The External Anatomy of Spiders 



easily distinguished. It is usually wedge-shaped behind; the 

 lateral portions of the thorax extending a considerable distance 

 on each side of it (Fig. 72). 



The Eyes.— The eyes are all simple, resembling in appear- 

 ance the ocelli or simple eyes of insects; in none of them is the 



Fig. 71. A SPIDER. SHOWING THE DIVISION OF THE BODY 

 INTO CEPHALOTHORAX AND ABDOMEN 



outer layer divided into facets as in the compound eyes of insecb. 

 They are usually situated near the front end of the head; in some 

 cases they are grouped upon a tubercle (Fig. 72); in others, they 

 are separated so as to occupy nearly the whole width of the 

 head (Fig. 73).* The normal number of the eyes is eight; but 

 two, four or six of them may be wanting and certain cave spiders 

 are blind. The number and the arrangement of the eyes furnish 

 characters which are much used in classification, as is shown in 

 later portions of this book. 



In works on the classification of spiders, two types of eyes 

 are distinguished, the nocturnal eyes and the diurnal eyes. The 

 so-called nocturnal eyes are found in spiders that live in the 

 dark or that frequent shady places; they are distinguished by 

 being pearly white in colour. The so-called diurnal eyes lack 

 the pearly lustre and are variously coloured. This distinction 



* See page ioon the primitive position of the eyes. 



96 



