296 Alexander Petrunkevitch 



objects in front of the spider. In the same eyes in Heteropoda 

 the direction of the axes changes in the opposite sense, i. e., out- 

 ward and this change serves to bring about a better discernment 

 of objects considerably at the side of the median hne, while the 

 same eyes still guard the front sufficiently. In the posterior 

 middle eyes of Lycosa, the most sensitive ones in this spider, the 

 direction of the axes changes to one more downward and inward 

 thus serving to protect better the front. The direction of the 

 axes in the posterior side eyes of the same spider changes in such 

 a way that the adult eyes look farther backward. In both PME 

 and PSE this change takes place at the expense of the field 

 protected in the young spiderling, which now becomes relatively 

 exposed. And herp tVip advantag-e of a slower growth of the eye- 

 area as compared with the growth of the rest of the cephalothorax, 

 becomes evident. Indeed if the eye-group should occupy in the 

 adult spider relatively the same portion of the cephalothorax as 

 it does in the youngest spiderling, the unprotected field would 

 become in consequence of the change in the direction of the axes, 

 so large that the presence within it of an object even larger than a 

 spider of the same species, would remain entirely unnoticed. In 

 Heteropoda the change in the direction of the axes of the PME 

 is in exactly the opposite sense to that in the same eyes in Lycosa 

 and affords more protection to the dorsal surface. At the same 

 time in the axes of the PSE in Heteropoda, the change of direction 

 is in the same sense as in Lycosa and Phidippus. But this change 

 is considerably more marked in Heteropoda with the result that 

 in the adult spider the eyes cover fully 267 degrees instead of the 

 (probable) 166° in the spiderling. This advantage cannot be 

 gained however, without the formation of an unprotected field. 

 Again, as in Lycosa, this field would have been much larger but 

 for the difference in growth between the eye-group and the 

 cephalothorax. 



THE LIMIT OF VISION 



If we examine under small magnifying power at once all eight 

 eyes of a cephalothorax, freshly prepared and suspended on a 

 drop of glycerin-albumen as I have described, we shall remark 



