214 



THE EYE IN EVOLUTION 



Tegenaria 



Aratieus 



Salticus 



Whip-scorpiou 



spider, Tegenaria domestica, has two rows of 4 ocelli, those of the 

 anterior row being slightly smaller than those of the posterior (Fig. 213); 

 the common garden spider, Araneus diadematus, has 4 median and 4 

 small lateral eyes (Fig. 214). The more active hunting species which 

 construct no web have larger eyes ; thus the wolf-spider, Lycosa, 

 has an anterior row of 4 small ocelli, two large posterior median 

 and two smaller posterior lateral ocelli (Fig. 215) ; while the jumping- 

 spider, Salticus, with a visual capacity more fully developed than 

 the wolf-spider, has an anterior row of two large and two smaller 

 ocelli on the front of its square-shaped cephalothorax, and two very 

 small posterior median and two posterior lateral ocelli on the top 

 (Fig. 216). With all its variations the general plan is thus consistent ; 

 the anterior median eyes (the two central eyes in the front row) have a 

 verted retina, the remainder are inverted provided with a crystalline 

 tapetum ^ and since these latter glow in the dark the former are some- 

 times called " diurnal eyes." The nerve-fibres from the two anterior 

 median eyes travel — with a partial decussation at a chiasma — to the 

 ganglion of the first cephalic segment, from the remaining eyes to that 

 of the second (Figs. 107, 126). 



It is interesting that the anterior median eyes of spiders are 

 equipped with muscles attached to their posterior aspect rendering them 

 motile so that they can increase their visual field ; thus web-spiders 

 have one muscle, Lycosids two, and Salticids six. These are absent in 

 the lateral and posterior median eyes. 



Curious anomalies to this general arrangement exist, but they are rare ; 

 thvis in the female of a spider found in France, Walckenaera acuminata, the eyes 

 are arranged on a dumpy tubercle on the cephalothorax, while in the inale they 

 are perched on a long stalk-like periscope, 4 on the tip and 4 half-way down 

 (Millot, 1949). It is interesting that among spiders the lens, which is part of the 

 outside covering of the animal, is cast at the time of moulting and thtis it would 

 appear that the spider may be ren-dered temporarily blind. 



PSEUDO-SCORPIONS (pseudoscorpionidea), minute animals resem- 

 bling miniature scorpions but without the long tail and sting, found 

 burrowing in books or under stones, the bark of trees and the wing- 

 covers of insects, are provided with two pairs of simple eyes (when 

 they exist) on either side of the cephalothorax ; the^e are typically 

 equipped with an inverted retina and a tapetum (Scheuring, 1913) 

 (Fig. 217). 



w^HiP-TAiLED SCORPIONS (PEDiPALPi). The eycs of this order are 

 not well known (Scheuring, 1913 ; Versluys and Demoll, 1923 ; Millot, 

 H'49). They are entirely absent in some species ; in others there are 

 two median eyes only ; but the typical arrangement consists of two 

 median (principal) eyes and two groups of 3 lateral eyes. 



' Except Salticus, the eyes of which lack a tapetum and are therefore " diurnal ". 

 See fr '-■ p. 1.50. 



