The Sense of Sight in Spiders 303 



The image of 14 cm. placed at a distance of 27.5 cm. from the 

 ASE corresponds to five divisions of the scale or the image of i 

 sq. cm. = 0.006- mm. The distance between the centers of 

 the rods in this eye being equal to 6,a, the image will occupy 4 

 rods (2 X 2). We have then in Lycosa also, four pairs of eyes 

 of different sensitiveness in the following sequence from more to 

 less sensitiveness: PME, PSE, AME, ASE. 



Unfortunately I have no material of Heteropoda venatoria 

 fixed in the manner described, but from comparison with Lycosa 

 and Phidippus we should expect to find that in this spider the 

 most sensitive eyes are the posterior side eyes followed by the ASE, 

 PME and AME. 



In order to form a clear conception of the acuity of vision of 

 the spider's eye, we shall compare the anterior middle eye of 

 Phidippus and the posterior middle eye of Lycosa, the two most 

 sensitive eyes, with the average human eye. The distance between 

 the centers of the cones in the yellow spot of a human retina is, 

 according to measurements made by various scientists, somewhere 

 between 4 and 5/<. One square centimeter placed at a distance 

 of 30 cm. occupies somewhere in the neighborhood of 114^ = 

 12996 cones in the human eye, 13^ = 169 rods in the AME of 

 Phidippus and only 2^ = 4 rods in the PME of Lycosa (Fig. 

 12). The difference becomes still more tangible if we find the dis- 

 tance at which i sq. cm. will fall on one rod in the spider's eye, 

 with other words, if we ascertain the smallest angle or hmit of vision. 

 This may be done either by direct calculation from the data obtained 

 or by measuring the image of a centimeter scale that is gradually 

 moved away from the eye until the image becomes smaller than 

 the distance between the rods. In this way I found that the 

 minimal angle of vision, using glycerin-ablumen for suspension 

 of the eye, equals 8' for the AME of Phidippus and about 60' 

 for the PME of Lycosa, while from observation of the double 

 stars it is known that the smallest angle of vision in man equals 

 i^ Thus a creeping insect about i sq. cm. in size would be per- 

 fectly visible to the human eye, even perhaps to the extent of 

 recognizing the species, at a distance of about 3 m., while it would 

 appear merely as an indefinite, tiny moving speck to Phidippus 

 and would be entirely beyond the range of vision of Lycosa. 



