The Sense of Sight in Spiders 295 



10° in front of the posterior middle eyes and about 55° of unpro- 

 tected field at the back behind these same eyes. In the projection 

 on the plane of symmetry (Fig. 7) the eyes cover 152° or in the 

 normal position of the spider, on a wall, 125°, as may be readily 

 understood from the drawing. In the vertical plane (Fig. 8) the 

 eyes of Heteropoda cover a field of 193°. This spider lives in build- 

 ings where it runs along the walls and ceilings hunting insects and 

 other spiders and it is distinctly crablike in motion. The com- 

 paratively large fields of vision in this species are possibly to be 

 accounted for in connection with the habit of the spider to remain 

 quiet during the day and to begin its activity at dusk. But this 

 does not obscure the fact that the sides of this spider are better 

 protected than the front. 



It was next necessary to ascertain whether or not the fields of 

 vision vary in spiders of the same species at different ages. With 

 this object in view many spiders were examined, always with the 

 same result, /. e., from the time when the eyes assume their 

 permanent position on the head of the spiderling, the maximal 

 angles of vision and the fields covered by these eyes are the same 

 as in the mature female. As to spiderlings taken directly from 

 the cocoon, I am sorry to say that I was unable to make any 

 observations upon them. They are so small and their chitin so 

 soft that it is impossible to prepare them in the manner described 

 and I have not as yet devised another method. But assuming 

 that their maximal angles of vision are the same, which is indeed 

 very probable, we may readily see the advantage in the changes of 

 direction in the eye-axes as I have described them. A glance at 

 the accompanying drawings will make this clear. To attain their 

 permanent position the axes of the AME in Phidippus move 

 upward and inward. This slight upward change makes it possible 

 for an image to be formed of an object on the central part of the 

 retina of an adult on the same plane, a good deal farther away 

 than is possible in the eye of the spiderling. We shall see farther 

 on that the central part of the retina is much more sensitive than 

 the periphery. The change inward tends to the same end as the 

 change upward and the final position of the anterior middle eyes 

 in Phidippus allows therefore of a more perfect distinction of 



