The Sense of Sight ifi Spiders 28 1 



tion of the two planes intersecting the plane of symmetry of the 

 bodv. After experimenting for a long time I decided upon the 

 following method. The spider is killed in alcohol and then kept 

 in it for several days. When the muscles have become entirely 

 rigid, so as to allow of no change in the shape of the cephalo- 

 thonax while drying, the abdomen is severed from the cephalo- 

 thorax through the petiolus with a sharp pair of scissors. Next 

 the pars labialis, together with the laminae, the palpi and the chelae, 

 is carefully removed with forceps. At the same time care must 

 be taken not to tear the chitinous bridge connecting the opposite 

 sides of the cephalothorax immediately behind the chelae since 

 this would lead to a flattening of the cephalic part and consequent 

 distortion of the true angles. The legs are now carefully removed 

 leaving the coxae alone in their normal position attached to the 

 sternum. Xext a thm Ime is drawn with chinese ink on a very 

 thin layer of Canada balsam that has been spread with the finger 

 over a small cover-glass. It is best to use rather a thick cover- 

 glass and to draw the line quite across it parallel to two edges. 

 A drop offish glue is now put in the center of the glass and on this 

 the cephalothorax is placed with the sternum toward the glue and 

 gently pressed until all the coxae and the sternum are in contact 

 with the glass. The glass now represents one of the two planes 

 intersecting the plane of symmetry at right angles. I shall" call 

 it the horizontal or foundation plane. It is only approximately 

 parallel to the surface of the earth or to that of any object upon 

 which the spider may be, since the spider is able to raise itself up 

 on either front or hind legs, in this way changing the angle between 

 the foundation plane and the horizon. At the same time it is 

 essential that the plane of symmetry should coincide with the 

 black line on the cover-glass. This is accomplished by using 

 a needle under the microscope at a magnifying power of about 

 twenty diameters. It is now easy to determine the plane of sym- 

 metry by taking the pomt midway between the front middle eves, 

 the central, longitudinal groove of the cephalothorax when it is a 

 species possessing this groove, and the point at an even distance 

 between the two chitinous plates of that part of the petiolus which 

 remains with the cephalothorax after the abdomen has been 



