The Sense of Sight in Spiders 283 



cover-glass is held in a vertical position with the aid of two straight 

 angles which are removed as soon as the wax is hard enough 

 to keep the glass in position. Another method, much simpler 

 and perhaps just as good, consists in adjusting the cover-glass 

 until its upright edge forms a straight line with its own reflection in 

 the slide. The angles are now measured in the same way as 

 before. The drawing is of course made under the same magnifi- 

 cation and the upper edge of the glass holding the cephalothorax 

 is likewise drawn. It represents a cross-section through the 

 horizontal or foundation plane. 



To make a drawing of the projection on the vertical plane one 

 proceeds in the same manner with that difference that the cover- 

 glass carrying the cephalothorax is placed on its other edge. 



Two facts become immediately apparent upon comparing the 

 drawings made in this way, of eyes of spiders belonging to 

 different famihes. First, that there is not a single pair of eyes 

 which are focused upon a single point like, for instance, the eyes 

 of man. On the contrary, the axes of all eight eyes are so directed 

 as to form divergent angles luith each other. Second, that not only 

 do the positions of the eyes on the cephalothorax of spiders belong- 

 ing to different families differ from each other, but the axes of the 

 same eyes in different spiders do not he in the same direction but 

 form with the three planes of the body, the planes of projection, 

 angles differing considerably from each other but fixed for each 

 species. To make this clear let us look at the drawings (Figs. 

 1-9) representing the eyes of three hunting spiders, Lycosa 

 nidicola^a common large ground spider of northern America, 

 Phidippus tripunctatus — a large jumping spider belonging to the 

 same region and Heteropoda venatoria — a cosmopolitan tropical 

 and subtropical spider of very large size, belonging to the family 

 Heteropodidae and resembling in habitus the crab spiders (Thomis- 

 idae) among which it was wrongly placed by earlier systematists. 



Translating the results of these measurements into common 

 language, we may thus describe the positions of the eyes in the 

 three spiders. The anterior middle eyes in the jumping spider, 

 Phidippus tripunctatus, are directed forward and a little outward 

 and downward. In Lycosa nidicola more outward and consider- 



