SIGHT IN INVERTEBRATES 



i6i 



no more than light-gatherers, but those that are more com- 

 plex are almost certainly organs of sight that give a clear 

 image, although perhaps only at certain distances. In the 

 ocelli of some insects there are even two layers of retinal cells 

 — on the outer layer distant objects may be focused and on 

 the inner layer nearer ones. 



Most spiders have eight eyes, each an ocellus, but some 

 have only six, and yet others that live in the darkness of 

 caves have none. The eyes are placed at the front end of 

 the "head" part of the cephalothorax and are grouped in a 

 great diversity of patterns so different from one another that 

 their arrangement is one of the most important characters 



Fig. 5. The cephalothorax in three different sorts of spiders to show the ocelli 

 in differing patterns at the "head" end. The views are looking down on the 

 spiders' backs and the head is at the top. The bases of the eight legs are indi- 

 cated and the abdomens would join the centres of the lower edges. 



used in the classification of spiders. Some of the eyes look 

 upwards, some sideways and some forwards, and sometimes 

 they are mounted on turrets that give a wide field of view ; 

 those directed forwards are usually the largest. It is difficult 

 to imagine how the world appears to a spider provided with 

 so many eyes. The images received by the different eyes 

 may be combined in the brain of the animal to form a 



