the image is produced by a lens ; in 

 the insect's eye, by the elongated cones. How this is effected 

 will be readily seen with the aid of the diagram. At a b 

 are a number of trans- 

 parent rods, separated by 

 pigmented material absorb- 

 ent of light. They repre- 

 sent the crystalline cones. 

 At c d is an arrow placed 

 in front of them ; at e f is 



Fig. 38. Diagram of mosaic vision. 



a screen placed behind 

 them. Eays of light start 

 in all directions from any point, c, of the arrow ; but of these 

 only that which passes straight down one of the trans- 

 parent rods reaches the screen. Those which pass obliquely 

 into other rods are absorbed by the pigmented material. 

 Similarly with rays starting from any other point of the 

 arrow. Only those which, in each case, pass straight down 

 one of the rods reach the screen. Thus there is produced a 

 reduced stippled image, c' d', of the arrow. 



There has been a good deal of discussion as to the 

 relative functions of the ocelli and the facetted eyes of 

 insects. The view generally held is that the ocelli are 

 specially useful in dark places and for near vision ; while 

 the facetted eyes are for more distant sight and for the 

 ascertainment of space-relations. How the two sets of 

 impressions are correlated and co-ordinated in insect-con- 

 sciousness, who can say ? * 



The interesting observations of Sir John Lubbock seem 

 to show that insects can distinguish between different 

 colours. " Amongst other experiments," he says,f " I 



* We must remember how largely the antennae are used when an insect is 

 finding its way about. Watch, for example, a wasp as it climbs over your 

 plate. If the antennae be removed, it seems to stumble about blindly. The 

 antennae seem almost to take the place of eyes at close quarters. 



t " Senses of Animals," p. 194. 



