502 ALFRED O. GROSS 



1. Material. The material used in these experiments was 

 obtained from the dark basement store rooms of a sugar refinery. 

 No attempt was made to rear the cockroaches in the laboratory, 

 but a fresh lot was secured each time it was desired to make 

 a series of tests. They were always dark-adapt6d before they 

 were used in the experiments. 



2. Methods. A plain piece of slate 28 cm. square was used 

 in exposing the cockroaches and thus the strong thigmotrophic 

 response of these insects called forth by sharp corners or con- 

 cave surfaces was avoided. The slate supported by a small table 

 16 cm. in height was placed inside a glass box 40 cm. square by 

 15 cm. in height. The purpose of the box, which in no way 

 interfered with the rays of light, was to entrap the cockroaches 

 when they ran off the edge of the slate. When a large number 

 of specimens had thus accumulated they were re-collected but 

 were not used in a second series of tests until they had become 

 dark-adapted. The entire apparatus including the outside of the 

 glass box was painted dead black to reduce to a minimum the 

 reflection of any light. 



A cockroach to be tested was placed at the middle of the 

 surface of the slate, with its axis perpendicular to the direction 

 of the rays. In this position there was an equal chance of its 

 going to one side or the other unless influenced by the light. 

 The direction of movement of the cockroach was not always 

 directly towards or away from the source of light, but was fre- 

 quently to one side of the illuminated field to the darkness. 

 Some of the erratic movements of these highly excitable and 

 nervous insects may be explained in part by what seems to be 

 their intense fear while in the light. In such cases, where the 

 orientation was not direct, the response was considered positive 

 or negative according as the cockroach left the slate on the side 

 toward or away from the source of light. If it left the slate on 

 the side of the median line away from the light, the reaction 

 was considered negative no matter if the course taken by the 

 insect was in the direction of the rays or at an angle with them. 

 If the response was in any direction toward the light it was 

 counted as positive. 



