INTRODUCTION. Vii. 



Their disposition, though uniformly symmetrical, is exceed- 

 ingly varied ; they are very generally placed in pairs, and almost 

 always present some modification of two transverse rows of 

 four eyes each; and their size, both actual and relative, 

 differs greatly. The portion of the caput occupied by the 

 eyes is called the ocular area (pi. i., fig. 5a',«") ; on some 

 eccentric forms of this area (see above). The interval between the 

 anterior row of eyes and the lower margin of the caput is called 

 the olypeus (pi. i., fig. 5J') ; and to the whole area, from the lower 

 margin to the posterior eyes, is given the name of the facial 

 tpace (pi. i., fig. 5c'). 



Spiders are in some measure compensated for inability to 

 move the eyes, by having their visual axis variously direoted, 

 owing partly to the greater or less convexity of the surfaoe of 

 the ocular area, and partly to the presence of tubercles, upon 

 or around which the eyes are seated : thus the eight-eyed spidors 

 are frequently able to look eight different ways at once. 



The eyes of spiders vary in shape as well as in colour ; some 

 are pearly white, others (in the same individual) are dark 

 grey, and even black ; -while among the Salticides eyes of most 

 brilliant hues may be seen, showing the colours of the amethyst, 

 emerald, opal, and other precious stones. Some (the greater 

 part) are round, others are oval, and some of a sub -triangular, or 

 irregular form. Mons. Eugene Simon (of Paris) calls the pearly 

 white eyes " leg yeux nocturnes," and the dark-coloured ones "let 

 ytux diurnes ;" laying great stress upon the systematic value of 

 the difference of function supposed to exist between these two 

 kinds of eyes ; but this value may well be questioned. 



In certain spiders some of the eyes have a flattened and 

 abortive appearance, and are probably, in such cases, of little, or 

 perhaps no use for visual purposes. Examples have occasionally 

 been met with in which one, or more, of the eyes is wanting. A 

 very curious instance of this kind is at present before me, in an 

 adult male example of Tegenaria Derhamii, Scop., received from 

 Belfast. No outward trace of an eye is visible in this specimen, 

 but I can discern a faint appearance of the four central ones 



