l62 THE SENSES OF ANIMALS 



single broad picture, or the creature may pay attention 

 only to one set, top, side or front, at any one moment. 

 Those species with very large front eyes probably use them 

 in the second way and disregard the images in the others 

 when looking at something close ahead. It is possible, too, 

 that different pairs of eyes have different focal lengths so 

 that some are used for distant vision, others for near. 



In some spiders some of the eyes are dark in colour, others 

 pearly white. It has been suggested that the dark eyes are 

 for day vision, the others for seeing at night or when the 

 light is dim. In the jumping spiders a peculiar flickering 

 and change of colour in the large front eyes is seen when 

 the animal is stalking its prey. This is probably caused by 

 a movement of the retina to alter focus, or by a slight move- 

 ment of the base of the eye-cup to give convergence. What- 

 ever may be the explanation it is certain that the eyes of 

 these spiders are highly efficient ; they evidently give a clear 

 image for these animals carefully stalk their prey and then 

 suddenly jump upon it from a distance of several inches. 

 Most of us have experience of the quickness of sight in 

 spiders. If a large house-spider is cautiously walking along 

 the base of the skirting-board in the room where you are 

 sitting quietly reading and you catch the movement out of 

 the corner of your eye (the image on the rods at the side 

 of the retina, not on the fovea) and you look at it (bring 

 the image on to the fovea) the spider at once notices your 

 slight movement and starts running for shelter. It seems 

 almost uncanny that the spider knows you are going to rise 

 and swat it — if you are one of those people that must swat 

 man-eating spiders — and it appears to have much too much 

 perception and intelligence for an invertebrate. But if you 

 can forget your emotions you realize that the unfortunate 

 creature is merely reacting to your own involuntary startled 

 reaction, though it is only a movement of your eyes, through 

 its acute powers of sight. 



Although the eyes of some spiders are such excellent 

 organs of vision, the ocelli in many arthropods are less 

 efficient. The caterpillars of butterflies and moths have no 

 compound eyes but are provided with ocelli which resemble 

 the eyes of spiders in fundamental structure. Yet they are 



