28o Alexander Petrunkevitch 



The disadvantages are: 



1 That beyond some experiments of Plateau and the Peck- 

 hams, nothing definite is known in regard to the acuity of vision 

 in spiders and no method has been brought forward for its study. 



2 That the eyes are compHcated organs, consisting of a refrac- 

 tion and a perception apparatus, each of which and the separate 

 parts of which may possibly be capable of adaptation and which 

 have therefore to be considered separately. 



Let us now begin with a closer study of the ocular group on the 

 cephalothorax of various adult spiders. 



THE POSITION OF THE EYES ON THE CEPHALOTHOR.\X 



The number of eyes in the great majority of spiders is eight 

 and the group which they form on the cephalothorax is so 

 characteristic for the different families that it was for a long time 

 used as a systematic character of great value. The group con- 

 sists usually of two or three rows, more rarely of two or three 

 smaller groups containing three or two eves each. Of importance 

 for systematics are the length of each row, the distances between 

 the eves taken in relation to their diameters, the form of the row, 

 whether straight or bent in the middle forward or backward, as 

 well as the shape and to some extent the color of the eyes. Even 

 a superficial observer will notice in addition, that both in the 

 jumping and the ground spiders (Attidae and Lycosidae), two of 

 the eyes on the forehead are larger than the others, especially in 

 the former. But the reason for such a configuration in the eye- 

 group was never sought for and the apparent similarity in the 

 eve-groups of spiders belonging undoubtedly to different families, 

 led to the conclusion that the value of the eye-group as a sys- 

 tematic character had been overestimated. 



The position of the eyes on the cephalothorax becomes more 

 comprehensible when we begin to study the directions of their 

 respective axes and the angles that these axes form with the three 

 chief planes of the body. Unluckily we at once meet with great 

 difficulties even though we choose species having perfectly round 

 eyes. The first of these is to ascertain with exactitude the posi- 



